THE DIGESTIVE AND RESPIRATORY SYSTEM 261 



tissue, needed to give support to the more delicate mucous 

 layer. The musculosa, or muscular coat, may vary much in 

 the different regions, but consists typically of involuntary mus- 

 cular cells arranged in two layers, circular and longitudinal, 

 the former internal. By the contraction of the circular layer 

 the caliber of the tube is lessened and its length increased, 

 while by the contraction of the longitudinal fibers the tube 

 is shortened and thickened. Various combined actions of these 

 fibers produce the peristaltic movements which occur during 

 active digestion, and furnish an important mechanical aid in 

 the process. The serosa, or serous covering for the tube, is in 

 reality a reflexed portion of the peritoneum, which lines the 

 coelom, and which is attached in such a way that, besides 

 covering the canal itself, its reflexions form broad, supporting 

 membranes known as mesenteries, which attach the tube 

 loosely to the body wall and hold it in place. 



In no living vertebrate is the canal, when fully developed, 

 in the form of a straight, undifferentiated tube, but becomes 

 modified in several ways. In the first place, through the nor- 

 mal process of digestion, it necessarily becomes divided into 

 regions, each of which is devoted to the performance of a 

 certain physiological function, either mechanical or chemical. 

 These portions are furthermore differentiated from one 

 another in shape and size, and vary from long, attenuated 

 tubes, to short and wide sacs; some grade into one another 

 without definite boundaries ; others are quite sharply set apart 

 by a sudden change of external shape, by a localized restric- 

 tion in the caliber of the tube, or by the entrance at a definite 

 point of some new digestive juice. 



A second cause for modification in the primary simple 

 digestive tube lies in the mathematical law of the ratio of 

 surface to mass, whereby the surfaces of two homologous 

 solids are as their squares, the masses as the cubes, of their 

 homologous dimensions. If, for example, an animal posses- 

 sing a straight alimentary canal with a smooth mucous lining 

 were to increase to twice its original length, its bulk would 

 be increased eight times, but the square surface of the in- 



