Biology 241 – Lab
LAB #13
(13th/21 Lab Sessions for Fall Quarter, 2008)
TOPICS TO BE COVERED:
»Demonstration/Dissection/Identification of the Muscles of the Abdomen
»Demonstration/Dissection/Identification of the Superficial Muscles of the Back and Shoulder
»Demonstration/Dissection/Identification of the Muscles of the Upper Forelimb (“Arm’)
»Demonstration/Dissection/Identification of the Muscles of the Lower Forelimb (“Forearm”)
DESIRED OUTCOMES:
After completing the activities described for this lab session, students should:
»Be able to identify the four abdominal muscles:
Rectus abdominis External oblique
Internal oblique
Transversus abdominis
»Be able to identify the four superficial back muscles:
Spinotrapezius Acromiotrapezius Clavotrapezius
Latissimus dorsi
»Be able to identify the four superficial shoulder muscles:
Spinodeltoid
Acromiodeltoid
Clavobrachialis
Levator scapulae ventralis
»Be able to identify the four upper forelimb muscles:
Epitrochlearis
Biceps brachii
Brachialis
Triceps brachii
»Be able to identify the eight lower forelimb muscles:
Brachioradialis Palmeris longus Flexor carpi radialis Flexor carpi ulnaris Pronator teres
Extensor carpi ulnaris Extensor digitorum communis Extensor digitorum lateralis
Extensor carpi radialis (longus and brevis)
MATERIAL NEEDED:
»Gloves
»Safety Glasses (or your own glasses)
»Dissection Trays (large, flat pans)
»Preserved cats
»Dissection Tools (large forceps, scissors, blunt probes, dissection needles, scalpels)
»Photographic Atlas, Ch. 1, Ch. 8, Ch. 19
»Cat Atlas
Activity #1: Demonstration/Dissection/Identification of the Muscles of the Abdomen
There are four “abdominals”, which are arranged in three layers to constitute the abdominal wall. Each
muscle is very thin, and great care must be taken in separating them. Working synergistically, these muscles act
to compress, flex, and “twist” the anterior of the abdomen.
EXTERNAL OBLIQUE: This is the most superficial of the lateral abdominal muscles. It originates
from the last nine or ten ribs by slips (which interdigitate with two deeper muscles, the serratus ventralis and serratus
dorsalis) and from the lumbodorsal fascia and inserts via a broad aponeurosis on the linea alba ( aka tendinous
ventral midline) of the abdominal wall from sternum to pubis. Its fascicles run ventrally and posteriorly (in the
direction of the fingers while “putting hands in pockets”). You will need to use great care to free up the medialmost border of this muscle, which “overshoots” the lateral-most border of the rectus abdominis, in order to reflect it laterally to view the next deeper layer. Be careful NOT to damage the rectus abdominis muscle belly
while freeing up the medial boundary of the external oblique. Deep to this muscle there is a fair amount of
areolar connective tissue, which should be appreciated for its bright white, “airy” consistency and its ability to
hold muscle layers together while still allowing muscle masses to operate efficiently. Gentle blunt dissection
with an angled probe or gloved finger will do the job nicely.
Biology 241 – LAB #13 – continued
Page Two
INTERNAL OBLIQUE: Deep to the external oblique lies the internal oblique, with its fascicles running ventrally and anteriorly. Note that its fascicles are oriented 90º (perpendicular) to those of the external oblique. It originates from the pelvis and the lumbodorsal fascia and inserts via a broad aponeurosis on the linea
alba. This muscle constitutes the second layer of the abdominal wall. The medial-most margin of the fascicles
does not come as far medially as the external oblique; therefore, the belly of this muscle seems less “vast.” You
will need to use even more care to reflect the medial-most border of this muscle laterally in order to view the
next deeper layer. Proceed slowly and meticulously so as to NOT do harm to either the external oblique (which
you are reflecting) nor to the transversus abdominis which you are trying to reveal.
TRANSVERSUS ABDOMINIS: The transversus abdominis constitutes the third and deepest layer of
the abdominal wall. It originates from the posterior ribs, lumbar vertebrae, and ilium, and inserts on the linea
alba via a “window-like, clear aponeurosis. Its fascicles are oriented transversely across the abdomen; but, remember, no muscle crosses the midline and the fascicles of this particular muscle end quite a long ways from
the midline. Also, the transversus is very thin and may be difficult to distinguish from the internal oblique. The
external oblique, internal oblique, and transverses abdominis act together to compress the abdominal viscera (as
in defecation and in forced expiration) , to arch the back and “twist” side to side.
RECTUS ABDOMINIS: The rectus abdominis is a long, flat, ribbon-like muscle in the midline of the
ventral abdominal wall. It is enclosed in a tough sheath formed by the aponeuroses of the external oblique, internal oblique, and transversus. The rectus abdominis originates from the pubic symphysis and inserts on the
sternum and all the costal cartilages (all the way up to Rib #1 *). It is considered to be in the same plane as the
external oblique (thus part of the most superficial layer of the abdominal wall); and it acts, together with the other
muscles of the abdominal wall, to flex the trunk and to compress the abdominal viscera.
*Note: You have to “peek” under the Pectoralis group muscles to view the anterior-most extent of the rectus abdominis.
Activity #2: Demonstration/Dissection/Identification of the Superficial Muscles of the Back and Shoulder
In the cat, the superficial muscles of the back include the trapezius group (consisting of the clavotrapezius, acromiotrapezius, and spinotrapezius) and the latissimus dorsi. The muscles of the trapezius group work synergistically, but are three distinct and separate muscles; whereas, in humans, the trapezius is one single, very extensive muscle. For the trapezius group, the prefix of the name describes the insertion of each muscle:
CLAVOTRAPEZIUS: The clavotrapezius is a broad muscle anterior to the acromiotrapezius. It originates from the superior nuchal line of the skull and from the median dorsal line of the neck and inserts on the
clavicle. It acts to draw the scapula cranially and dorsally, so as to allow for forward extension of the humerus,
as in running. Its fascicles are continuous with those of the clavobrachialis muscle (studied in Activity #3 below),
but their innervations are separate.
ACROMIOTRAPEZIUS: The acromiotrapezius originates by a thin aponeurosis from the spinous processes of the cervical and anterior thoracic vertebrae. It inserts onto the acromion process (of the spine of the scapula). It is a large, almost square muscle that acts to stabilize the scapula, that is, hold it in place.
SPINOTRAPEZIUS: The spinotrapezius originates from the spinous processes of the posterior thoracic vertebrae. It inserts on the spine of the scapula and the surrounding fascia. Together the spinotrapezius and
the acromiotrapezius act to hold the two scapulae together across the midline. The spinotrapezius also draws
the scapula posteriorly.
LATISSIMUS DORSI: (aka the “lats”) This large, flat muscle that wraps around the lateral aspect of
the torso, lies posterior to the trapezius group. Its origin is from the spinous processes of the posterior thoracic
and lumbar vertebrae, and it inserts on the medial aspect of the humerus. It acts to pull the forelimb posteriorly
and dorsally.
Biology 241 – LAB #13 – continued
Page Three
In the cat, the superficial muscles of the shoulder include the deltoid group (consisting of the acromiodeltoid, spinodeltoid and clavobrachialis) and the levator scapulae ventralis. The muscles of the deltoid group work synergistically, but are three distinct and separate muscles; whereas, in humans, the deltoid is one single, large muscle. All of these muscles work together in forward extension of the humerus, as in running. This time, for the
deltoid group, the prefix of the name describes the origin of each muscle.
ACROMIODELTOID: The acromiodeltoid originates from the acromion process (of the spine of the
scapula), and inserts on the proximal end of the humerus along with the insertion of the spinodeltoid. Together
with the spinodeltoid, the acromiodeltoid acts to raise (aka “flex”) and rotate (laterally) the humerus.
SPINODELTOID: The spinodeltoiod originates from the length of the spine of the scapula and inserts
On the proximal end of the humerus along with the insertion of the acromiodeltoid. As stated above, together
with the acromiodeltoid, the spinodeltoid acts to raise (aka “flex”) and rotate (laterally) the humerus.
CLAVOBRACHIALIS: The clavobrachialis originates from the clavicle and inserts on the proximal
end of the ulna. It functions with the clavotrapezius to enable forward extension of the humerus (as in running);
it also assists in turning the head and in flexing the elbow. (The clavobrachialis is homologous in man with that
portion of the deltoid which originates from the clavicle. In fact, this muscle – in the cat - is sometimes known
by the name clavodeltoid.)
LEVATOR SCAPULAE VENTRALIS: A flat, straplike muscle, the levator scapulae ventralis, lies on
the side of the neck between the clavotrapezius and the acromiotrapezius. It originates from the occipital bone
and transverse processes of the atlas and inserts on the ventral surface of the vertebral border of the scapula.
This muscle pulls the scapula forward. This muscle has no homolog in man. (The full view of this muscle is exposed when the clavotrapezius and the clavobrachialis muscles are reflected.)
Activity #3: Demonstration/Dissection/Identification of the Muscles of the Upper Forelimb (“Arm”)
To begin a dissection of the upper portion of a cat’s forelimb, there is a most superficial muscle seen
covering the medial surface of the appendage. One must incise the lateral-most border of this thin “sheet” of
muscle in order to gain access to the flexor and extensor compartments of the “anatomical arm” (aka: portion of
the forelimb from shoulder to elbow). Once reflected, the biceps and triceps brachii muscles come into view.
Just a word here about “compartments” of the upper extremities: for the upper extremities, the anterior compartment contains the flexor muscles and the posterior compartment contains the extensor muscles. This “arrangement” is just the opposite for the lower extremities. In the lower extremities, the anterior compartment
contains the flexor muscles and the posterior compartment contains the extensor muscles. In truth, all extremities had the flexors and extensors aligned the same; it’s just that embryologically, when the lower limb buds
start elongating, they also rotate medially and the surface that was originally posterior (dorsal) is now anterior
(ventral). Thus, the “compartments” become reversed.
EPITROCHLEARIS: The epitrochlearis is a thin, superficial muscle which appears to be an extension
of the latissimus dorsi. In fact, it originates from the lateral surface of the ventral border of the latissimus dorsi
and inserts by a thin aponeurosis which is continusous with the fascia of the lower forelimb. It acts in common
with the triceps brachii as an extensor of the elbow, and once extended, it helps stabilize the elbow for weight
bearing. It has no homolog in man.
BICEPS BRACHII: This is a convex muscle interior to the pectoralis major and minor on the ventromedial surface of the humerus. It originates just above the glenoid fossa of the scapula and inserts on the radial
tuberosity near the proximal end of the radius. It functions as a flexor of the forelimb. In man, the biceps has
two heads of origin (as the name indicates): one above the glenoid fossa, as in the cat, and one from the coracoid
process of the scapula. In cats, the two heads are fused into one.
Biology 241 – LAB #13 – continued
Page Four
BRACHIALIS: The brachialis is located on the ventrolateral surface of the humerus. It originates from
the lateral surface of the humerus and inserts on the proximal end of the ulna. It functions with the biceps to flex
the forelimb at the elbow.
TRICEPS BRACHII: This is the largest superficial muscle in the upper forelimb. The triceps brachii
has three divisions: long, medial and lateral heads. The long head orginates from the axillary border of the scapula just below the glenoid fossa. The lateral head originates from the deltoid ridge of the humerus. The medial
head lies deep to the lateral head and originates on the shaft of the humerus. (In the cat, the medial head is divisible into three smaller slips which need not be individually identified. ) All three heads of the triceps insert on the olecranon process of the ulna. The triceps acts to extend the elbow. In your dissection, it is easy to identify the distinct boundaries of the lateral head and dissect its belly free; however, this is not the case for the medial and lateral heads. The fascicles of these two heads interdigitate and thus the two heads are very difficult to tease apart.
Activity #4: Demonstration/Dissection/Identification of the Muscles of the Lower Forelimb (“Forearm”)
As we proceed into the lower forelimb region, it is well to check your dissection to make sure that the
wrist “cuff cuts” that you made previously are, indeed, distal to the duclaw. This is the only way you will have
a clear view of the tendinous insertions of the next eight muscles. These muscles are much easier to find and
identify if their tendons are accessible.
As was stated earlier, in the cat’s forelimb (homologous to the upper extremity in man), the anterior
compartment contains the muscles that bring about flexion when they contract. Except for the brachioradialis
muscle (technically, not belonging to either flexor nor extensor compartments) which rotates the radius and supinates
the paw/hand (in the cat/man), and the pronator teres which pronates the radius, the following muscles are all
flexors:
BRACHIORADIALIS: The brachioradialis originates about the middle of the humerus and inserts on
the distal end of the radius. It rotates the radius and supinates the foot.
PALMERIS LONGUS: This muscle arises from the medial epicondyle of the humerus and passes under the transverse carpal ligament. It is easy enough to trace its tendons, which insert on the pads of the forepaw
and on the proximal phalanges of the digits. It is a flexor of the paw at the wrist and and of the digits. (The palmaris longus of the human is a slender muscle which inserts on the fascia of the palm. Interestingly, it is absent in about 10
percent of humans!)
FLEXOR CARPI RADIALIS: This muscle originates from the medal epicondyle of the humerus and
inserts on the second and third metacarpals. It flexes the paw at the wrist.
FLEXOR CARPI ULNARIS: This muscle arises as two heads. One originates from the medial epicondyle of the humerus; the other originates from the olecranon. About the middle of the ulna the two heads
join and pass along the ulnar border of the lower forelimb to insert on the ulnar side of the carpals. This muscle
acts as a flexor of the paw at the wrist.
PRONATOR TERES: This muscle originates from the medial epicondyle of the humerus and inserts
about the middle of the radius. It is quite a “fleshy” muscle, being cone-shaped proximally and tapering to just
its tendinous insertion distally. It rotates the radius to the prone position.