88 THE HUMAN BODY 



by the whole cell before any division of labor had occurred 

 in it. 



That the movements depend upon the structure and composi- 

 tion of the cells themselves, and not upon influences reaching them 

 from the nervous or other tissues, is proved by the fact that they 

 continue for a long time in isolated cells, removed and placed in a 

 liquid, as blood-serum, which does not alter their physical consti- 

 tution. In cold-blooded animals, as turtles, whose constituent 

 tissues frequently retain their individual vitality long after that 

 bond of union has been destroyed which constitutes the life of the 

 whole animal as distinct from the lives of its different tissues, the 

 ciliated cells in the windpipe have been found still at work three 

 weeks after the general death of the animal. 



The Physico-Chemistry of Skeletal Muscle. The activity of a 

 muscle is the sum of the activities of its individual fibers. To un- 

 derstand the operation of the muscle engine, therefore, we must 

 analyze the activity of the muscle fiber. Some help toward this 

 may be gained by studying the structure of the fiber from the 

 physico-chemical standpoint. Throughout this study the funda- 

 mental fact of muscular activity should oe kept before the mind, 

 namely, that the muscle is a device for executing a forcible shorten- 

 ing at the expense of energy derived from the food (p. 22). 



From the account of the anatomical structure of the muscle- 

 fiber, given previously (p. 85), we have learned to think of the 

 fiber as consisting of a large number of tiny, longitudinal cylinders, 

 the sarcostyles, inclosed in, and attached by their ends to, a sheath, 

 the sarcolemma, and surrounded by a fluid, the sarcoplasm, which 

 occupies all the space within the sarcolemma not taken by the 

 sarcostyles. Physico-chemical studies indicate that the sarco- 

 styles are colloidalm nature. In fact there is reason to think that 

 the colloids of which they are composed may be quite dense, con- 

 taining less water than do most of the colloidal tissues of the body. 

 The sarcoplasm, on the other hand, is thought of as a very watery 

 fluid, with salts and other relatively simple substances dissolved 

 in it, but containing little if any colloid. When we reach the con- 

 sideration of the precise manner in which the forcible shortening 

 of the muscle comes about we shall find that the densely colloidal 

 consistency of the sarcostyles and the watery nature of the sarco- 

 plasm are of great significance. 



