518 



COLLEGE ZOOLOGY 



Goblet cell — 



Epithelial cell 

 Villus 



Tunica propria 



Intestinal gland 



Muscularis 

 mucosae 



Artery ■ 

 Vein — 



Lymph vessel 



Circular muscle 

 Longitudinal muscle 



Submucosa 



Muscle layer 

 -(muscularis) 



Serosa 

 (peritoneum) 



Figure 380. Small intestine of man. Diagrammatic section showing villus at left with only 

 blood vessels; that at right with only lacteal vessels and nerves; but all these structures occur in 

 all viUi. 



of the concentration of each kind of amino 

 acid in the blood is one of the most impor- 

 tant functions of the Hver. Other functions 

 of the liver are the storage of glycogen, 

 vitamins, iron, and copper; fat metabolism; 

 production of heparin; protein detoxifica- 

 tion; and secretion of bile, a liquid that may 

 be stored in the gall bladder until needed. 

 Bile contains bile salts that aid in the diges- 

 tion of fat by bringing about physical divi- 

 sion of fat droplets (globules) into smaller 

 particles; this enables the lipase to act upon 

 it more rapidly. 



Chemical digestion 



We have now reached the subject of what 

 happens to different types of foods during 

 the digestive processes. 



Water 



Most of our tissues contain from 75 to 90 

 per cent of water. It is the medium in which 

 digestive changes occur, and in which dis- 



solved food substances are absorbed, as well 

 as the medium in which wastes are dissolved 

 and removed by excretion. Water may be 

 absorbed without change. 



Mineral salts 



Inorganic salts make up about one per 

 cent by weight of the protoplasm and body 

 fluids. Nevertheless, they are very important 

 since they must be present in the proper 

 concentration or our normal condition is 

 greatly disturbed. For example, if the cal- 

 cium content of the blood in mammals is 

 greatly lowered, convulsions and death fol- 

 low; and if calcium, sodium, and potassium 

 are not present in the proper relative con- 

 centration, our hearts will not beat normally. 

 The principal mineral salts contained in 

 protoplasm are listed on page 16. 



Calcium is a constituent of all proto- 

 plasm; 99 per cent of it is contained in the 

 bones and teeth. Milk and leafy vegetables 

 are the best sources of this mineral. Milk is 

 also the best source of phosphorus, which 



