External Features and Muscle
System
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Introduction
You will be forming groups of 6 to dissect the three animals, which are compared in the remaining labs. Not every animal will be equally easy to dissect and some labs will spend more time on one animal than another. You will have to work as a team to spread the load and make sure that you all have equal time to learn the material. Discuss your progress as you are going along, what your approach will be and how to modify it if something is more complex or time consuming than you thought. This lab examines the external features and muscular system of three animals, the cartilaginous dogfish shark,
the amphibian mud puppy Necturus,
and a mammal, the laboratory rat or domestic cat. Although we will be only
looking at one type of muscle, that of the skeleton, it is important to note
that there are three types of muscle fibers in the body and they are all
displayed in the lab under microscopes. Look at the demonstration slides of smooth and skeletal, or striated muscle.
Smooth muscle is usually from hypomere mesoderm and forms the visceral or
involuntary muscles. They have no striations, central nuclei and
spindle-shaped fibers that taper and dovetail together. Skeletal muscles are
usually from myotome except the branchiomeric muscles in the head, which are
from hypomere mesoderm. They are voluntary muscles and the muscle is
striated, the many nuclei are peripheral and the fibers are long, cylindrical
and unbranched. Cardiac muscles,
found in the heart, are striated but involuntary. They have branching strands
and come from mesenchyme. Muscles can only contract. Relaxation is
the lack of contraction. Muscles have origins (fixed or the most fixed part) and insertions (less fixed). They often work in pairs called agonists and antagonists. Some examples are abductors (away from midline) and adductors (towards midline), flexors (reduce angle) and extensors ((increase angle), protractors (move limb forward) and retractors (move limb backward). Deeper muscles may insert by faschia or by tendons onto bone. These are of dense collagen fibers, which also form ligaments, which connect bone to bone. We will be studying the muscles of the
skeleton and eye in this lab. These muscles come from embryonic myotomes, which are segmented, in the embryonic body. The
first three segments in the head region are located in front of the ear (pre-otic). The epimere mesoderm in these segments survives
as the muscles of the eyes; the hypomere mesoderm does not develop. Each
segment has split to form different eye muscle masses, which have then moved
apart. We can follow these masses because their innervation (the nerve going
to them) remains the same. We will look in detail at the eye muscles of the
shark, but every vertebrate, including man has a similar pattern of eye muscles
and innervation. The hypomere mesoderm in the head region
does not contain a coelom and instead forms a solid sheet called
branchiomere. This branchiomeric
muscle becomes the muscles of the
jaw, hyoid arch and gills. It is innervated by somatic sensory, visceral
sensory and visceral motor nerves, all from the dorsal roots. These dorsal
nerves in the first three segments are the nerves passing through the orbit
of the eye to serve the branchiomeric muscles of the head, jaws and spiracle
in the dogfish. They perform a similar function in Necturus, but in mammals, with the loss of gills, the more
posterior muscles are lost or transformed into larynx muscles and the
trapezius muscles of the shoulder. The axial muscles of the body are from mesodermal somites and
form the skeletal muscles of the trunk and tail. The segmented origin of
these muscles can be easily traced in the dogfish and mud puppy but becomes
obscured by massive limb muscles in the mammal. In the head region, hypobranchial
muscles are formed from somites
behind the gills that move (as embryonic mesenchyme) forward to the ventral
region of the pharynx (between the gills). Appendicular muscles are formed in the shark by muscle buds in
the embryonic myomeres. They serve to operate the pelvic and pectoral fins. The
axial muscles of the body do much of the shark locomotion. The advent of
limbs in Necturus, with
elbow/knee and wrist/ankle joints makes limb musculature far more complex. In
mammals, limbs have become more powerful for increasing speed on land and
appendicular muscles lie over many of the body (axial) muscles. In addition,
some branchiomeric muscles formerly operating the gills have become shoulder
muscles (trapezius). DOGFISH EXTERNAL ANATOMY Note the shape and how it is designed for
swimming. Feel the placoid
scales and note they point backward. Examine the fins - 2 dorsal (with
spines), caudal (note shape of lobes), pectoral (1 pair), pelvic (1 pair),
claspers (present or not?). Follow the lateral line along each side of body. It is used for hearing; interconnected
neuromast cells relay the pressure wave. The head has a sub terminal mouth, a
pair of lidless eyes, pair of spiracles, 5 pairs of gill slits, pair of
nares. Examine the surface of the head. Note the continuation of the lateral
line onto the skin of the head in three principal lines, the supra orbital,
the sub-orbital and the mandibular. Look at the ampullae of Lorenzini, fluid filled pits that sense low-level electrical
stimulus and sense depth. Find the two endolymphatic ducts, which connect to
the inner ear, which is for balance only. Examine the cloaca, a chamber between the pelvic fins and note the
urinary papilla, and anus. Skin the left side of the head to about 10 cm behind the fins. The skin is closely
adhering to the muscles, so be patient. Use a blunt probe to separate the
skin from the muscles; a scalpel will only cut the muscles. You will have to
cut through the dorsal cartilage in the region of the left eye and remove the
gelatinous packing material and blood behind the eye. You should identify the
eye muscles in situ from above
and below the eye, before you remove the eye. DOGFISH MUSCLES The eye muscle dissection is concerned with
the first three pre-otic segments. In these segments the three myotomes (epimere mesoderm) form the six muscles of the eye.
These muscles are innervated by somatic motor nerves, which in the head
region come from the ventral roots. Table: Summary of structures present in the head
segments.
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1st preotic segment |
2nd preotic segment |
3rd preotic segment |
4th and 5th otic segment |
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Name |
Premandibular |
Mandibular |
Hyoid |
Otic |
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Arch |
neurocranium |
jaws |
hyoid |
gills |
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opening |
brain cavity |
mouth |
spiracle |
gill slit |
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myotome |
4 eye muscles |
1 eye muscle |
1 eye muscle |
none |
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nerve |
III oculomotor |
IV trochlear |
VI abducens |
none |
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hypomere |
none |
jaw muscles (adductor mandibulae, intermandibularis) |
spiracle muscles (interhydoideus, levator
hyomandibulae, levator hyoideus etc.) |
gill muscles (cucullaris, superficial constrictors) |
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Dorsal nerve roots |
V1 deep opthalmic |
V2+3 infraorbital (ss) mandibular superficial |
VII spiracular superficial infraorbital VIII auditory |
IX glossopharyngeal roots X vagus (ss, vs, vm) |
The 4-7 gill slits are served by nerves X -
XI and are branchiomeric muscles:
levators (cucullaris, interarcuals) and superficial constrictors. Behind the
gill arches, the myotomes grow down and forward as hypobranchial muscle which
fills the ventral area between the gill arches with coracoid muscles and is innervated
by the hypobranchial nerve XII. Posterior to these muscles we have typical
myotomes forming axial muscles in
the cervical thoracic and lumbar areas and special appendicular muscles for the pectoral and pelvic limbs.
Muscles are bundles or sheets of fibers
attached to the skeleton or other parts of the body at either end. One end,
designated the origin is attached
to a more or less rigid part of the animal (in the case of eye muscles, to the
cartilaginous wall of the orbit). The opposite end, designated the insertion, is attached to a more freely movable part (in this
case, to the eyeball).
Pull the eye gently away from the orbit in order to expose the six muscles, which connect the
eye to the orbit. These muscles can be divided into two groups: the two
oblique muscles which have their
origins very close together in the anterior medial corner of the orbit and the four rectus muscles which have their origins in the posterior medial
corner of the orbit. It is important
to identify these muscles correctly as they are supplied by cranial nerves that
are encountered later.
Origin: anterior medial corner of orbit
Insertion: dorsal surface of eyeball.
Origin: posterior medial corner of orbit.
Insertion: dorsal surface of eyeball.
These two muscles insert side by side on the
dorsal surface of the eye. Their fibers run at right angles to each other, the superior
oblique anteriorly and the superior rectus posteriorly. Be careful not to
damage the fine trochlear nerve,
which runs from the middle of the super oblique muscle to the medial wall of
the orbit.
Origin: posterior medial corner of orbit.
Insertion: dorsal posterior surface of eyeball.
This is the only muscle on the dorsal
posterior surface of the eye. Its fibers run medially from the point of
insertion on the eyeball.
Origin: posterior medial corner of orbit.
Insertion: mid-ventral surface of eyeball.
Origin: anterior medial corner of orbit.
Insertion: mid-ventral surface of eyeball.
These two muscles can be seen by tilting the
eyeball upwards and looking on its ventral surface. They insert side by
side, their fibers running at right angles to each other. The origin of this
muscle is right beside that of the superior oblique muscle. Be careful not to
confuse the inferior oblique muscle, which runs from the eyeball to the medial
wall of the orbit, with the optic nerve.
Origin: posterior medial corner of orbit.
Insertion: medial surface of eyeball, near its
anterior end.
This muscle can best be seen by looking from
the dorsal surface, between the eyeball and the medial wall of the orbit. Its
insertion is beneath the superior oblique muscle. Its fibers run posteriorly.
Dogfish Branchiomeric Muscles
The first arch contains the jaw muscles. The adductor
mandibulae, a large muscle below the
spiracle, closes the jaw. The intermandibularius is on the ventral surface, posterior to the jaw. It
originates on the Mechel's cartilage (lower
jaw) and inserts on the central raphe. Its function is to elevate the floor of
the mouth for swallowing.
In the hyoid arch, the levator
hyomandibulae can be located
posterior to the spiracle where it compresses the gill pouches. The
interhyoideus is another ventral constrictor muscle, but it is hidden by the
intermandibularius.
In the gill arches, there are a variety of
muscles to operate the gills. The levator muscle of note is the cucullaris, lying above the gills, which results from a fusion
of all the levators and serves to elevate the scapula and gill arches. Other
muscles (constrictors, adductors and interacuals) are located deep within and
between the gills.
Dogfish Axial Muscles
These somatic (myotomal) muscles are the
skeletal muscles of the trunk and tail. The anteriomost myotomes form hypobranchial muscles, which move anteriorly between the gills to
form some of the muscles of the throat. Superficially, between the gills and
attached to the scapulocoracoid bar can be seen the triangular coracoarcual muscles which open the mouth.
The epaxial muscles are found dorsal to the horizontal septum and
the hypaxial muscles are ventral
to it. There is a myomere for each vertebra. The horizontal septum in anchored to the dorsal ribs and the ventral ribs
develop in between, in the myosepta.
The ventral hypaxial muscles meet at the midline forming a connective tissue
raphe, the linea alba.
Dogfish Appendicular Muscles
Skin the left pectoral fin near its base and
look for the dorsal extensors (elevators) that lift the fin, the pectoral
abductor muscle. On the ventral side
is found the flexors (depressors) that pull it down, the pectoral adductor muscle.
Necturus
External Anatomy
The flattened head has a mouth with lips,
small lidless eyes and external nares. There are three pairs of lateral gills and
two pairs of gill slits. The head is separated from the trunk by a gular
fold. In the head and trunk regions
small indents of the lateral line system
can be seen. The cloaca is
surrounded by a large cloacal gland
in males. The skin aids in respiration and has a thin stratum corneum with
mucous and poison glands. It is easily removed on the left side to the midline by cutting around the gills and
turning the skin on the limbs inside out. Start with a dorsal incision and do
not go very deep or you will cut into the muscles.
Necturus
Muscles
Necturus Eye Muscles
These muscles are the same as those of the
dogfish and will not be studied in Necturus.
Necturus Branchiomeric Muscles
In the mandibular arch, the adductor
mandibulae of the shark splits into the temporalis and masseter muscles. They form a sling for the lower jaw to close the mouth. On
the ventral side lies the intermandibularis, extending from the mandible to the median raphe. It
elevates the throat.
In the hyoid arch, the branchiohyoideus can be found lateral and posterior to the jaw. It
waves the gills back and forth in the water for respiration. Ventrally,
posterior to the intermandibularius, lies the interhyoideus, which elevates the throat. The posterior portion of
this muscle is the sphincter coli
which becomes most of the facial muscles in mammals.
In the gill arches, directly above the gills,
lie the levatores arcuum and
cucularis (trapezius), which elevate
the gills. Most of the other gill constrictor muscles have disappeared or moved
to the larynx.
Necturus Axial Muscles
Anteriorly, the hypobranchial muscles can be
seen in the center of the throat, behind the gular fold. Their segmentation can
be seen in the rectus cervicis, which
retracts the hyoid and gills and depresses the head.
The epaxial and hypaxial muscles separated by the horizontal septa of the myotomes are similar to those of the dogfish.
Dorsally the epaxial muscles are fused into a long, straplike dorsalis
trunchi. Ventrally the hypaxial
muscles meet at the linea alba.
Lateral to the linea alba lie the strap-like longitudinal rectus abdominis muscles. There are three layers of abdominal muscles,
the external oblique, internal
oblique and transversus abdominis,
all of which will be observed in the
mammal.
On either side of the cloaca are located the ishiocaudalis muscles and external to them, the caudocrualis muscles which serve to flex the tail.
Necturus Appendicular Muscles
Pectoral muscles developed from the fin
muscles of fish. The dorsal, pectoral abductor, muscles will be examined first.
Note the latissimus dorsi, a large
triangular muscle originating on the faschia of the dorsalis trunchi and
inserting on the humerus, which is used to retract the limb. The trapezius
(cucullaris) also originated from the dorsalis trunchi and inserts on the
scapula to pull it anteriorly. This muscle is branchiomeric in origin and not
an appendicular muscle. The dorsalis scapulae originates on the superscapular cartilage and inserts
on the humerus to pull it anteriorly. It is related to the deltoid muscles of
mammals. Laterally lies the procoracohumeralis, which pulls the humerus anteriorly. The triceps
brachii lie on the dorsal surface of
the upper arm with three heads from the coracoid, scapula and humerus. They
insert together on the ulna and serve to extend the forearm.
The ventral, pectoral adductor muscle has
become large, ventral pectoral muscle masses that adduct (move inward) the
humerus. The anterior muscle, the supracoracoid originates on the caracoid cartilage of the pectoral
girdle. The pectoralis muscles
originate on the linea alba. The upper forearm is flexed by the humero-antebrachialis,
which originates on the humerus and
inserts on the radius and the coracobranchialis, which originates on the coracoid process and inserts
on the humerus.
The distal forelimb has dorsal extensors
homologous with the shark abductors, which extend the limb. It also has ventral
flexors homologous to adductors to flex the limb.
The pelvic dorsal abductors have also been
highly modified. They have given rise to small muscles on the anterior of the
limb. The iliotibialis and the iliofibularis, which form the sartorius and gluteus muscles of
mammals, originate on the ilium and insert on the tibia and fibia to abduct the
hind limb.
The pelvic adductor muscle from the ventral
surface of the shark fin becomes even more highly diversified in Necturus. On the ventral side, between the limbs, note the
large anterior pubo-ischio-tibialis, which along with the pubo-tibialias adduct the hind limb. The more posterior ischio-flexorius flexes the shank and foot.
The distal hind limb, like the forelimb, has
dorsal extensors to extend the shank and foot, which are homologues to the
shark pelvic abductors. There are ventral flexors, which adduct and flex the
shank and foot, which are homologues to the adductor of the shark pelvic fin.
Rat/Cat External Anatomy
Note the skin with hair and the mammary glands, characteristic of mammals. The head has a mouth with
lips. The upper lip is split by a groove, the philtrum. The nose has nares and the eyes bear upper and lower eyelids and a
reduced nictitating membrane. The external ears or pinna direct sound. The whiskers or vibrissae are sensory. The trunk has an anterior thorax and posterior abdomen. The anus is at the base of the tail. The urinary and
genital openings are separate in females, but in males they run as one duct to
the tip of the penis. Males have a double pouch, the scrotum containing the testis. In the female, between the skin and muscles lies
granular, mammary gland tissue in a band under the lines of nipples.
Cut through the skin of the rat/cat from the
pelvic region to the throat and the upper limbs on the left hand side. Remove the skin from the left had side of
the head. Skin the animal by inserting the handle of the scalpel or a blunt
probe between the skin and the muscles. The muscles are connected to the skin
by fascia and muscles attached to the fascia are used to twitch the skin. The
cat will be much more difficult to skin than the rat and will probably have
large fat areas under the skin which can be removed. Try and work quickly so
you have time to view the muscles.
Rat/Cat Muscles
Mammal Eye Muscles
These muscles are the same as those of the
dogfish and will not be studied in mammals.
Mammal Branchiomeric Muscles
In the mandibular arch, the temporalis
muscles lie from behind the eye to
the ear, occupying the temporal fossa of the skull. It elevates the mandible
(lower jaw). Behind and below the eye is the large masseter muscle, which elevates the jaw and allows for complex
chewing. Ventrally and medially lies the digastric muscle, which depresses the mandible.
In the hyoid arch, the most important muscle
in mammals is the spincter coli, which is the posterior portion of the
interhyoideus muscle. This muscle spreads onto the neck and becomes the platysma in mammals. It then spreads over the head and face to
form the facial muscles of higher
mammals.
The gills are now gone and the muscles of the
gill arches have for the most part disappeared. Some muscles remain on the
larynx and for swallowing. The cucullaris muscle has survived as the trapezius muscles of the pectoral girdle. The trapezius complex
consists of three muscles, two of which originate on the spine and insert on
the scapula to adduct and move it dorsally and forward. The third muscle
inserts on the clavicle and protracts the humerus. The sternomastoid muscle is also derived from the cucullaris muscle and
serves to turn or flex the head.
Mammal Axial Muscles
In mammals the hypobranchial muscles form the
deep muscles of the throat and tongue.
Much of the primitive metamerism is lost due
to the appearance of complex appendicular muscles for the movement of limbs and
the development of broad muscular sheets of parietal muscles in the abdomen.
The epaxial muscles have become long bundles to straighten and flex the body.
These are deep muscles on either side of the spine and are hidden by the limb
muscles. The hypaxial muscles are three sheets of muscles joined ventrally at
the linea alba. It is formed by a muscle-to-muscle attachment of fascia
called an aponeurosis. Cut out a rectangle of abdominal muscle about 8cm2,
which includes the mid-line. Pull apart the layers of muscle from the lateral
side towards the mid-line. On either side of the linea alba is a band of longitudinal
muscle, the rectus abdominis,
where the three muscle layers sit in a common sheath. The three muscle layers
from the outside to the inside are: external oblique, internal oblique, and
transversus abdominis. These muscles
support the abdominal viscera in a muscular sling. Other, deeper hypaxial
muscles are not seen. These are the intercostals and supracostals of the rib
cage and the diaphragm, which acts
as a suction pump in respiration. Tail muscles are continuations of the epaxial
and hypaxial muscles bundles.
Mammal Appendicular Muscles
In the pectoral region, the pectoral abductor
muscles of the dogfish now are represented by a number of forelimb muscles.
Note the triangular latissimus dorsi,
which now originates from the neural spines of most of the thoracic vertebrae
and inserts on the humerus, to pull the forelimb dorsally. Anteriorly it is
overlaid by the trapezius complex, which is branchiomeric in origin. From the
scapula there are five deltoid muscles, which insert on the humerus to adduct
and rotate it. Locate the spinodeltoideus. The triceps brachii consists
of three muscles originating on the scapula or humerus and inserting together
on a process of the ulna, to extend the forearm.
The ventral pectoral adductor muscles of the
dogfish have also become complex in mammals. The biceps brachii is located on the anterior of the forearm. They
originate on a tendon inserting on the scapula and insert by a tendon on the
tuberosity of the radius, to flex the forearm. Turn the animal on its back to
see the large pectoral muscles.
There are several muscles in this group in mammals, some of which adduct or
pull the forelimbs towards the midline, others retract the forelimb backwards.
These muscles originate on the sternum and insert on different parts of the
humerus. The most obvious of these muscles will be the anterior pectoralis
major and the more posterior pectoralis
minor.
The pelvic abductor muscles of the dogfish
are highly specialized for powerful locomotion in the mammal. In the thigh is found
the large sartorius muscle, which
runs from the ilium to the knee to adduct and rotate the femur and extend the
shank. More posteriorly lay the thin, wide, gluteus muscle of the hip, originating on the last sacral and
first caudal vertebrae and inserting on the femur to abduct the thigh.
The pelvic adductor muscles now occupy both
surfaces of the hind limb. Anterior on the limb is the tensor fascia latae, a triangular muscle originating on the ilium and
inserting on the knee by a fascia lata. It extends the shank. Posterior to it lays
the caudofemoralis, partly hidden
by the biceps femoralis. It originates on the tail (caudal) vertebrae and
inserts on the shank. The posterior thigh has the large, thick biceps femoralis, which runs from the ischium to the tibia and knee to
abduct the thigh and flex the shank. The posterior most muscle is the semitendinosus, which is stuck to the biceps femoralis and runs from
the ischium to the tibia to flex the shank. Ventrally can be seen the large gracilis muscle which occupies the posterior half of the
thigh. It runs from the pubic symphysis to the tibia and adducts and retracts
the leg.
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SHARK |
MUD PUPPY |
MAMMAL |
|
Eye Muscles (myotomes) |
inferior oblique superior rectus anterior rectus inferior rectus superior oblique posterior oblique |
inferior oblique superior rectus anterior rectus inferior rectus superior oblique posterior oblique |
inferior oblique superior rectus anterior rectus inferior rectus superior oblique posterior oblique |
|
Branchiomeric muscles |
adductor mandibulae
intermandibularis levator hyomandibulae hyoid constrictors
cucullaris |
masseter & temporalis intermandibularis
branchiohyoideus & interhyoideus levatores arcuum & cucullaris |
masseter & temporalis digastric
platysma, digastric trapezius & sternomastoideus |
|
Hypobranchial muscles |
coracuals |
rectus cervicus |
tongue & throat muscles |
|
Axial muscles
|
epaxial |
dorsalis trunchi epaxial |
deep longitudinal muscles |
|
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hypaxial |
rectus abdominus external oblique internal oblique transversus abdominis ishiocaudalis caudocruals |
rectus abdominus external oblique internal oblique transversus abdominis |
|
Appendicular muscles |
pectoral abductors |
latissimus dorsi dorsalis scapulae triceps brachii procoracohumeralis |
latissimus dorsi deltoids triceps brachii
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pectoral adductors |
supracoracoid pectoralis humero-antebrachialis coracobranchialis |
pectoralis biceps brachii (coracobranchialis) |
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pelvic abductors |
iliotribialis (iliofibularis) |
sartoris gluteus |
|
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pelvic adductors |
pubo-ischio-tibialis pubo-tibialis (iliofemoralis) ischioflexorius |
gracilis (adductor femoralis) tensor faschia latae biceps femoralis & semitendinosus |
updated by Sandra Millen, December, 2003