# A Laboratory Manual of the Anatomy of the Rat/The Muscular System

THE MUSCULAR SYSTEM
The function of muscular tissue is to produce movement of parts by contraction. Contraction may or may not be under the control of the will. The action of voluntary muscles is consciously controlled; involuntary muscle is not so controlled, and a person is usually not cognizant of its activities. These two types of muscle tissue differ also in microscopic structure, form, and distribution. They are alike in that the structural unit in each case is the muscle fiber, which is a cell highly differentiated for the function of contraction.
Involuntary, or nonstriated, muscles are found in the walls of the digestive tract, arteries and veins, bronchial tubes, uterus, bladder, in the glands, etc. The fibers are either more or less scattered or disposed in sheets. Each fiber is an elongated, spindle-shaped cell, containing a single, centrally located nucleus, and a longitudinally striated cytoplasm. They are under the control of the sympathetic nervous system, and they contract much less rapidly than voluntary muscles.
The striated muscle tissue comprises the skeletal muscles for the movement of the arms, legs, trunk, jaw, and head, the eye muscles for rotating the eyeballs, the muscles of the face, etc. With the exception of the striated muscles of the esophagus, the somewhat differently constructed muscles of the heart, and a few others, striated muscles are voluntary. The fibers are very long as compared with their diameter. The sarcolemma, or thin outer wall of the fiber, incloses the sarcoplasm, in ​which the longitudinal fibrils are imbedded. Alternate light and dark regions in these fibrils produce the cross striated appearance. Each fiber contains many nuclei, but it is believed by some to be a single cell. The nuclei occur near the surface in the "white" muscles, or deeper in the fiber between the fibrils in the "red" muscles. "Red" muscles contract more slowly than the "white," and are more resistant to fatigue.
Striated muscle fibers are aggregated into bundles, fasciculi, and these in turn into masses of varying size and shape, known as muscles. In general these fibers run parallel to one another, and are bound together into bundles by connective tissue. The contraction of the muscle is initiated by a nervous stimulus. The energy for muscular work is derived from chemical changes in carbohydrates, proteins, and fats. These are conveyed by the blood to the muscles. Heat is generated by muscular contraction. Carbon dioxide and other waste products of muscular work are carried away by the blood.
As a rule a muscle is attached at each end, the less movable attachment being the origin; the more movable is the insertion. The attachment may be by means of a tendon, a non-elastic cord sometimes of considerable length, or by a sheet of connective tissue, an aponeurosis. A compound muscle has several origins or insertions. Sometimes parts of a tendon may ossify, producing a sesamoid bone, such as the patella. The fleshy portion of a muscle is called the belly.
Most muscles move bones and cartilages, though some may induce movement in soft parts only, such as the facial muscles which pass from a bone to the easily movable skin. Some which lie beneath a concave or convex surface merely tend to straighten out this surface when they contract. The processes of many of the bones act as levers moved by ​the muscles attached to them, while adjacent articulations of the bones are fulcra. There are three classes of these levers in the body: (1) those in which the fulcrum lies between the muscular attachment and the resistance; (2) the fulcrum at one end, muscle at the other, the weight between; (3) or the fulcrum may be at one end, with the muscle attached to the middle, and the weight at the other end of the bone. Examples of case 2 are rare in man. The nearer the muscle attachment is to the fulcrum, the greater must be the force of contraction to move the lever. Mechanical advantage is secured in machines by having a small force act through a great distance at the end of a long lever, moving a heavy object a short distance. In the musculo-skeletal system this arrangement is generally reversed, a powerful muscular contraction acting at the end of a shorter lever to cause rapid movement of a small weight at the end of a longer lever. Efficiency is sacrificed for speed.
The extensor muscles straighten a member, like the arm or finger. Flexor muscles bend it. Rotators turn it on its axis. Abductor muscles bend the part away from the median line of the body or limb ; adductor muscles cause the reverse movement. As suggested by the names, a levator muscle lifts a structure, a depressor depresses it. A sphincter muscle surrounds an opening, which is closed when the muscle contracts. The function of dilators is to expand such an orifice.
To prepare a rat for the study of the muscles, lay it on its back and make a midventral longitudinal cut through the skin extending from the external genitalia to the tip of the jaw. Cut no deeper than the skin, to avoid injuring the underlying muscles. In the same way slit the skin on the median surface of the front and hind legs, extending the cuts to the midventral incision. Remove the skin. ​Great care should be exercised to avoid removing the cutaneous muscles, which are located immediately under the skin. The student should be provided with two rats, one for the study of the muscles, the second for the dissection of the remaining soft parts. Only the larger superficial muscles are included in the following account.
MUSCLES OF THE SKIN
The cutaneous maximus muscle spreads out fanwise from the region of the arm pit. It covers the back from the shoulders to tail, the sides of the trunk posterior to the front leg, and a large part of the ventral surface of the trunk. Its function is to move the skin, in which it is inserted.
The platysma is a thin sheet of muscle which radiates from the posterior angle of the mouth, covers the posterior side of the face and the lateral surface of the neck, and sends a branch up in front of the ear. The broad cervical portion is attached along the middorsal line of the neck.
Exercise X. Sketch the cutaneous maximus and platysma muscles into an outline drawing of the lateral view of the rat.
Carefully remove the two preceding muscles.
SUPERFICIAL MUSCLES OF THE HEAD
(LATERAL VIEW)
Masseter. This muscle consists of two main parts, an external and an internal. Anteriorly these two are distinct, but they blend posteriorly.
Internal part. Origin, the whole length of the zygomatic arch. Insertion, the mandible, posterior to the diastema. Anteriorly the fibers run nearly at right angles to the long axis of the jaw ; posteriorly they form an acute angle with this axis.
​External part. Origin. It is attached anteriorly to the skull in an elongated pit ventral to the base of the zygomatic process of the maxillary bone. From here a strong flat tendon passes backward over the outer surface of the internal part of the muscle. It originates posteriorly from the posterior half of the zygomatic arch. The anterior and posterior regions blend posteriorly vvdth one another and with the internal part. Insertion, the mandible.
This muscle is a powerful elevator, raising the lower jaw. The anteroposterior fibers, especially of the external part, presumably draw the jaw forward in gnawing.
Temporal Muscle. This is a powerful muscle occupying the posterior part of the orbito-temporal fossa and the side of the head behind it. Origin: the temporal line, the post-tympanic hook, the lateral surfaces of the parietal and squamosal bones, and the posterior wall of the orbito-temporal fossa. Insertion: the coronoid process and the other parts of the mandible. It elevates the lower jaw.
The Quadratus Laibii Superioris Muscle is a triangular structure whose apex is at the anterodorsal angle of the orbito-temporal fossa; it is inserted at its broad end into the whisker pad.
SUPERFICIAL MUSCLES OF THE NECK, SHOULDER, AND UPPER FORE LIMB (LATERAL VIEW)
Levator Auris Longus Muscle. It extends from the middorsal line of the neck as far forward as the back of the head, to the base of the auricle of the ear. It pulls the auricle dorsocaudad.
Acromiotrapezius Muscle. Origin, the spines of all the cervical and first four thoracic vertebrae. Insertion: (1) the clavicle; (2) the acromion process and lateral edge of the spinous process of the scapula.
Clavotrapezius Muscle. Origin, the medial half of the ​clavicle. Insertion, the lateral part of the crest that runs dorsally from the external auditory meatus.
Sternomastoideus Muscle. This muscle is anterior, and parallel to, the clavotrapezius. Origin, the anterior end of the manubrium. Insertion, by a tendon to the mastoid bone.
Spinotrapezius Muscle. Origin, (1) the spines of the fourth to the thirteenth thoracic vertebrae; (2) from the lumbodorsal fascia from the level of the thirteenth thoracic to the third lumbar vertebra. Insertion, the scapular spine. It draws the scapula dorsocaudad.
Latissimus Dorsi Muscle. This is a triangular muscle attached at one of its angles to the medial side of the humerus. Origin: the spines of the thoracic vertebrae from the eighth to the twelfth, inclusive, and the lumbodorsal fascia to the level of the third lumbar vertebra. Insertion, by a strong tendon to the medial surface of the humerus. It is also strongly united to the muscles lateral to it in the axillary region. The fibers converge to the tendon of insertion. It pulls the arm caudodorsad.
Acromiodeltoideus Muscle. Origin, the acromion and metacromion processes of the scalpa, and the ventral border of the clavicle. Insertion, the deltoid process of the humerus. The acromiodeltoideus is bipinnate ; the fibers from the clavicle form an acute angle with those from the acromion and metacromion processes where these two groups of fibers join a connective tissue septum ventral to the humerus. This septum forms the connection of the muscle with the deltoid process.
Spinodeltoideus Muscle. Origin, the infraspinous fascia and anterior two-thirds of the spine of the scapula. Insertion: it is united ventrally by an aponeurosis with the free edge of the deltoid tuberosity of the humerus. It flexes the humerus and rotates it outward.
​Triceps Brachii Muscle. This muscle has three heads—the caput longum, caput laterale, and caput mediale.
Caput longum. This head lies on the dorsal side of the upper arm. Origin, the anterior third of the axillary margin of the scapula. Insertion, the olecranon.
Caput laterale. This head lies on the lateral surface of the upper arm. Origin, the larger tubercle of the humerus. Insertion, the olecranon.
Caput mediale. It lies along the dorsal surface of the humerus. It is wedge-shaped, being pointed at the anterior end, but greatly increasing in size posteriorly. Origin, the dorsal and dorsolateral surfaces of the humerus throughout practically the whole ventral surface of the muscle. The pointed anterior end of the muscle reaches nearly to the head of the humerus. Insertion, the olecranon. The caput mediale is not seen superficially, but is found when the caput laterale and caput longum are removed.
SUPERFICIAL MUSCLES OF THE TRUNK
SUPERFICIAL MUSCLES OF THE HIND LEG
Biceps Femoris Muscle. This muscle has three portions, or heads—the anterior, posterior, and accessory. The origin of the anterior head is the last sacral and the first caudal vertebra. The posterior and accessory heads originate at the sciatic tuber of the ischium. All three are inserted by an extensive fascia into the distal end of the femur and the proximal two-thirds of the tibia.
Glutaeus Maximus Muscle. This triangular muscle originates by fascia from the dorsal border of the ilium, from the last three sacral vertebrae, and the first caudal vertebra. It is inseparably united anteriorly with the tensor fasciae latae muscle. Its origin is covered posteriorly by the anterior head of the biceps femoris. Insertion, the third trochanter of the femur.
Tensor Fasciae Latae Muscle. Origin, iliac crest. Insertion, fascia lata of the thigh. This muscle is exposed antero-ventrad to the glutaeus maximus.
Semitendinosus Muscle. Origin, the posterior part of the sciatic tuber. Insertion, by a tendon on the tuberosity of the tibia. It joins the tibia from the medial side. It passes from its dorsal origin down around the posterior side of the thigh on to the medial surface of the shank. Action, flexes and rotates the shank.
Exercise XI. Outline the whole body of the rat as seen in the lateral view, and in this outline draw and label the muscles visible after the removal of the cutaneous maximus and platysma muscles.
MUSCLES ON THE VENTRAL SIDE OF THE HEAD, NECK, THORAX, AND ABDOMEN
Place the skinned animal on its back, then dissect and identify the following muscles.
Digastricus Muscle. This muscle lies between the mas​seters on the lower jaw. Locate its origin on the skull and its insertion on the mandibles at, and near, the symphysis.
Sternahyoideus Muscle. The two sternohyoideus muscles lie on the ventral side of the neck behind the digastricus. They are in contact with one another along the sagittal plane of the neck. Origin, the sternum. Insertion, the hyoid bone.
The sternomastoideus and clavotrapezius muscles have already been mentioned in connection with the lateral view. Identify them here.
Omohyoideus Muscle. This muscle will be observed on the ventral surface of the neck in the triangular space bounded by the digastricus, sternohyoideus, and sternomastoideus muscles. Origin, the superior margin of the scapula. Insertion, the hyoid bone.
The Pectoral Muscles are a prominent group on the ventral side of the thorax. The pectoralis major partially conceals the pectoralis minor. The pectoralis major consists of a superficial and a deeper portion. The origin of the superficial portion is the anterior half of the sternum's manubrium; it is inserted on the deltoid ridge. The deeper portion originates on the manubrium of the sternum and the first and second sternebrae; the insertion is likewise on the deltoid ridge.
Pectoralis Minor Muscle. Remove the pectoralis major to study this muscle. It comprises three parts. The origin of the first, or most anterior, part is the second to the fifth sternebrae, inclusive; insertion, on the coracoid process of the scapula and the lesser tuberosity of the humerus. The second, or middle, portion originates at the fifth sternebra and is inserted on the deltoid ridge. The origin of the third portion is the xiphoid process of the sternum; the insertion is on the coracoid process of the scapula.
Rectus Abdominis Muscle. This consists of a pair of ​thin strap-like muscles along the ventral side of the abdomen and the ventrolateral surface of the thorax. Over the greater part of the abdomen each muscle unites with its fellow by means of the linea alba, along the midventral line. Posterior to the xiphoid process the muscles diverge, running forward ventrolaterally along the thorax. It retracts the sternum and compresses the abdomen. Origin, the posterior part of the pubic symphysis, and the coxal bone in the immediate vicinity. Insertion, the first rib, the manubrium of the sternum, and the clavicle.
Exercise XII. Outline the ventral aspect of the body and draw the muscles into this outline.