48 HUMAN PHYSIOLOGY 



" division of labor" which is used with reference to the 

 latter. The cells are evidently of different orders and 

 specially adapted to particular functions. Those of 

 muscle are eminently contractile. Those of the nervous 

 tissues have most highly developed the property of 

 conduction. Some, as in the skin, have it their chief 

 duty to provide hosts of descendants whose dead remains 

 may form a protective covering. The specific service of 

 the cells in the connective tissues is to elaborate inter- 

 cellular deposits. 



FIG. 5. To suggest the confinement of the blood within definite 

 vessels (66). The lymph occupies the dotted region between these 

 blood-vessels and the tissue-cells. 



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 



