GENERAL ZOOLOGY 



tract, substances actually enter the body by absorption. The parts of the 

 tract are the mouth cavity, pharynx, esophagus, stomach, small intestine, and 

 large intestine. 



Teeth and tongue are characteristic structures of the mouth cavity, although 

 birds do not have teeth (Fig. 3.1). Salivary glands empty their product, the 

 saliva, into the mouth cavity in terrestrial vertebrates by way of the salivary 

 ducts (p. 61). The mouth, or opening of the mouth cavity, is the place of 

 food intake or ingestion, and teeth either hold the food in the mouth cavity 

 or initiate its mechanical breakup. Various functions are performed by the 

 tongue in diflferent vertebrates: in frogs and lizards it serves to capture food; 

 in mammals it functions to manipulate food, as a site for the taste buds 

 (p. 1 10) and, in man, as an important adjunct to speech. 



No obvious landmark separates the mouth cavity and pharynx; they bear 

 different names because of the manner of their origin during development. 

 Both serve jointly as passageways for food and air. During evolution an 

 important change occurred in these regions, as can be seen by comparing 

 man with the frog. In man a horizontal partition, the hard palate, separates 

 oflT an upper portion of the original undivided mouth cavity, such as is found 

 in the frog. This upper part, the nasal cavity, opens externally by the 

 nostrils and is exclusively an air pathway (Fig. 3.1). Posteriorly, the nasal 

 cavity is continuous with the nasal pharynx, which is only incompletely sepa- 

 rated from the oral pharynx by the soft palate. On each side the Eustachian 

 tube leads from the anterior part of the pharynx to the cavity of the middle 

 ear (Fig. 4.4, p. 90). Posteriorly, the pharynx is continuous with the esoph- 

 agus and, ventrally, connects with the air passages through a slit-like open- 

 ing, the glottis. This crossing, so to speak, of the food and air paths is not 

 a very efficient arrangement, as everyone is well aware. 



The esophagus varies in length with the neck of the vertebrate. Usually, it 

 only carries the food from the pharynx to the stomach, but in birds a part of 

 it, the crop, is expanded to serve for food storage. In some mammals, such 

 as the ruminants, expanded regions of the esophagus form part of what is 

 ordinarily referred to as the "stomach," again functioning as a storehouse 

 of food. 



The stomach and the small intestine, which are distinctly separated by the 

 pyloric sphincter, are essentially comparable in structure in all vertebrates. 

 They, together with the large intestine, are suspended in the coelom, or body 

 cavity, by the mesenteries. These are formed by two layers of the peritoneum 

 which lines the coelom and covers the stomach and intestines (p. 69). The 

 mesenteries serve to anchor the gut and to hold the liver, pancreas, and spleen. 

 Mesenteries function as bridges by means of which the blood and lymph 

 vessels, as well as nerves, reach the organs of the body cavity; they are also 

 important fat depots (p. 77). 



The coelom has only two compartments in the lower vertebrates such as the 

 frog, the pericardial cavity, containing the heart, and the pleuroperitoneal cavity. 

 In mammals this cavity is divided into three, two pleural cavities, each con- 



48 



