118 THE HUMAN BODY. 



portion of the primitive protoplasmic mass, which forms the 

 cilia. These, being supplied with food by the rest of the 

 cell, are raised above the vulgar cares of life and have the 

 opportunity to devote their whole attention to the perform- 

 ance of automatic movements; which are accordingly far 

 more rapid and precise than those executed by the whole 

 cell before any division of labor had occurred in it. 



That the movements depend upon the structure and com- 

 position 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 constitution. 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 m the windpipe have been found still at work 

 three weeks after the general death of the animal. 



The Muscles. These are the main motor organs ; their 

 general appearance is well known to every one in the lean 

 of butcher's meat. While amoeboid cells can only move 

 themselves, and (at least in the Human Body) ciliated 

 cells the layer of liquid with which they may happen to be 

 in contact, the majority of the muscles, being fixed to the 

 skeleton, can, by alterations in their form, bring about 

 changes in the form and position of nearly all parts of the 

 Body. With the skeleton and joints, they constitute pre- 

 eminently the organs of motion and locomotion, and are 

 governed by the nervous system which regulates their activ- 

 ity. In fact skeleton, muscles, and nervous system are 

 correlated parts: the degree of usefulness of any one of 

 them largely depends upon the more or less complete de- 

 velopment of the others. Man's highly endowed senses and 

 his powers of reflection and reason would be of little use to 

 him, were his muscles less fitted to carry out the dictates of 

 his will or his joints less numerous or mobile. All the 

 muscles are under the control of the nervous system, but 

 all are not governed by it with the co-operation of will or 



