ALIMENTARY TISSUES 283 



its extension is increased by folds and villi and other evaginations arising 

 from the inner surface of the enteron. The cells lining the posterior 

 half and more of the alimentary tube of invertebrates and those found 

 upon the villi and folds of the intestine of vertebrates furnish good 

 examples of absorptive cells constituting an absorptive tissue. 



The essentially digestive tissues are mostly glandular. We meet 

 with such diversity among them that we find ourselves at a loss for a mor- 

 phological basis by which to classify them. As all structures exist 

 purely for the purpose of performing certain functions in some particular 

 manner, we may be justified in adopting a physiological basis for the 

 classification of these tissues. The physiological study of the digestive 

 tissues of invertebrates does not as yet afford a basis for an extensive 

 classification of these tissues. All glands belonging to the alimentary 

 tube and the anterior third or less of an alimentary tube may be con- 

 sidered as digestive tissues. 



Among the vetebrates the digestive tissues may be classified for our 

 purpose as: pancreatic, gastric, serous, and hepatic. This physiological 

 basis holds only for the higher vertebrates, where these functions have 

 been assigned to particular tissues. If the experiments of physiologists 

 are to be considered as final, the cells of the villus region of the midgut 

 folds of insects are both hepatic and pancreatic, and the hepatic cells 

 of mollusks are also pancreatic. The digestive cceca of the starfish 

 and Amphioxus have been shown physiologically to be pancreatic, 

 though it is quite probable that they have other functions as well. By 

 these conclusions we may appreciate the fact that among the lower ani- 

 mals where digestive tissues have been differentiated from absorbing 

 tissues, there has not yet arisen a differentiation of the digestive 

 tissues into pancreatic and gastric tissues. The tissues of the gastric 

 cceca of the invertebrates are spoken of as hepato- pancreatic tissues. 

 In the higher forms, where this fourfold specialization has been effected, 

 most digestive tissues are glandular. 



The pancreatic tissues are those that secrete ferments that are active 

 in an alkaline medium. These ferments are ptyalin, trypsin, and steap- 

 sin. These are the most active digestive tissues. The pancreatic epi- 

 thelium of mammals resembles somewhat a serous epithelium. These 

 tissues are assembled to form the vertebrate organ called a pancreas. 



The gastric tissues elaborate a ferment that is active in an acid 

 medium. This ferment is pepsin. These tissues in mammals are com- 

 posed of two kinds of cells : the ordinary gastric cells that elaborate 

 the ferment, and the acid cells which supply the acid necessary for the 

 action of the ferment. The alimentary tissue found in the gastric glands 

 of a mammalian stomach shows these two kinds of cells. In a bird they 

 are found in separate tissues. 



