262 HISTORY OF THE HUMAN BODY 



terior of the alimentary canal but four times ; in other words, 

 it would have but half as much digestive power, and would 

 be in danger of starving were not some means employed to 

 proportionately increase its digestive surface. As the physio- 

 logical digestive membrane is the mucosa, this layer is the one 

 primarily concerned in these modifications, and increase in its 

 surface is gained, ( i ) by lengthening the entire canal and al- 

 lowing it to fold or coil in some more or less definite fashion, 

 (2) by the formation of diverticula, or blind pockets, usually 

 long and narrow like the canal itself, and (3) by various meth- 

 ods of folding or wrinkling the mucosa itself, with or without 

 the other modifications. Independently of the above law, vari- 

 ations in the amount of digestive surface, and especially in 

 the capacity of those portions of the canal used as temporary 

 reservoirs, are dependent upon the quality of the food habitu- 

 ally taken, an innutritions food requiring a greater capacity 

 and probably a greater mucous area than a more concentrated 

 one. 



A third necessary tendency of the mucosa is the formation 

 of glands for the elaboration of the various digestive juices 

 needed in the case of different kinds of foods and in different 

 stages of the process; and in this are shown again the prin- 

 ciples of gland formation as treated above in the case of in- 

 tegument. Thus there is a widespread occurrence of beaker 

 cells and of simple tubular glands, which dip a short distance 

 below the surface, and as they are usually placed close to- 

 gether they form a thick mucosa, the thickness of which is 

 that of the length of the glands composing it. In more com- 

 plicated cases the glands may become too large to be included 

 within the mucosa and push their way outward to the serosa, 

 beneath which they appear as localized swellings, as in the case 

 of the pancreas of many fishes; a still farther extension of 

 this principle produces an accessory organ like the liver or 

 like the pancreas of higher vertebrates, beyond the bounds of 

 the alimentary canal, but connected to it by one or more ducts 

 and still invested by the serosa (peritoneum). 



The application of these principles and 'the gradual attain- 



