122 ESSENTIALS OF ZOOLOGY 



these papillae. The roof of the mouth is composed of the hard 

 palate at the posterior margin of which is the soft palate, a muscular 

 flap hanging down to separate the buccal cavity from the pharynx 

 just behind. The space between the base of the tongue and the 

 palate, which provides the opening between mouth and pharynx, is 

 known as the fauces. 



The pharynx also receives the naixs just dorsal to the palate and 

 this portion of it is called the nasopharynx. It contributes then to 

 both digestive and respiratory systems. The two Eustachian tubes 

 from the middle ears enter the pharynx. There are two passages 

 continuing posteriorly from the pharynx. The aperture leading to 

 the esophagus which is usually constricted except when food is pass- 

 ing through it, and the glottis, which is located just posterior to the 

 tongue, leads to the larynx. The glottis is guarded by the flaplike 

 epiglottis to prevent food material and water entering it. The epi- 

 glottis is quite important in swallowing. 



The esophagus is a distensible tube extending from the pharynx 

 to the stomach. It passes through the dorsal portion of the thoracic 

 cavity, and immediately posterior to the diaphragm it enlarges to 

 become the stomach. The esophageal end of the stomach is known 

 as the cardiac portion, and the more inflated, left-hand portion 

 which follows the cardiac portion is the fundus. The tapering pos- 

 terior portion which comprises the right-hand side of the organ is 

 the pyloric portion. In the epithelial lining of the internal wall are 

 many fine foldings in which are imbedded the gastric glands. They 

 secrete gastric juice into the cavity. This is an acid solution con- 

 taining the enzymes (ferments), pepsin and rennin. The former 

 converts complex proteins to peptones, an intermediate stage of 

 digestion of this class of food, and the latter coagulates (clots) the 

 protein portion of milk to concentrate the casein. The pyloric por- 

 tion of the stomach leads directly into the small intestine through a 

 sphincter valve located at the junction of these two. This is the 

 pyloric valve, and it is usually closed. When the chyme (food in 

 process of digestion) reaches a certain level of acidity (0.4 to 0.5 

 per cent), the nerve endings controlling the valve are stimulated 

 and cause it to open, allowing the chyme to rush into the intestine. 



The small intestine is the longest division of the alimentary canal, 

 being approximately six times the length of the body from snout to 

 anus. Coiled in a manner which almost defies description, it occu- 

 pies a large portion of the abdominal cavity between the liver and 



