116 



GENERAL ZOOLOGY 



nitrogenous residue which can be readily oxidized. Deaminiza- 

 tion occurs principally in the liver, though there is evidence that 

 it also occurs in the tissues generally. Urea, which has the 



/NH 2 

 formula C=0, is the chief end product of nitrogen metabolism 



N \NH 2 

 in the body. The role of the products of protein digestion is 



m. 



m.m. 

 s.m. 



m&km * 



m.pX 



A B C 



Fig. 71. — A, diagram of the blood vessels of the small intestine; the arteries 

 appear as coarse black lines; the capillaries as fine ones, and the veins are shaded. 

 (After Mall.) B, diagram of the lymphatic vessels. (After Mall.) C, diagram 

 of the nerves. (After Cajal.) The layers of the intestine are m, mucosa; m.m., 

 muscularis mucosa; s.m., sub-mucosa; cm., circular muscle; i.e., intermuscular 

 connective tissue; l.m., longitudinal muscle; s, serosa; c.l., central lymphatic; n, 

 nodule; s.pl., submucous plexus; m.pl., myenteric plexus. (From Stohr, Text- 

 book of Histology, by Lewis. P. Blakiston's Son and Company. By permission.) 



therefore twofold: (1) to build tissue, and (2) to provide fuel. 

 Of the two the first is the more important. There is evidence 

 that the liver is involved in the formation or in the mechanism 

 of regulation of the blood proteins, especially fibrinogen. 



It is possible to keep a dog in nitrogen equilibrium (in which 

 intake and outgo of nitrogen are equal) and even with a plus 

 balance of nitrogen if it is fed on certain amino acids without 

 fats or carbohydrates. This means, of course, that some part 



