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THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Voluntary Muscle 



Voluntary muscles are here presented before the simpler involuntary ones 

 because they are the ones we most often see and eat. They are characteristic 

 of animals with endoskeletons, that is, the backboned animals and their im- 

 mediate ancestors. 



General Structure and Arrangement. Each voluntary or skeletal muscle 

 consists of bundles of slender cells. Each bundle is held together by very deli- 

 cate connective tissue and the whole muscle is also sheathed by connective 

 tissue, the white strands visible in roast beef and ham (Fig. 7.5). The older 

 the animal, the thicker and tougher these are. Blood and lymph capillaries 

 and nerves run throughout the muscle actually in touch with the muscle cells 

 (Figs. 10.5, 10.6, 10.7). 



Although the form of muscles differs with their functions, most of them 

 are spindle shaped and the ends are drawn out to their points of attachment, 

 the origin and insertion. The origin is usually on a firmly fixed part of the 

 body; that of the biceps muscle which bends the arm is on the shoulder. Its 

 insertion is on the radius of the forearm, the bone to be moved, and the at- 



Clavicle (collar bone) 



Tendon of biceps 

 (origin) 



Biceps muscle 



Triceps muscle 



Fig. 10.3. Voluntary muscle. The biceps muscle takes the main part in lifting 

 and bending the arm; the triceps acts in lowering and straightening the arm. These 

 and other voluntary muscles work in pairs independently, e.g., as the biceps 

 contracts, the triceps relaxes. The nicety of nervous control which is essential for 

 such synchronous action occurs in many regions and at the same time. The 

 shoulder joint adapted for flexibility should be compared with the hip joint adapted 

 for support. (Redrawn from Haggard: The Science of Health and Disease. New 

 York, Harper & Bros., 1927.) 



