Biomechanics of Ventilation in Boa Constrictor

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Honors Theses
5-19-2013
Biomechanics of Ventilation in Boa Constrictor
John George Capano
Dickinson College
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Capano, John George, "Biomechanics of Ventilation in Boa Constrictor" (2013). Dickinson College Honors Theses. Paper 7.
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Biomechanics of Ventilation
in Boa constrictor
By
John Capano
Submitted in partial fulfillment of Honors Requirement
for the Department of Biology
Dr. Scott Boback, Thesis Advisor and Collaborator
Dr. Charles Zwemer, Collaborator
Dr. Anthony Pires, Reader
May 16, 2013
The Department of Biology at Dickinson College hereby accepts this senior honors thesis by
John Capano, and awards departmental honors in Biology.
Scott Boback (Advisor)
Date
Charles Zwemer (Committee Member)
Date
Anthony Pires (Committee Member)
Date
Charles Zwemer (Department Chair)
Date
Department of Biology
Dickinson College
May 2013
ABSTRACT
Boa constrictor possess an elongate lung containing an anterior vascular section that abruptly
changes into a saccular lung incapable of gas exchange. Measurements and analyses of data
have shown that the anterior vascular portion may be compromised for ventilation during
acts such as constriction and prey ingestion and that the saccular lung may actively
participate in ventilation. Five snakes were instrumented with electromyograph electrodes to
record the ventilatory muscle activity throughout the various regions of the lung. It was
determined that the animals in this study were capable of activating the inspiratory levator
costa muscle independently in the anterior and/or posterior regions and could effectively shift
the site of ventilatory pumping between the vascular and saccular lungs. It was also found
that locomotion precludes the levator costa from being used for ventilation, and that this
muscle is co-opted for both locomotion and ventilation. These findings support the
hypothesis that the saccular lung functions as a caudal bellows during times of compromised
activity and indicates that the levator costa in snakes is utilized for multiple tasks.
INTRODUCTION
Snakes are characterized by an extreme elongation the body, which poses significant
anatomical and biomechanical challenges. One the most important features of this character
are the housing of the visceral organs in an elongate cylinder, with many of the organs
assuming attenuated shapes, including the lungs. In all snake species, the right lung functions
as the primary lung and is considerably longer than the left lung, which often has reduced
functional capacity or is almost entirely vestigial (Wallach 1998). The elongate lung of
snakes is heterogeneous in nature, possessing various levels of vascularization. The anterior
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portion is highly vascularized while the distal posterior section consists of a non-septate sac,
a saccular or semi-saccular lung, which is incapable of gas exchange. The function of this
avascularized saccular lung is of considerable interest. In this study, I look at the lung
anatomy and function of the ventilatory muscles in Boa constrictor in an attempt to elucidate
how this saccular lung contributes to the ventilation of Boa constrictor.
Ventilation and Lung Morphology of Snakes
All sauropsids, the lineage containing Boa constrictor, respire using costal ventilation
with faveolar lungs; a lateral movement of the ribs with costal musculature to increase or
decrease the thoracic volume. The faveolar lung system of sauropsids are composed of many
faveoli, small chambers (faveolus means cup or honeycomb), all connected to a central
chamber, for gas exchange (Pough et al 2012). Sauropsids consist of two major clades,
archosaurs and lepidosaurs, which possess considerable variation in the mechanics and
physiology of their ventilation. Most lepidosaurs—squamates, which includes snakes, and
tuataras—have a simple, septate lung with shallow depressions on the sides of the lung that
contain the faveoli used for gas exchange.
The lung morphology of snakes follows the lepidosaur pattern but has been
elongated. Mathematically this is of particular interest because of the ratio between the
surface area (of faveoli) to volume (of the lung) ratio necessary for efficient gas exchange.
Dividing the radius of the lung by a factor of X, the length of the lung must be multiplied by
the same factor of X in order to maintain surface area. Surface area and volume are not
proportionally related and it is necessary to multiply the length of the lung by X² in order to
preserve the original volume (McDonald 1959). Presumably, snakes have accounted for this
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mathematical relationship through a considerable increase in lung length and a modest
increase in the faveolar region. This results in heterogeneous lung morphology including the
vascular, semi-saccular and saccular regions. This presence of a distinct, concentrated
vascularized region may be explained by a need to balance the percentage of total blood
volume between pulmonary and systematic circuits. There also exists a relationship between
lung diameter and volume of air ventilated where the saccular lung, by increasing volume,
serves to reduce the diameter change necessary to ventilate the volume of air that fills the
vascular section of the lung (McDonald, 1959). The ability of the organism to meet the gas
exchange demands of its body requires a balance between all of these factors.
The elongated right lung contains several distinct regions: the vascular lung, the
semisaccular lung, and the saccular lung. The vascular region is defined as the cranial tip of
the right lung to the point where the faveoli noticeably change in size and density per unit
area as well as a reduction in septal wall thickness and parenchyma (Wallach 1998). The
semisaccular region is the transition between the vascularized and saccular regions, from the
posterior end of the vascular lung to the point of the last visible trabeculae. The saccular
region, occupying the posterior 60% of the lung, is distinguished by a lack of vascularization
with very thin, translucent walls (Grant 1981; Wallach 1998). In Boa constrictor, the saccular
lung contains smooth muscle ridges; the few capillaries present beneath the smooth muscle
do not connect to the air space (Grant 1981). Therefore the saccular regions appear to be
incapable of significant gas exchange. This inability to participate in gas exchange makes it
of particular interest to determine how the saccular lung participates in ventilation.
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Axial Musculature in Snakes
Snakes utilize their highly developed axial musculature to perform actions
necessary for locomotion, constriction and ventilation. The axial muscles can be
broken up into two groups, epaxial—dorsal to the horizontal septum—and hypaxial—
ventral to the horizontal septum (Figure 1). Constricting species of snakes, especially
those in Boidae—the family including Boa constrictor, have highly developed
epaxial muscles—the group responsible for controlling the axial bending used for
application of pressure during constriction as well as locomotion (Moon 1998, Gasc
et al 1989, Lourdais et al 2005). The three major muscle groups of this epaxial
bundle are the spinalis-semispinalis, the longissimus dorsi, and the iliocostalis. The
epaxial muscle group is of particular importance for Boa constrictor as they
participate in constriction and their semi-arboreality requires a complex locomotory
pattern with these muscles (Byrnes, 2010; Jayne, 1988).
Ventilation in snakes is accomplished via costal ventilation and is powered
almost entirely by hypaxial muscles (Rosenberg 1973). Inspiration is accomplished
by contracting the levator costa and retractor costa which pull the ribs laterally,
increasing thoracic volume. Specifically, the levator costa pulls the ribs laterally and
craniad while the retractor costa pulls the ribs laterally and caudad, resulting in an
overall lateral forward movement. The retractor costa attaches dorsally to the ribs
near the lateral connection of the levator costa and indirectly to the vertebral column
due to its origin from the longissimus dorsi, which is epaxial and just dorsal to the
vertebrae. The retractor costa is innervated by the lateral branch of the dorsal ramus,
as all of the epaxial musculature is innervated by the dorsal ramus in snakes (Fetcho
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1986, Wood 1976). The levator costa lies beneath the retractor costa as part of the hypaxial
muscle group. The levator costa originates on the prezygopophyis of the vertebrae and runs
ventro-lateral to their terminal connection on the anterior surface of the adjacent posterior rib.
It is innervated by the anterior portion of the lateral branch of the ventral ramus nerve—the
ventral ramus being responsible for innervation all of the hypaxial musculature in snakes
(Fetcho 1986, Figure 2).
Exhalation uses the dorso-lateral and ventro-lateral sheets to decrease thoracic
volume. The dorso-lateral sheet consists of the transversus dorsalis and the obliquus
internus dorsalis, both of which connect most anteriorly to the midventral surface of the
vertebral column and terminate on the medial surface of the ribs and are innervated by the
medial branch of the ventral ramus (Fetcho 1986).The ventro-lateral sheet is comprised of the
transversus ventralis and obliquus internus ventralis that connect on the medial rib surface
and mid-ventral connective tissue of the skin and are innervated by the posterior portion of
the medial branch of the ventral ramus nerve (Figure 2, Figure 3, Wood 1976, Fetcho 1986).
During exhalation the dorso–lateral sheet contracts to pull the ribs medially and caudad while
the ventro-lateral sheet contracts to pull the ventral surface of the snake dorsad and push the
organs, primarily the liver, against the ventral surface of the lungs, facilitating exhalation
(Rosenberg 1973).
Constriction and Prey Ingestion in Boa constrictor
Boa constrictor is a primitive snake species of Boidae that use constriction to subdue
and kill their prey, organisms such as lizards, birds, and mammals. The momentum of the
initial strike and contact is carried through as the snake’s body twists and applies two or more
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loops around the prey and begins to apply force (Boback 2012). After the strike, the snake
first makes contact with its prey with its ventral surface and then progressively more laterally
as it applies coils around the prey animal, producing what is referred to as a ventral-lateral
coil (Mehta and Burghardt, 2008). Boa constrictor almost always wrap their coils with their
left side, viewed dorsally, in contact with the prey—the side containing the elongate right
lung, when viewed ventrally. When snakes are constricting their prey, the process of
ventilating becomes more difficult as much of the ventilatory musculature, especially the
levator costa on the left side of the snake, must work against the epaxial muscles that are
attempting to reduce the diameter of the coil around the prey. It has also been hypothesized
that the levator costa participates in body wall movements involved during constriction and
may be co-opted for use during both constriction and ventilation (Moon 2000). Constriction
coils are applied with the anterior portion of the snakes trunk and in Boa constrictor the
vascular lung extends between 26-42% of the snake’s snout-vent length—SVL—and the
saccular lung extends 43-60% SVL (Wallach 1998). Therefore, the act of constriction and the
potential overlap seems to preclude the necessary volumetric changes needed for ventilation.
This compromised function is interesting as constriction is an energetically costly activity
that increases aerobic metabolic demands nearly seven times more than basal levels (Canjanji
et al 2003). It is possible that the posteriorly positioned saccular lung, that is not located
within the typical coil application region, could function in ventilation when the anterior
musculature is devoted to constriction.
As many as seventeen different hypotheses have been proposed for the
function of the saccular lung (Wallach 1998). Possibilities include its use as a
buoyancy device, a gas reservoir, a storage chamber for deoxygenated air and others.
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One of the most intriguing and plausible is that the saccular section of the lung may be
utilized as a caudal bellows when normal lung function is compromised in the anterior
portion of the body. This may be during prey ingestion or constriction, where thoracic rib
movements are hindered in the anterior trunk typically associated with ventilation (Wallach
1998). Locomotion may also compromise ventilatory function of levator costa, as it has been
shown to contribute to the axial bending necessary for movement in snakes (Moon 1998).
The caudal bellows function would allow for the caudal portion of the lung to expand by use
of caudal hypaxial ventilatory muscles, creating negative pressure to pull air into the lungs
and across the vascular lung and would obviate the need to use more cranially positioned
hypaxial muscles that are occupied with constriction or locomotion, or are unable to expand
thoracic volume due to the presence of a food bolus.
Previous research investigated the ability of Boa constrictor to respond to the heart
rate of its prey (Boback et al 2012). During this work, observations were made of the snake
apparently ventilating with the section of its body distal to the constriction coil, the area
associated with the saccular lung. These observations are similar to those described by
McDonald et al (1959) who suggested the shifting of ventilation to posterior regions when
the anterior, vascular lung was compromised. In this study, a 75 cm Lampropeltis getula
ingested a 50 cm Thamnophis elegans and during ingestion, all ventilatory movements were
observed posterior to the distended region—which was apparently suffering from
compression by the food bolus. Only after the prey moved caudally to the distal end of the
stomach and the approximate terminal end of the right lung, did the ventilatory movements
move anterior to the prey, presumably as the compression on the vascular lung lessened
(McDonald 1959). McDonald concluded that ventilation may have been controlled by the
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saccular region during swallowing but that this saccular portion was also subject to
compression once swallowing was complete and the food bolus compressed the saccular
region.
These observations indicate that the posterior portion of the snake associated with the
saccular lung is participating in ventilation, potentially functioning as a caudal bellows. We
are interested in determining whether the saccular lung of Boa constrictor could function this
way. This hypothesis may be particularly relevant in Boa constrictor, a species known to
constrict and ingest relatively large prey animals, up to 100% of the snake’s body mass, for
up to 45 minutes (Boback et al 2012). The reduced ability to ventilation due to constriction
may then be temporally lengthened as the ingestion of the prey item will continue to preclude
ventilation in the anterior portion of the animal.
Justification
Considering the lack of knowledge about how this large portion of the lung
functions, we do not have a complete understanding of the ecology or evolution of
Boa constrictor. Understanding the function of the saccular lung will increase our
knowledge of how Boa constrictor effectively prey upon and ingest their food,
leading to a better understanding of the evolution of the saccular lung in Boa
constrictor and snakes in general. Through understanding the use of the saccular lung
during constriction and prey ingestion, we will increase the knowledge of snake
evolution by learning whether the saccular portion of the lung is indeed a necessary
adaptation to allow for sustained constriction as well as ingestion of large prey items
in Boa constrictor. This research seeks to establish whether the saccular lung
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functions as a caudal bellows to facilitate ventilation during constriction and/or prey
ingestion.
Anecdotal observations on Lampropeltis and preliminary observations on Boa
constrictor support the idea that Boa constrictor have the ability to shift the location of
ventilation. I predict that when the region of the body around the vascularized lung is
compromised, Boa constrictor will shift ventilatory movements to the saccular region
utilizing the saccular lung as a bellows. In this study, I test whether Boa constrictor are
capable of spatially controlling ventilatory muscle activity. I predict that Boa constrictor are
capable of independently controlling ventilatory muscle activity in various regions of their
body. To test this, I instrumented snakes with electromyography electrodes on the levator
costa and obliquus internus ventralis placed at three locations along the length of the entire
lung: anterior (over the vascular region), posterior (over the saccular region) and in the
middle (at the junction of the two). The null hypotheses are that Boa constrictor will activate
levator costa along the entire length of its body for inspiration (all three electrodes will show
simultaneous muscle activity), and will activate obliquus internus ventralis along the entire
length of its body for expiration (all three electrodes will show simultaneous muscle activity).
The alternative hypotheses are that Boa constrictor will activate the levator costa in either the
anterior or posterior region of its body independently of the other regions and ; and that Boa
constrictor will activate obliquus internus ventralis in either the anterior or posterior region
of its body independently of the other regions. If I find that Boa constrictor are capable of
independent control of their ventilatory musculature, this would support the hypothesis that
Boa constrictor could use the saccular lung as a caudal bellows.
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MATERIALS AND METHODS
Dissection and Morphology
An understanding of the morphology of the Boa constrictor and its internal
anatomy was accomplished through detailed dissection of deceased Boa constrictor.
Measurements were recorded for snout-vent length, and the anterior and posterior
measurements of the following: trachea, heart, left lung, right lung, the vascular
section of the right lung, the saccular section of the right lung, esophagus, stomach,
liver, gall bladder, heart, left kidney, right kidney, and small intestine. The lung was
carefully inflated by inserting a straw into the trachea to determine the length of the
vascular and saccular regions. The vascularized region was measured by observing
the change in parenchyma and faveoli and the visible difference in wall thickness
(Figure 4).These values were then used to determine the relative locations of the
visceral organs in relation to one another, in particular in reference to the right lung.
Constriction Video Analysis
I analyzed data collected in previous constriction experiments in 2010 to
determine average coil application length as a percentage of SVL, as well as average
coil start and end locations as a percentage of SVL. These data were used to estimate
the relative location and percentage of the body any given Boa constrictor would use
during a constriction event. The data collected contained SVL and coil length
measurements but lacked start or stop measurements. I therefore used the videos of
constriction events for 5 snakes where the start of the coil was visibly detectable and
counted which blotch on the dorsum of the snake this start of the coil correlated with.
10
I then measured the distance to that blotch on the animal and added the coil application
length data previously recorded to produce a coil start length, coil end length, and total coil
length for each snake. Using each snake’s SVL, we calculated each of these values as a
percentage of SVL and then calculated the averages of these percentages. The results were
analyzed and no outliers were found. These data gave me the average length and application
start and stop locations of constriction coils for Boa constrictor.
Electromyography
An effective way to determine independent control of ventilatory muscles was
through the use of electromyography. This allows the monitoring of ventilatory muscle
activity in different regions of the body associated with the vascular and saccular sections of
the lung. Previous research on Thamnophis elegans determined the major ventilatory
muscles. I used the measurements and observations made during dissection to determine
which muscles were most appropriate to instrument as well as to measure the relative sizes of
the muscles for electrode fabrication sizes.
Patch electrodes were used to monitor the ventilatory muscle activity. The electrods
were made from 0.5mm thick Down Corning reinforced Silastic sheeting (Dow Corning,
Midland, Michigan, USA). Each electrode measured 4.00 mm X 3.00 mm with electrode
bipole spacing of 1.5 mm and bare recording surface areas of 2.0m Two sets of patch
electrodes were constructed with 0.003 mm stainless steel wire (California Fine Wire, Grover
Beach, CA, USA) while four other sets were constructed with 0.01 mm silver-silver chloride
wire (Grass Technologies, Warwick, RI, USA).
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Each snake had six electrodes implanted, totaling twelve separate wires to
completely instrument the animal. The use of a live animal over an extended period
of time required the ability to freely connect and disconnect the EMG recording wires
from those attached to the animal. Two 12 pin connections were used to create a plug
and play recording system, with female connector attached to the snake and a male
connector attached to the EMG machine. The EMG connections were made by using
Grass Featherlite Disposable Ag-AgCl Disc Electrodes (Model DE-48), removing the
disc end and soldering the wire to Samtech 2.00 mm Low Profile Terminal Strip
(Series TMM) connectors (Grass Technologies, Warwick, RI, USA; Samtech, New
Albany, IN, USA). Electrodes wires from the snake were soldered to Samtech 2.00
mm Tiger Eye™ High Reliability Socket Strip (Series SMM) connectors and sutured
to the dorsum of the snake at approximately 60% SVL. The soldering of the wires to
the connector pin was done post-operatively. For recording, the EMG leads were
plugged into the connector pin attached to the snake. The EMG leads were connected
to and amplified by BIOPAC EMG100B Preamplifiers (BIOPAC Systems, Goleta,
CA, USA). The analog signal was then digitized to 800-1000 Hertz and amplified
with the BIOPAC Systems MP100CE and data was recorded on AcqKnowledge
Software (BIOPAC Systems, Goleta, CA, USA).
Surgical preparation
All surgical procedures were approved by the IACUC Committee. Induction
of anesthesia was accomplished using isoflurane vapor of up to 5% concentration.
The snake was placed into a nylon bag, which was knotted and then placed into a
large plastic bag. Underneath a fume hood, a cotton gauze pad was soaked with the
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5% isoflurane and placed into the plastic bag containing the snake. All of the remaining air
was forcibly removed from the plastic bag and the bag was sealed with a knot. The snake
remained in the induction bag until loss if its righting reflex occurred. Once adequately
anesthetized by this method, the snake’s airway was secured using an endotracheal tube.
Anesthesia depth was maintained throughout the surgical procedure using a variable
concentration of isoflurane and oxygen. Each snake typically remained at a 4-5%
concentration of isoflurane/oxygen balance to maintain surgical depth.
The operating room was set up the day prior to each surgery day. All surgical
instruments were sterilized by autoclave and the operating room was cleaned with bleach. At
the start of each procedure, the total ventral scale count was recorded and a small suture was
then placed through the scale correlating to the location of the electrodes to be implanted at
33%, 43%, and 53% of SVL or at 33% and 48% of SVL. In preparation for surgery, the
snake was positioned in lateral recumbency post anesthetization. Three separate surgical sites
were isolated and were prepared using a commercial preparation of Nolvasan® scrub
(chlorohexidine 2%) as a cleansing agent.
Five snakes were unilaterally instrumented with EMG electrodes (Table 1).
Electrodes were sterilized in 2% chlorohexidine solution before being placed into the snake.
After disinfecting the skin with Novalsan®, short longitudinal incisions were made with a
#15 scalpel blade along the lateral side of the snake for each mark electrode implantation site.
These incisions were made along the dorsal lateral side of the snake along the boundary
between the longissimus dorsi and iliocostalis. The successive layers of tissue and fascia
between the longissimus dorsi and iliocostalis were separated using a combination of blunt
and sharp dissection in order to expose the desired muscle, the levator costa. A patch
13
electrode was placed dorsal to the levator costa and lateral to the obliquus internus
ventralis at each implantation location. Securing sutures were placed through each
corner of the sheets and underlying fascia, angled diagonally away from the sheet.
The wire leads from the more cranial patch electrodes were cannulated via a shared
tunnel to the most caudal incision point (48% or 53% SVL) where the wire bundle
was drawn to the external surface and sutured to the skin with 4-0 PBS.
Post-operative pain was managed by use of the NSAID, meloxicam 0.4mg/kg
IM, once after surgery. Body posture and general activity were used to determine the
degree of prescription intervention. Each day for two days post-operative, snakes
were checked for infection and all incision sites were covered in antibiotic ointment.
Wire exit sites were checked and sealed with cyanoacrylate.
Monitoring of Ventilation
The EMG electrodes were sufficient to record muscle activity, but it was
necessary to correlate these data with ventilatory movements. Video recordings were
taken for each experiment conducted in order to visually confirm ventilatory
movements and correlate these movements with the muscular activity being recorded.
A GoPro Hero Naked (Silver Edition; GoPro, Half Moon Bay, CA, USA) Camera
was used to record high definition video of ventilation at 30 frames per second.
Ventilation Recording
I monitored the muscular activity of Boa constrictor during active ventilation
in order to determine if they were capable of independent ventilatory muscle control.
Researchers entered the snake room as quietly as possible to avoid over-agitating the
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snake being recorded. The GoPro Hero video camera was set on a tripod above the snake
container and angled to approximately 40 degrees to monitor lateral ventilatory movements.
For recording, the EMG leads were hung from a mobile IV stand that was wheeled next to
the snake housing units, where they were plugged into the socket connector strips. The
recording software and the GoPro Hero video camera were then simultaneously started to be
as closely synchronized as possible. Video and electromyograph recordings were taken for
each snake with functioning electrodes at various levels of activity.
Analysis
In order to determine if Boa constrictor was capable of independent ventilatory
muscle control, video footage was correlated with electromyograph data. The video was
analyzed using video frame analysis, where each inspiration and expiration was recorded on
a frame by frame basis. VideoLAN Multimedia Player was used to zoom in on the
ventilatory site and count the frame at which the snake’s body wall moved. Inspiration was
recorded as the frames from the start of the expansion of the body wall movement laterally
until the body wall began to retract medially. The electromyographs were then analyzed,
recording the duration of concentrated positive and negative deflections from baseline
electrical signal. The times of visual body wall movements for inspiration were correlated
with the times of inspiratory ventilation muscle activity. These values were then analyzed
using the Pearson product-moment correlation coefficient. This analysis gives a value
between 1 and -1 representing the strength of the linear relationship between two variables.
The Pearson Correlation (v1.0.6) in Free Statistics Software (v1.1.23-r7) by the Office for
Research Development and Education was used to perform these tests
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RESULTS
Measurements
The results of the measurements of the deceased Boa constrictor showed the
relationship of the visceral organs to one another (Figure 5). The right lung of the
dissected specimen extended 32% of the total SVL, from 38—60% SVL. The
vascular region extended from 28 – 38% SVL with an abrupt change to the saccular
region, which extended from 38 – 60% SVL. The data show that the esophagus and
stomach that extend from 2% – 38, and 38—52% SVL respectively, overlap the
vascular region of the lung that extends 28—38% SVL. This overlap continues for
most the saccular lung which extends from 38—60% SVL. These measurements
support the plausibility that a large food bolus could compromise the function of the
anterior lung by compressing the lung, precluding the ability to expand the thoracic
cavity, potentially promoting the shift of ventilation to the posterior, saccular region
not suffering from compression.
The results of the constriction data analysis of five snakes were combined
with their corresponding anatomical data to determine the relative position of average
constriction coil application in relation to visceral organs (Figure 5). The total region
engaged in constriction was found to be the anterior 40.5% of the snake’s body,
starting at approximately 17.2% (±4.86) of total SVL and extending an additional
23.9% (±5.52) of total SVL. I then correlated all of these data to determine the
plausibility of the anterior vascular lung being compromised and unable to ventilate
during constriction. Based on the data, the average application of constriction extends
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17—40% SVL, with the coils beginning before the vascular lung, which extends 28—38%
SVL, but extending slightly past the distal most portion of the vascular lung. I concluded that
the sustained use of the epaxial muscles throughout this region during constriction may
inhibit the ability of the hypaxial musculature to draw the ribs laterally to ventilate. Therefore
constriction coil application may compress the vascular lung and preclude the snake from
ventilating this portion of its body. This overlap of the vascular region and constriction coil
supports the plausibility that it could be beneficial to shift ventilation to the saccular region
during constriction utilizing independent ventilatory muscle control. The dissection data in
conjunction with the constriction data showed that the vascularized section of the lung could
be compromised during both prey ingestion and constriction.
I concluded after muscular dissection that the levator costa, the major inspiration
muscle, was accessible from the lateral aspect of the snake by lifting the iliocostalis ventrally,
exposing the ventral aspect of the levator costa. For expiration, I determined that the ventrolateral sheet was the most easily accessible expiration muscle, as it was the most lateral and
ventral of the expiration muscles. Specifically, I determined that the obliquus internus
ventralis was the part of the ventro-lateral sheet to instrument as it extended as a sheet on the
most lateral aspect and could be accessed from a lateral incision without damaging the
muscle. The other part of the ventro-lateral sheet, the transversus internus would require
actually cutting the muscle tissue itself to access it, thereby altering our recordings. I
concluded that it was best to monitor the muscle activity of the levator costa and the obliquus
internus ventralis (Figure 6). In order to test the ability of Boa constrictor to independently
control ventilatory muscle firing and potentially utilize the saccular lung as a bellows I
placed electrodes on the levator costa and obliquus internus ventralis at approximately 33%,
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43%, and 53% SVL corresponding to the middle of the vascular region, a third of the
way into the saccular lung, and two-thirds into the saccular lung, respectively. With
this design I could monitor ventilatory muscle firing along the entirety of the lung and
determine if regions could function independently of each other.
Experimental Results
Data from two specimens, 38N10-8 and 37-6 was used for analysis. Two
recordings of both video and electromyograph data were used for 38N10-8 during
active ventilation. Electromyograph data was obtained from the levator costa
electrodes placed at 33% and 48% SVL as well as the obliquus internus ventralis
electrode at 48% SVL. One recording of both video and electromyograph data was
used for 37-6 during active ventilation. Electromyograph data was obtained from the
levator costa electrodes placed at 33% and 53% SVL as well as the obliquus internus
ventralis electrode at 43% SVL.
Completion of time frame analysis of the video footage of each specimen with
associated electromyograph data displayed correlation between the muscle activity
recorded and inspiratory ventilatory movements observed. Pearson product-moment
correlation coefficients of the times recorded for ventilatory movements and muscle
activity for all tests and showed strong statistical significance (Table 2, Figure 7). The
analysis of the video footage showed that specimen 38N10-8 definitively ventilated
independently in both the anterior and posterior regions of the body with correlating
electromyographs. In Trial 1, 38N10-8 was recorded utilizing the levator costa at 33%
SVL during ventilation without activating the levator costa at 48% SVL and did not
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activate the obliquus internus ventralis electrode at 48% (Figure 8). In Trial 2, specimen
38N10-8 was recorded utilizing the levator costa at 48% SVL without activating the levator
costa at 33% SVL, and did not activate the obliquus internus ventralis electrode at 48%
(Figure 9). In Trial 3, specimen 37-6 was recorded ventilating using both levator costa at
33% SVL and levator costa at 53% SVL simultaneously, as well as an inspiration where only
levator costa at 53% SVL activated but inspiration was still visually observed, with one only
moment of activity recorded in the obliquus internus ventralis electrode at 53% (Figure 10).
DISCUSSION
The data collected support the hypothesis that Boa constrictor are capable of
independent ventilatory muscle control. All Pearson product-moment correlation coefficients
were almost exactly 1. The p-values of all correlations analyzed are < .001, well below the
0.05 level of significance, and the correlations show strong statistical significance. This is not
surprising as ventilatory muscle activity is responsible for the visual ventilatory movements,
and should be linearly correlated with them. Determining that the muscular activity observed
was accurately synchronized with the ventilatory movements observed allowed me to
interpret the findings further. The data shows that the anterior and posterior levator costae
are capable of muscular activity independent of or in conjunction with one another. In this
study, the animals tested have displayed muscular control over location of ventilation. The
following discussion will attempt to elucidate the implications of these findings.
This study pioneers the qualitative assessment of the location of ventilation in snakes.
The results of this study are consistent with the findings of Rosenberg (1973) that the levator
costa is a major muscle of inhalation. My results also agree with his observations of detecting
19
ventilation in the middle third of the snake’s body but go a step further in determining
the specific location of ventilation within this region (Rosenberg 1973). I have shown
that ventilation does not necessarily occur with the entirety of this middle third of the
trunk but is capable of being shifted or ventilated throughout the length of the trunk.
These results are consistent with the observations of McDonald (1959) and
qualitatively support his claim of the ability to shift ventilatory pumping.
The results of this experiment have demonstrated that the two Boa constrictor
recorded are capable of independently controlling their inspiratory ventilation
muscles and can ventilate with the vascular, saccular, or both portions of the lung
simultaneously. At rest, I have observed Boa constrictor ventilating almost
exclusively in the vascular region of the lung. It may be most efficient to use the
anterior vascular lung during normal ventilation, expanding the volume of the lungs
directly on the vascular lung to only draw in the volume of air necessary to fill the
vascular region and only as distally as that region. Therefore, possessing the ability to
independently control ventilation location could allow Boa constrictor to ventilate
with maximum efficiency when capable and to use various other ventilation patterns
in different circumstances. There exist many possibilities as for the use of this ability
such as during defensive displays, prey ingestion, constriction, and locomotion.
Interestingly, in all tests, no significant activity was recorded in the obliquus
internus ventralis muscles instrumented. Although it appeared that the snake was
actively expiring, no muscular activity was recorded. This could be attributed to the
low-frequency output of the muscle, making it difficult to detect. There is also the
potential that the snake was not actively expiring and was passively expiring through
20
the elastic recoil of the walls of the lungs through relaxation of the levator costa as described
by Rosenberg (1973). Grant (1981) also determined that Boa constrictor possess
considerable smooth muscle in the saccular lung region that is not capable of gas exchange
and Rosenberg (1973) hypothesized that the passive expiration he observed could have been
attributed to elastic recoil and the contraction of smooth muscle. The lack of
electromyograph data for the obliquus internus ventralis responsible for expiration and the
observed medial body wall movements after inspiration in this study support the use of
passive expiration during these tests. The only muscle activity recorded in the obliquus
internus ventralis was during an inspiration event, and therefore could not be interpreted as
expiration. It is unclear what this muscle firing indicated. Further study involving gas
exchange rates, gas flow, and muscle activity could further determine the capability of
passive expiration in Boa constrictor.
In Trial 1, specimen 38N10-8 was observed participating in ventilatory movements in
the anterior section of its body. This observation matched the muscular firing of the
anteriorly placed levator costa electrode located at 33% of the snakes SVL, in the section of
the body associated with the vascularized region of the lung. During the segment of the video
and electromyograph analyzed the snake was ventilating at ~ 80 breaths per minute,
considerably higher than the typical resting value of 4 – 16 breaths per minute for snakes
(Klauber, 1972). This could be attributed to the snake being agitated by the presence of the
researchers. Previous research by Rosenberg (1973) has observed Boa constrictor increasing
ventilation rate as well as depth of breathing in response to a visual disturbance and my
results support this claim. Regardless of the ventilatory rate, it was determined that the snake
was ventilating solely with the anterior levator costa at 33% SVL in the portion of its body
21
associated with the vascular lung, with no activity recorded or observed in the levator
costa at 48% SVL in the saccular region.
In Trial 2, the data recorded for specimen 38N10-8 only showed activity in the
levator costa electrode placed at 48% SVL, the section containing only the saccular
lung. The snake also appeared to be agitated in this experiment and breathed more
rapidly than normal, at a rate of`~ 60 breaths per minute. It is unclear why the snake
would choose to ventilate in the saccular region rather than with the vascularized
region of the lung. I predicted that the snakes would ventilate in the posterior saccular
region of the lung when ventilation in the anterior portion of the body was
compromised by constriction or a large food bolus, neither of which occurred during
this time. However, during this test, the snake appears to have retracted and elevated
the anterior portion of its body into an S-shaped defensive posture prepared to strike.
It may be that the epaxial musculature responsible for locomotion is being used to
hold the upright posture and the contraction of these muscles precludes the lateral rib
movement necessary for ventilation. Studies testing locomotion of snakes have
indicated that the majority of the axial bending for locomotion is done by hypaxial
musculature but Gasc (1967, 1974) set forth the hypothesis that the levator costa
contributed to axial bending and locomotor movements as well. The findings of
Moon (1998) testing the muscular activity of locomotion in Pituophis melanoleucus
indicate that the levator costa contributes to axial bending and locomotion and are
consist with the hypothesis of Gasc. The act of striking is a locomotory act, and the
results of this study have shown that before a strike the snakes have ceased
ventilatory firing with the anterior levator costa at 33% SVL, presumably because the
22
levator costa is co-opted for both ventilation and locomotion. Therefore the snake ceased use
of the levator costa for ventilation because of the more immediate need of a defensive
locomotory strike. My findings indicate that locomotion itself may be an act that
compromises the ventilatory function of levator costa in the region used or anticipated to be
used for locomotion. In Trial 3, the electromyography data showed that during the last
inhalation in the recording, only the posterior levator costa fired. The snake stopped
activating the levator costa at 33% SVL and continued to inhale using only the levator costa
at 53% SVL (Figure 11). Interestingly, the snake struck a few seconds after this final
recording. The researcher was directly in front of the snake for the duration of this recording
and was manipulating a snake bag in preparation to handle the animal just before this strike.
The snake may have seen the movement as the researcher prepared to manipulate the animal
and ceased using the anterior musculature for ventilation in preparation for a defensive strike
requiring locomotion. The implication of the cessation of muscle activity in only the levator
costa at 33% remains unclear, as electromyograph data of strikes observed during these
experiments have shown that during a strike, levator costa located at all regions of the body
activate simultaneously to project the snake forward. This was seen in 38N10-8 using
levator costa at 33% and 48% SVL and in 37-6 using levator costa at 33% and 53% SVL. It
may be the distance of the strike that is important, where a further strike requires more
posterior muscular firing and the snake was preparing for a shorter strike. Regardless, the
results of this study support the hypothesis that the levator costa is used as a locomotor
muscle, as activation of levator costa was observed for each locomotory strike and activation
was inhibited, apparently in preparation for a strike. The ability to shift the location of
23
ventilation and independently control the function of levator costa would be of
particular use in order to utilize the levator costa for the most immediate need.
The independent muscular control of the levator costa, its participation in
ventilation, as well as the support for the hypothesis that levator costa functions
during locomotion make it of particular interest to determine how snakes control the
use of this muscle. Snake locomotion involves complex epaxial musculature
coordination, and this musculature activity may limit the ability of the ribs to expand
laterally to ventilate or preclude the use of the levator costa for ventilation in the
region locomoting because of the co-opted use of this muscle for both ventilation and
locomotion. Boa constrictor are semi-arboreal and are constantly bracing themselves
with various regions of their body when contacting tree limbs as they engage in a
form of concertina locomotion to climb (Byrnes, 2010). When spread between two
tree limbs, the points of contact must brace the snake, potentially inhibiting
ventilation in this region. During the experiments I fortuitously encountered one of
the study animals exhibiting this behavior. While soldering wires to the socket
connections, I observed a Boa constrictor with a short (approximately 10 cm) medial
section of its body contacting the table, but the sides immediately anterior and
posterior of this were elevated and not touching the table. While the medial portion
was visibly tense, bracing the snake against the table, I observed clear ventilatory
movements in both elevated trunk regions. No electromyography data was collected
during this observation, but this qualitatively supports the hypothesis that Boa
constrictor are capable of independently controlling ventilatory muscle activity, and
apparently in response to the conflicting muscle activity of the epaxial/hypaxial
24
muscles used for bracing itself. If the muscular activity of bracing precludes ventilation, the
ability to independently control ventilatory muscle firing and shift the site of pumping could
be of particular importance in arboreality, as the section of the snake contacting the limb
constantly change as it moves through the branches. Additional tests recording ventilation
during both terrestrial and arboreal locomotion would help elucidate whether this is a
generalized behavior. The constant relocation of the point of bracing during locomotion and
the need to change ventilatory pumping sites in response to the compromised ventilation
ability suggest the need for further inquiry into the innervation of the musculature and
locomotory patterns used.
The levator costa has been shown to be innervated by the lateral branch of the ventral
ramus nerve but considering its function in locomotion and ventilation, further inquiry could
be made into seeing if there is any difference in the innervation segmentally or otherwise
(Fetcho 1986, Moon 1998, Rosenberg 1973). The findings of Fetcho (1986) show that the
epaxial muscles—those associated with locomotion and constriction—are innervated by the
dorsal ramus while the hypaxial—those associated with ventilation—are innervated by the
ventral ramus. All of the motorneurons innervating these axial muscles are located in the
ventral part of the ventral horn, with the epaxial occupying the ventromedial portion of the
motorcolumn and spatially distinct from the hypaxial in the dorsolateral portion (Fetcho
1986). Interestingly, the results of Fetcho (1986) have indicated that the levator costa is
innervated differently than the other axial muscles. In rats, the levator costa is innervated by
the dorsal ramus and the motorneurons are located in the ventral portion of the ventral horn
(Fetcho 1986). In snakes, Fetcho (1986) concluded that the levator costa is innervated by the
ventral ramus, yet its motorneurons are located in both the ventromedial and dorsolateral
25
portions of the ventral horn. Although it is hypaxial and innervated by the ventral
ramus, some of its motorneurons are located in the ventral parts of the motorcolumn
associated with the epaxial motorneurons, but are longitudinally segmented from the
epaxial ones (Fetcho 1986). This is interesting because when compared to the
motorneuron arrangement of a rat, if the levator costae are homologous in each, it is
apparent that the pathway taken by the axons innervating the muscle has changed in
snakes—potentially as a function to co-opt the use of the muscle. The findings of my
study support the hypothesis that levator costa is co-opted for both locomotion and
ventilation, and the innervation of the levator costa as discussed by Fetcho (1986)
may be consistent with this finding. The levator costa is innervated through the
ventral ramus associated with the hypaxials but its motorneurons are in regions
associated with both epaxial and hypaxial. This complex innervation appears to
connect the levator costa to both epaxial and hypaxial motorneurons and further
supports the hypothesis that the levator costa functions in locomotion—as an epaxial,
and ventilation—as a hypaxial. A more detailed study into the specific innervation
and neurophysiology of levator costa is necessary to elucidate this further.
It remains unclear how the snake effectively manages to control this
independent firing of the co-opted levator costa. Arboreal locomotion in Boa
constrictor involves complex concertina motions that require unilateral muscle
activity in adjacent muscle segments simultaneously (Jayne 1988). The locomotion of
Boa constrictor involving alternating contralateral muscle application may be
mediated by central pattern generating circuits utilizing coupled oscillators that output
to motorneurons producing rhythmic body movements, with activity alternating as in
26
the locomotion circuits of lamprey or leeches (Chen 2007; Cohen 1982). The complexity of
the arboreal habitat of Boa constrictor results in continuous muscular adjustment to account
for shifting surface diameters, inclines, discontinuities and substrate compliances (Byrnes,
2010). This constant adjustment as well as the co-opted use of the levator costa may cause
the site of ventilation to shift in a rhythmic fashion but at varying levels throughout the body,
as different regions use different rhythmic patterns to locomotion that section on varying
substrates. As the levator costa is recruited for ventilation and locomotion at various regions
of the body in response to arboreal complexities, it is unclear how Boa constrictor manage
this simultaneous shifting of locomotion and ventilation. I have determined that the levator
costa is indeed capable of shifting muscular firing and further study could elucidate how Boa
constrictor control ventilation and locomotion simultaneously, potentially through a central
pattern generator utilizing coupled segmental oscillators modulated by proprioception or
another mechanism. It is also of interest to determine the stimuli that stimulate the shift of
ventilation from one region to another. Rosenberg (1973) proposed that the shift of pumping
due to prey ingestion may be a reflex analogous to the gastroabdominal inhibitory reflex in
dogs, where an increase in intragastric pressure decreases the activity of abdominal muscles.
My findings have shown the capacity to shift ventilatory pumping but further work is
necessary to determine how Boa constrictor interpret the stimuli and reflexively control the
act of shifting ventilation in regards to any stimuli, be it prey ingestion, constriction, or
locomotion.
In Trial 3, specimen 37-6 was recorded and observed ventilating using both the
anterior and posterior regions of its body. The anterior levator costa instrumented at 33% of
SVL in the vascular portion of the lung was activated simultaneously with the posterior
27
levator costa instrumented at 53% in the saccular portion of the lung. Immediately
prior to recording, the snake was offered a rat in an attempt to record ventilation
during constriction and swallowing. However, after repeatedly presenting the snake
with the rat, the snake refused and became extremely agitated. During the recording
the snake exhibited a defensive display of prolonged hissing—the production of
sound by forcibly expelling air through the glottis (Gans, 1973). It is possible that the
snake engaged the entirety of its lung as a means of inflating its entire body for
increased body size as part of a threatening display or in order to have increased gas
volume for the prolongation of the defensive hiss I observed. Wallach (1998) and
many previous studies have proposed this as one of the hypotheses for the functions
of the saccular lung. The findings of this study have preliminarily supported this
hypothesis as the saccular region was always involved in the inhalations used for
hissing, as determined by the muscular activity recorded at the levator costa at 53%
SVL. The snake’s size visibly increased and the volume of gas inhaled presumably
increased, although this was not measured. The extended inspirations observed were
immediately followed by long bouts of defensive hissing followed by a deep
inhalation and another prolonged hiss. The snake inspired with full body inhalations
in durations averaging approximately three seconds. It may also be possible that in
such an agitated state, the snake was also attempting to inhale as deeply as possible in
order to increase oxygen uptake in preparation for a potential defensive encounter.
The snake was observed striking just before the recording and throughout the
experiment and the perceived need for further defensive action could support the need
for increased gas exchange and extensive full body ventilation. Independent control
28
of this musculature could also function defensively when the snake is at its most vulnerable,
during prey ingestion. When swallowing, the snake has lost its primary defensive weapon, its
teeth and mouth but retains the ability to project its epiglottis past the prey item to ventilate.
It may be advantageous then to be able to ventilate in the saccular region of the lung,
utilizing this region as a bellows to draw air in to produce a hiss while the anterior region is
compromised by the prey item and this threatening display is the only means of defending
itself. Either way, additional tests are necessary to more completely understand the
importance of the saccular lung during defensive hissing.
Conclusions
The results of this study have shown that Boa constrictor are capable of independent
ventilatory muscle control. The findings of this study have failed to support the null
hypothesis that the ventilatory muscles of Boa constrictor activate along the length of the
body of the snake during ventilation and are consistent with the hypothesis that that Boa
constrictor are capable of independent ventilatory muscle control. This spatial control may
function to more efficiently ventilate by shifting ventilation to an uncompromised location,
the saccular lung, most likely be in response to the inhibited ventilation of the vascular lung
due to prey ingestion, constriction, or locomotion. The constriction data analysis as well as
the anatomical measurements of the visceral organs of Boa constrictor supports the
hypotheses that constriction or prey ingestion could compromise anterior vascular lung
function. Further research involving the monitoring the ventilatory muscle activity of the
various regions of the lung during constriction and prey is needed to test whether these acts
actively preclude ventilation in the vascular lung and cause the shift of ventilatory pumping
to the saccular region. If this indeed is the case, the utilization of the posterior saccular lung
29
to ventilate would support the caudal bellows hypothesis. The monitoring of gas
composition and gas exchange rates within the various regions of the lung during
these acts would further illuminate whether the saccular lung functions as a caudal
bellows in Boa constrictor. The findings of this study have supported the use of
passive expiration during ventilation in Boa constrictor, the role of the levator costa
as a locomotory muscle, the use of the entirety of the lung, including the saccular
lung in defensive hissing, and ability of Boa constrictor to independently control the
levator costa and shift the site of ventilatory pumping. Further study is necessary to
understand the complex innervations of levator costa and the mechanisms to control
its use. In this study the levator costa has been demonstrated to be independently
controlled throughout the body, but much clarity is still needed to understand its
complete role during ventilation, in particular during activities where its capacity to
participate in ventilation is compromised.
30
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musculature of vertebrates. II. Florida water snakes (Nerodidia fasciata pictiventris). J.
Comp. Neurol., 249:551–563.
Gans C., P. F. A. Maderson. 1973. Sound Producing Mechanisms in Recent Reptiles: Review
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Gasc, Jean-Pierre. 1974. L’interprétation fonctionnelle de l’appareil musculo-squelettique de
l’axe vertébral chez les Serpents (Reptilia). Mé Mus. Hist. nat. Paris A Zool. 83,
1–82.
Gasc, Jean-Pierre. 1981. Axial musculature. Pages 355-446 in C. Gans and T. S. Parsons,
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yellow rat snake (Elaphe obsoleta) J. Morph. 197, 159-181.
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32
Table Legend
Table 1. Table displaying snout-vent length, mass, instrumentation location as a percentage
of total SVL, and electrode wire material. SVL, snout-vent length; LC, levator costa; OIV,
obliquus internus ventralis; Ag-AgCl, Silver-silver chloride.
Table 2. Table displaying Pearson correlation coefficients. LC, levator costa; Electrode
location is as % of total SVL.
33
Table 1.
Snake
18-7
38N-4
37-6
18-14
38N10-8
SVL
(cm)
150
144
128
149
107
Mass (kg)
Muscles
Instrumented
Electrode Location
(% SVL)
Electrode Wire
Material
Side
Instrumented
2.04
2.24
1.64
2.28
0.83
LC, OIV
LC, OIV
LC, OIV
LC, OIV
LC, OIV
33%, 43%, 53%
33%, 43%, 53%
33%, 43%, 53%
33%, 48%
33%, 48%
Stainless
Stainless
Ag-AgCl
Ag-AgCl
Ag-AgCl
Left
Right
Right
Right
Right
34
Table 2.
Snake
Trial
Muscle
38N10-8
38N10-8
37-6
37-6
1
2
3
3
LC
LC
LC
LC
Electrode
Location
33%
48%
33%
53%
35
Correlation
Coefficient
0.999
0.999
0.999
0.999
P-value
< 0.001
< 0.001
< 0.001
< 0.001
Figure Legend
Figure 1. Depiction of axial muscular anatomy of snakes. The upper epaxial muscles are the
primary muscles used for constriction and locomotion. The lower hypaxial muscles are those
involved in ventilation.
Figure 2. Diagrammatic lateral depiction of muscular attachment of ventilatory muscles
Thamnophis elegans. Inspiration uses the levator costa (lc) to pull the ribs cranio-lateral
while the ventro-lateral sheet (vl), and the obliquus internus dorsalis (cis)—a portion of the
dorso-lateral sheet—that are used during exhalation (Rosenberg 1973).
Figure 3. Visual cross-sectional depiction of ventilatory muscles. The the dorso-lateral (dl)
and ventro-lateral (vl) sheet connections to the vertebrae, ribs and skin (s), in reference to
Thamnophis elegan (Rosenberg 1973).
Figure 4. Photographic image of the lung of dissected Boa constrictor, 02-29. The elongated
right lung—inflated—can be seen as well as the abrupt transition from vascular to saccular
indicated by the yellow arrow. The bottom image is a close-up of the top image showing the
extensive vascularization in the vascular lung and lack thereof in the saccular lung.
Figure 5. Visual representation of the relative sizes and positions of the visceral organs and
constriction coil of Boa constrictor. The bar at the top represents 100% SVL of a snake and
the image is scaled as percent of total SVL. Snake image drawn to scale.
Figure 6. Diagrammatic representation of the musculature of a snake in reference to
Agkistrodon. Number 8 is the levator costa and numbers 11 and 13 together constitute the
ventro-lateral sheet. The muscles indicated by hashed lines are those instrumented: the
levator costa and obliquus internus ventralis (Cundall, 1987)
36
Figure 7. Graphs of Pearson product-moment correlation coefficient analyses. Top-left is of
Trial 1 for 38N10-8 analyzing levator costa at 33% SVL. Top-right is of Trial 2 for 38N10-8
analyzing levator costa at 48% SVL. Bottom-left is of Trial 3 for 37-6 analyzing levator costa
at 33% SVL. Bottom-right is of Trial 3 for 37-6 analyzing levator costa at 53% SVL.
Figure 8. Electromyograph data for Trial 1 for specimen 38N10-8. The black bars indicate
measured length of time for correlated inspirations determined by video frame analysis. Each
column of the graph represents 0.6 seconds. The data is labeled LC1 for the first levator costa
instrumented at 33% SVL, OIV1 for the first obliquus internus ventralis instrumented at 33%
SVL, and LC3 for the levator costa instrumented at 48% SVL.
Figure 9. Electromyograph data for Trial 2 for specimen 38N10-8. The black bars indicate
measured length of time for correlated inspirations determined by video frame analysis. Each
column of the graph represents 0.4 seconds. The data is labeled LC1 for the first levator costa
instrumented at 33% SVL, OIV1 for the first obliquus internus ventralis instrumented at 33%
SVL, and LC3 for the levator costa instrumented at 48% SVL.
Figure 10. Electromyograph data for Trial 3 for specimen 37-6. The black bars indicate
measured length of time for correlated inspirations determined by video frame analysis. Each
column of the graph represents 1.0 seconds. The data is labeled OIV2 for the second obliquus
internus ventralis instrumented at 43% SVL, LC1 for the first levator costa instrumented at
33% SVL, and LC3 for the third levator costa instrumented at 53% SVL.
37
Figure 1.
38
Figure 2.
39
Figure 3.
40
Figure 4.
VASCULAR
SACCULAR
VASCULAR
SACCULAR
41
Head
Trachea
Esophag
Stomach
Liver
42
Average Constriction
Vascular Saccular Right
Heart
Left Lung
Snake
Gall
Tail
Figure 5.
Figure 6.
43
Figure 7.
44
Figure 8.
3 Seconds
LC1
OIV
LC3
45
Figure 9.
LC1
2 Seconds
OIV
LC3
46
5 Seconds
LC3
LC1
OIV
Figure 10.
47