338 THE MAINTENANCE OF THE INDIVIDUAL 



which move the boiios of tlie skeleton. There are approximately over 

 five hiindnnl sucli mviseles distinguishable in man. They form the 

 body wall, thus constituting, through a three-ply arrangement, the 

 chief means of keeping the viscera in position. They regulate the 

 position of the head and the degree of curvature of the backbone, as 

 well as the shape of the thigh and the calf of the leg, and the contour 

 of the arm. Since these muscles are responsible for all quick, con- 

 sidered movements, as well as simple reflex actions, they must be 

 built upon a plan whereby one set of muscles through contraction 

 may perform an opposite type of movement from the other, that is, 

 work in opposition to each other. 



Individual skeletal muscle fibers may reach something over an inch 

 in length, but average only about 5^^ of an inch in diameter. If a 

 single fiber of skeletal muscle is examined under the microscope, the 

 regular rows of striations become visible. Careful study reveals a 

 series of dense strands of protoplasm running the entire length of the 

 muscle fiber, between which are spaces filled with a watery proto- 

 plasmic material. It is believed that these delicate protoplasmic 

 strands are capable of forcible contraction which, by mass action, 

 results in the shortening of the entire muscle fiber. Each muscle 

 fiber is enclosed in a modified elastic connective tissue membrane 

 called sarcolemma, that bears scattered nuclei on its inner surface. 

 Practically every muscle fiber cell is stimulated by a nerve ending. 

 Groups of these muscle fibers are bound together with connective 

 tissue, numbers of these bundles forming the muscle proper, which 

 is then spoken of as a biceps, triceps, and so on. 



The ends of a muscle are usually tapered. One end is anchored 

 to an immovable portion of the skeleton, and is termed the origin, 

 while the opposite end, which is attached to the portion of the skeleton 

 to be moved, is termed the insertion. The helly of a fusiform muscle 

 is the mid-portion between origin and insertion which swells during 

 contraction. The tough sheath of connective tissue surrounding the 

 muscle becomes continued as a tendon merging into the periosteum of 

 the bone, thus giving a firm attachment. Striated muscles are also 

 arranged in flat, fan-shaped masses, or in thin sheets. 



Heart Muscle. This variety of muscle occurs in all of the higher 

 animals. Although it has characteristics similar to the muscles 

 previously described, cytological and physiological differences place 

 it in a category by itself. Notwithstanding the fact that the action 

 of the heart is involuntary, the cells composing heart muscle are 



