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CELLS OF THE VERTEBRATE BODY 



The mucous membrane is composed of simple columnar epithelium 

 which forms the lining of the tract and functions in secretion and 

 absorption. These several tissues are associated to form the small 

 intestine, in which digestion and absorption occur, and each tissue 

 contributes its part of the function of the whole. 



In addition to the grouping of cells to form tissues, and of tis- 

 sues to form organs, organs are associated to form the systems 

 described in the discussions of morphology and physiology. An 

 understanding of the vertebrate body as a whole is to be had in 



■ TH. \ cap \ m.m 

 era. ^ s-mu 



Fig. 83. — Wall of small intestine (semi-diagrammatic). 



cap, capillary; cm., circular muscle; g.c, goblet cell; i.g., intestinal gland; l.m., longi- 

 tudinal muscle; l.v., lymph vessel; m.m., muscularis mucosae, a more or less distinct layer 

 of non-striated muscle cells lying beneath the mucous membrane; mu, mucous membrane; 

 p, peritoneum; s-mu, submucosa; v, villus 



terms of the cells, which are the units of both structure and 

 function. 



Let us consider briefly, for the purpose of illustrating this state- 

 ment, some of the activities of a common vertebrate, such as a 

 frog. The structure of such an animal is familiar, and the general 

 relations of parts and their functions are well understood. If we 

 consider a frog that has not recently fed, sitting on the bank of a 

 stream, we know that, as a result of metabolism and the conse- 

 quent using up of the protoplasmic constituents, it will be necessary 

 for the animal to obtain food. At such a time, if an insect comes 

 within the frog's range of vision, the cells of the frog's retina receive 



