Muscle tissue may be divided into three types:
We will be concerned only with some of the more easily identified, superficial skeletal muscles.
Generally, a muscle is attached at each end. The less movable attachment is called the origin, the more movable attachment, the insertion. The fleshy central portion of a muscle is called the belly. The attachment may be to a bone by means of a narrow band of connective tissue called an aponeurosis. Most skeletal muscles move bones and cartilages, some also cause movement of soft parts, for example, facial muscles which originate on a bone and insert on the easily movable skin of the face.
As you examine the images of the dissected rat identify the muscles from their descriptions below and note especially the type of movement caused when each muscle contracts. It is these movements that make possible the wide range of behaviour necessary to the survival of the living animal. Muscles are covered by and separated from each other by thin layers of connective tissue called fascia.
Muscles that cause particular types of action can be described as follows:
Extensors - straighten members such as fingers, arms, etc.
Flexors - bend members such as fingers, arms, etc.
Rotators - turn members on their axis (e.g. turn your neck sideways)
Elevators - lift or raise parts or structures
Depressors - lower or depress parts or structures
Sphincters - surround openings which close when muscles contract
Dilators - expand openings
Rat Dissection Image 1
At the posterior angle of the jaw, locate the large masseter muscle, which elevates the jaw. Note the various neck muscles that elevate, depress and rotate the head. On the chest, notice the pair of flat, triangular pectoralis muscles, one on each side of the midline.The pectoralis major originates on the sternum and inserts on the humerus. The anterior margin of each pectoral muscle is marked by the disappearance beneath it of the external jugular vein, and by the lateral passage of a superficial vein from the shoulder.
Rat Dissection Image 2
The lateral border of the pectoral muscle is somewhat disguised in the region of the armpit by the cutaneous maximus muscle which originates in the region of the armpit and the outer surface of the latissimus dorsi muscle. The cutaneous maximus muscle inserts on the skin (image). This muscle is used for shaking the skin. See how the pectoralis maximus muscle attaches to the upper arm. Its chief action is to pull the arm ventrally and to rotate it somewhat.
Originating around the shoulder joint i.e. scapula (shoulder blade) and appearing from beneath the distal attachment or insertion of the pectoralis muscle is the biceps muscle. This muscle inserts on the radius just distal to the humerus. It flexes and rotates the forearm. On the back side of the arm, antagonistic to the biceps, is the triceps muscle. It originates from the humerus and the scapula and inserts on the ulna. The triceps muscle extends the forearm. Running back from the armpit region on the medial side of the humerus, and somewhat hidden by the cutaneous maximus, is the latissimus dorsi muscle which passes posterior and dorsal. The latissimus dorsi originates on the thoracic and lumbar vertebrae and inserts on the shaft of the humerus. Its action is to pull the arm backward and upward. For each of these muscles locate the origin and insertion on the rat skeleton.
On the forearm (distal to the humerus), note the fleshy muscles near the elbow which extend as long tendons over the wrist and attach to the digits. Most muscular control of the digits comes from these muscles. Wiggle your fingers and clench your fist and watch the play of muscles in your upper forearm. You can see and feel the tendons on the front of your wrist and the back of your hand. Muscles that move your fingers are mainly located proximal to the wrist, and are connected to the digits by long tendons.
Rat Dissection Image 3
The abdominal body wall is very thin and its covering fascia is quite tough. In the mid-ventral line find a narrow, longitudinal, whitish band. This is the tendinous linea alba. On each side of it lies a longitudinal strap-like muscle, the rectus abdominis. The rectus abdominis originates on the pelvic girdle (pubis) and inserts on the cartilage of the first and second ribs. The overlying connective tissue must be cleared to see the longitudinal direction of its fibers. Lateral to the rectus abdominis are three layers of muscle, extending over the rest of the ventral and lateral portions of the abdomen. From outside to inside, these are: