METABOLISM AND TRANSPORT IN ANIMALS 



517 



canines (cuspids), 4; premolars (bicuspids), 

 8; and molars, 12. The first set of 20 teeth 

 (incisors, 8; canines, 4; and molars, 8) are 

 called milk or deciduous teeth because they 

 fall out and are replaced during childhood. 

 Each tooth may be divided into three parts: 

 root, neck, and crown; and it consists of den- 

 tine, enamel which caps the crown, and ce- 

 ment which covers the root. The character 

 of the teeth is closely correlated with the 

 feeding habits of the respective animals. 



The pharynx dilates to receive food and 

 passes it on to the esophagus, where, as a re- 

 sult of peristaltic contractions, it is forced 

 down through the cardiac opening of the 

 stomach. The other opening of the stomach, 

 that into the duodenum, is called the pyloric 

 opening. Both openings may be closed by 

 ringlike muscles or sphincters. Three types 

 of glands occur in the stomach wall: (1) 

 cardiac glands, which secrete mucin, a con- 

 stituent of mucus; (2) fundic glands, which 

 secrete hydrochloric acid and the enzyme 

 pepsin, the latter being activated in the pres- 

 ence of the acid; and (3) pyloric glands, 

 which secrete pepsin and mucin. The stom- 

 ach retains the food until it becomes a 

 semiliquid chyme, which is then passed on 

 into the small intestine at frequent inter- 

 vals. Certain cells in the mucous membrane 

 of the stomach secrete a hormone called 

 gastrin, which stimulates the secretion of 

 gastric juice. 



Digestion takes place principally in the 

 small intestine. The chyme is here acted 

 upon by bile from the liver and pancreatic 

 juice from the pancreas; both usually enter 

 the duodenum together through the same 

 orifice. Simple tubular glands in the mucous 

 membrane throughout the small intestine, 

 and more complex duodenal glands in the 

 submucous membrane of the duodenum 

 secrete a digestive fluid named intestinal 

 juice. Certain cells in the mucous membrane 

 secrete the hormone secretin when an acid 

 food mass from the stomach enters the in- 

 testine. Secretin is carried by the blood to 

 the pancreas, which it stimulates to secre- 



tory activity. Peristaltic movements and 

 constricting movements carry the food along 

 the small intestine, but circular folds tend 

 to retard the movement, thus allowing more 

 time for digestion and absorption. Minute 

 fingerlike projections occur throughout the 

 small intestine; these are called villi (Fig. 

 380). Digested food is absorbed into the 

 blood and lymph vessels in these villi. 



The large intestine consists of 4 parts: 

 (1) cecum, with vermiform appendix, (2) 

 colon, (3) rectum, and (4) anal canal. An 

 ileocecal valve allows material to pass from 

 the small intestine into the large, but not 

 back again into the small intestine. A small 

 pouch at the beginning of the large intestine 

 is known as the cecum. Attached to the end 

 of the cecum is a narrow tube about three 

 inches long, the vermiform appendix; its 

 principal importance appears to be the finan- 

 cial support of the surgeons who remove it. 

 Material is carried through the colon by 

 peristalsis. Digestion is continued here by 

 digestive fluids that enter with the food 

 from the small intestine, and absorption also 

 takes place. Most of the water drunk with 

 food and some that is secreted into the 

 intestine in digestive juices is absorbed in 

 the colon. Due to the absorption of water, 

 the contents of the posterior end of the 

 colon are in a semisolid condition. This 

 fecal material consists largely of cellulose 

 and other indigestible substances, bacteria, 

 and excretions of the colon such as excess 

 calcium and iron. 



Besides the tongue, teeth, and salivary 

 glands, the pancreas and liver are impor- 

 tant glands of the digestive system. The 

 pancreas secretes three types of enzymes: 

 ( 1 ) trypsin * may convert proteins into 

 amino acids, (2) pancreatic amylase (amy- 

 lopsin ) converts partly digested starches into 

 sugar, and (3) lipase splits fat molecules 

 into glycerol and fatty acids. The regulation 



* Secreted in an inactive form, trypsinogen is 

 changed to active trypsin by enterokinase, a coen- 

 zyme which is contained in the mucous membrane 

 of the small intestine. 



