502 TEXTBOOK OF ZOOLOGY 



demim its highly alkaline secretion. In addition, this pancreatic juice 

 contains three digestive enzymes : trypsin, which continues the diges- 

 tion begun by pepsin in the stomach, converting proteins to amino 

 acids; an amylase, annjlopsin, which changes starches into sugars; 

 and a lipase, steapsin, which, aided by the bile, causes a splitting 

 of the fats into glycerol and fatty acids. Bile also contributes to the 

 alkaline condition here. 



The process of digestion is completed in the intestine and the 

 food products are taken up by absorption in its mucosa layer. These 

 foods in solution are taken by the blood stream and lymph vessels 

 to various parts of the body where they are utilized for building 

 tissue or for supplying energy, leaving as by-products urea and 

 carbon dioxide. Sugars that are not used are stored as glycogen in 

 the liver and in voluntary muscles. The liver also serves to store 

 fats and to secrete urea and sugar directly into the blood stream. 



Food that is not digested passes to the large intestine where it is 

 retained for a time and then passed to the outside through the anus 

 as feces. 



Other Glands. — Attached by a mesentery to the wall of the intes- 

 tine near the anterior end of the rectum is the spleen. It is a small, 

 reddish, spherical, Ijnnphoid organ, the functions of which are but 

 incompletely known. The destroying of red blood corpuscles is an 

 important duty, as possibly also is the formation in its tissues of 

 lymphocytes, one type of white blood corpuscle. In mammals the 

 spleen is also believed to accumulate iron freed by the metabolism 

 of other tissues. This iron is subsequently used in the formation 

 of hemoglobin. 



The two thyroid glands are small and lie in front of the glottis 

 under the floor of the mouth. There is one on each side of the hyoid 

 apparatus. The secretion and functions are discussed in the chapter 

 on Internal Regulation. 



A thymus gland lies under the skin behind the tympanic membrane 

 on each side. It is partly covered with muscle and is small. Further 

 discussion of it will be taken up in the chapter on Internal Regulation. 



Circulatory System 



The circulatory^ system comprises the Mood vascular system and the 

 lymphatic system. The two systems are closely interrelated in that 

 they both carry to the tissues of the body nutritive material neces- 



