48 HUMAN PHYSIOLOGY 



Along with specialization such as has been illustrated 

 the cells lose some of the primitive endowments. A cell 

 of muscle or of the nervous system cannot feed upon all 

 sorts of food particles like the roving infusorium. It is 

 limited to the use of dissolved foods which must be of a 

 few standard types. The power of movement is not 

 preserved in the majority of cells in the higher animals. 

 It is the peculiar property of the muscular tissues. 

 When the body is in motion it is these elements which 

 are at work and all the rest are moved by them. The 

 result of physiologic division of labor, combined with 

 the removal of most cells from direct relations with the 

 outside world, is the absolute dependence of each cell 

 upon the contributions of others for its continued 

 existence. 



One of the capacities which may be lost in connection 

 with specialization of structure and function on the part 

 of cells is that of reproduction. We have seen that this 

 is retained by epithelial units; its persistence in the hair 

 and the nails is most remarkable. But it is not retained 

 by the most conspicuous kind of muscle in the body. 

 The enlargement of a muscle in consequence of exercise 

 is due to increased size of the individual units and not 

 to their multiplication. So, too, in the nervous system: 

 the wonderful advances in the working of the brain from 

 infancy to maturity are not assisted by the addition of a 

 single cell to the number originally present but only by 

 the organization of this collection. 



Elements of Anatomy 



Our natural interest will center in the physiology of 

 man, and before we go farther the outlines of human 

 anatomy may be very briefly suggested. The skin, 

 which forms the surface of the body, is an epithelium 

 of many layers. Beneath it there is loose connective 

 tissue, more or less rich in fat. Deeper still we find the 

 muscles. The bones, articulated as the skeleton, give 



