360 SYSTEMATIC ZOOLOGY 



The teeth are always inserted into cavities or sockets in the jaw- 

 bones. The cheeks are very distensible in some cases, as in 

 rodents and some apes, and serve as temporary food pouches. 

 Salivary glands open into the mouth in almost all mammals, and 

 their secretion assists in the digestion of the food. A tongue 

 is always present, though it may be immovable, and is the prin- 

 cipal organ of taste, a special sense which is more highly de- 

 veloped here than in any other class. The oesophagus extends 

 backward from the mouth and, passing through the diaphragm, 

 enters the abdominal cavity, where it joins the stomach, a large, 

 saclike organ, which in the ruminants, animals which chew 

 their cud, such as the oxen and sheep, is divided into four com- 

 partments. The stomach connects with the small intestine, 

 which is generally very long and greatly coiled ; the portion of it 

 which immediately succeeds the stomach is called the duodenum, 

 and receives the ducts from two large glands, the pancreas and 

 the liver. The small intestine opens into the large, which is 

 both larger and shorter, and passes into the rectum, which ter- 

 minates in the anus; only in the lowest mammals is there a 

 cloaca. Attached to the large intestine near the point at which 

 the small intestine joins it is a blind sac, the vermiform appen- 

 dix. In addition to the organs just mentioned, the abdominal 

 cavity contains the spleen and the urogenital organs. 



At the back of the mouth is the glottis, which can be closed 

 during the passage of substances into the oesophagus by a lid- 

 like membrane, the epiglottis ; the glottis opens into the larynx, 

 the organ of voice, which forms the anterior end of the trachea, 

 or windpipe. At its posterior end the trachea divides into two 

 bronchi, which pass into the two lungs. Between the lungs lies 

 the heart, which has the same general structure as in birds, with 

 four chambers, and the course of the blood through it is the 

 same. Opening into the blood system is another system of ves- 

 sels, the lymphatic, containing a nearly colorless fluid; it is 

 found in all the other vertebrates as well. The color of the 

 blood is due, as in all the Yertebrata, to colored cells ; in all the 

 other classes, however, these cells are nucleated like any other 

 cells in the body, but in the Mammalia the colored blood cor- 

 puscles are without nuclei. The two kidneys lie in the lumbar 

 region in the abdominal cavity ; from each passes off a duct, the 





