Motor System — Muscles and Skeleton 95 



Certain terms are used in describing muscles. When a muscle con- 

 tracts, one end (of an elongated muscle) or edge (of a sheet of muscle) 

 remains relatively fixed and the structure to which the other end or 

 edge attaches is moved. The fixed end or edge is called the origin of the 

 muscle; the other is the insertion (Fig. 94). An elongated muscle is 

 usually more or less spindle-shaped, its muscle-tissue forming a thick, 

 "fleshy" middle region called the "belly," which tapers at each end 

 into a nonmuscular tendon by which the muscle is attached. Muscles 

 are usually attached to skeletal parts but in some cases attach to 

 fascia, sheets of tough, fibrous connective tissue. 



A distinction may be made between extrinsic muscles which ex- 

 tend between the body (in a narrow sense, exclusive of its appendages) 

 and the appendages, and intrinsic muscles whose extent is confined 

 within the territory of the appendage. 



Extrinsic muscles may arise from the vertebral column or other 

 parts of the body and attach to a girdle. Such are the dorsal muscles, 

 the trapezius and rhomboideus, which move the pectoral girdle in 

 relation to the body (Fig. 91). Other muscles arise from the body and 

 by-pass the girdle to attach on the proximal segment of the leg; e.g., 

 the latissimus dorsi has origin from numerous trunk vertebrae and 

 inserts on the humerus (in the upper arm); the ventral pectoralis or 

 "breast muscles" have origin upon or near the sternum and insert on 

 the humerus (Fig. 91C). There is an important distinction, however, 

 between pectoral and pelvic girdles. In modern tetrapods the pectoral 

 girdle is never rigidly attached to other parts of the skeleton, while the 

 pelvic girdle is immovably joined to the adjacent sacral region of the 

 vertebral column. (Slight movement may be possible in some am- 

 phibians.) Therefore, in the case of the pelvic appendage, such ex- 

 trinsic muscles as exist must by-pass the girdle and insert on the bone 

 of the upper leg, the femur — e.g., the gluteus muscles, having origin 

 mainly on the vertebral column (Fig. 91C). 



The intrinsic muscles, in general, have origin on one segment, 

 extend over one joint, and insert on the next segment. But there are 

 many exceptions. The biceps of cat and man has origin on the "shoul- 

 der-blade" or scapula and inserts on the radius of the forearm (Fig. 

 94). The thick belly of the muscle lies in the upper arm. The shoulder 

 and elbow joints are traversed by the tendons. It is true in general that 

 bellies of muscles lie within segments of the limb and only the tendons 

 cross the joints. Certain of the tendons which effect movements of the 

 digits belong to muscles whose origins are on the humerus or femur, all 

 of the joints intervening between origin and insertion being crossed by 

 the tendons. 



The numerous intrinsic muscles are described according to their 



