40 NATURAL HISTORY OF VERTEBRATES. 



markable recess of each nasal sac is well developed in the snakes and lizards, and in 

 some mammals. It is known as the organ of Jacobson, and is situated between the 

 roof of the mouth and the nasal sac. It is to be regarded as a very specialized part 

 of the olfactory neuro-epithelium, being abundantly supplied with branches from the 

 olfactory nerves. 



THE INTESTINAL SYSTEM. 



In all Vertebrata, part of the anterior region of the intestinal canal is devoted to 

 the function of respiration, the organs in connection \^ith which will form the subject 

 of a special section. Here only those parts of the intestinal system which are con- 

 nected more or less directly with the elaboration of the food will be discussed. Tlie 

 formation of the cavity of the mouth, by the turning in of a j)it of epiblast, has been 

 already referred to ; the anal end of the tract is similarly formed in many animals, 

 but the rest of the tract is lined by hypoblast in its whole extent, which gives rise to 

 the cells engaged in secretion. Associated with the hypoblast, which in the greater 

 part of the tract is only one cell thick, there is the layer of mesoblast (p. 5) from 

 which is formed the vascular connective tissue, on which the hypoblast rests, as well 

 as the muscular tissue which constitutes the greater part of the thickness of the in- 

 testinal wall. Covering the muscular tissue on the outside is a fibrous membrane, 

 clothed with flat endothelial cells, which is continuous by a double dorsal fold (the 

 mesentery), with the similar membrane which lines the inner surface of the body-wall. 

 This is the ccelomic or pleuro-peritoneal membrane, the ' parietal ' layer of which lines 

 the body-wall, while the ' ^•isceral ' layer clothes the viscera, which lie in the coelom. 



The following regions are recognizable in the alimentary canal of all vertebrates : 

 the cavity of the mouth and pharynx, the oesophagus and stomach, the small intes- 

 tine, and the large intestine. No well-marked boundary exists in many forms between 

 the pharynx and the oesophagus, but the stomach is generally marked off from the 

 small intestine by a distinct pyloric valve, while a similar valve (ileo-CEScal) between 

 the small and large intestines likewise prevents the backward passage of the food. 



From their position we should expect the aperture and cavity of the mouth to be 

 subject to extremely wide modifications. These being always such as are adapted to 

 the food, the modifications are as infinite in their variety as the food itself. To realize 

 this it is only necessary to think of the remarkable differences in this respect between 

 the animals with which we are most familiar. The aperture of the mouth or gape, 

 with its bony framework, is liable to just as much modification as the organs contained 

 in the mouth cavity. It is only in the mammals that we find fleshy lips forming a ves- 

 tibule in front of the bony framework, but this, in certain cases, where cheek pouches 

 are present, may be an important part of the alimentary tract. 



Within the cavity of the mouth are developed certain organs which secure the 

 food, or masticate it, or propel it into the oesophagus, or chemically alter it. These 

 are the teeth, the tongue, and the salivary glands. 



We have already seen how teeth are developed, and how cement bones formed in 

 connection with them contiibute in a most important manner to the formation of the 

 bony walls of the mouth. The tendency to be observed as we proceed from the lower 

 to the higher animal is to the restriction of these teeth to definite tracts, eventually 

 to the bony margins of the gape, and gradually to specialization not onl}- in num- 

 ber but also in form. The description of the teeth forms such a readily accessible 



