16 



NUTRITIONAL PHYSIOLOGY 



require their food to be in solution and of a few standard 

 forms. In other words, the primitive capacity to digest 

 and assimilate every kind of nutriment has been lost, but 

 by a wonderful co-operative activity the internal medium 

 has been made a depot of those particular foods which can 

 still be utilized. 



Fig. 2. Drawings like the above are almost always made from 

 tissues which have been prepared and colored by special means to 

 make clear, minute features: a Represents an ovum or egg-cell, the 

 typical cell may be assumed to tend toward this spheric form; b is a 

 cell from a compact tissue, to show how mutual pressure produces 

 a faceted or polyhedral form; c is a contractile element such as 

 occurs in the walls of the alimentary canal, it illustrates an elon- 

 gated cell; d is an epithelial or lining cell of the order found on the 

 inner surface of blood-vessels; this is an example of extreme flatten- 

 ing; e, from the nervous system, exhibits the possibility of a branch- 

 ing development. 



As an animal grows larger its directly exposed surface 

 becomes smaller in proportion to its weight. The trans- 

 fers which must take place between the organism and the 

 external world require ample surfaces, and they are secured 

 by infoldings of the body wall at different places. The 

 lining of the alimentary tract is an example of such an 

 infolding and provides a large area for absorption. Among 

 the higher forms the lining of the lungs constitutes a vastly 



