RAT, A REPRESENTATIVE MAMMAL 125 



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the midventral line from the hyoid almost to the manubrium. A sub- 

 maxillary duct (Wharton's) leaves the anterior end of each gland 

 and leads anteriorly along the medial border of each mandible, enter- 

 ing the floor of the mouth just posterior to the incisor teeth. Joining 

 the anterior ends of the submaxillary glands and covering their ducts, 

 the sublingual glands are found, the ducts of which enter the floor of 

 the mouth also. The salivary glands secrete saliva, which contains 

 mucus and the starch-digesting enzyme ptyalin. This enzyme con- 

 verts starches in the food to the intermediate sugar, maltose. 



The Liver. — This large gland is located ventrally in the cavity, 

 just posterior to the diaphragm at about the level of the junction of 

 esophagus and stomach. It is divided into four parts : the two lateral 

 lohes; the median (cystic) lobe; and the caudate lobe, which is smaller 

 and partially encircles the esophagus. No gall bladder is present. 

 The bile duct (ductus choledochus) is formed by tributaries (hepatic 

 ducts) from various lobes of the liver. This duct in the rat seems not 

 to substitute for a gall bladder as it does in some other animals where 

 the bladder is absent. The bile duct enters the duodenum a little more 

 than an inch from the pylorus. The liver not only secretes the bile 

 which is important in alkalizing the chyme and emulsifying the fats 

 in the intestine as well as possibly producing some fat and carbohy- 

 drate digesting enzymes, but it is also an organ for the storage of 

 digested carbohydrates and fats. Furthermore, it is an important 

 organ in the formation of urea which is then carried by the blood to 

 the kidneys for excretion. 



The gastric glands located in the fine folds (crypts) of the mucosa 

 of the stomach have been mentioned already. The mucosa or inner 

 lining of the small intestine also has numerous simple glands which 

 secrete intestinal juice. This juice protects the walls of the intes- 

 tine, lubricates them to assist in the movement of food, contains the 

 protease enzyme (protein-splitting) erepsin for digestion of peptones, 

 and probably produces maltase for the conversion of maltose to glu- 

 cose. It may be that there are even other enzymes produced here. 



Of course, the digestive action of the three pancreatic enzymes oc- 

 curs in the small intestine. These are as follows: trypsin, a pro- 

 tease, converting complex proteins and peptones to amino acids; amy- 

 lopsin, a diastase, converting starches into maltose; and steapsin, a 

 lipase, converting complex fats into fatty acids (oleic, stearic, butyric, 

 palmitic) and glycerin. 



