484 ANIMAL BIOCHEMISTRY 



work has been reported on the point, observed levels should parallel 

 changes in protein intake and metabolism. One hereditary disease 

 called cystinuria leads to the minary excretion of relatively large 

 quantities of certain amino acids, including ornithine, cystine, argin- 

 ine, lysine, and isoleucine. Since cystine in particular is not very 

 soluble, stones in the urinary tract are common in persons having 

 this disease. Other animals are subject to the same sort of disability. 



A greater quantity of nitrogen appears in peptides than in free 

 amino acids. However, little is known of the origin and amino acid 

 compositions of these materials. Traces of proteins are also normal, 

 mostly as serum albumin and globulins and material from the lining 

 of the urogenital system. A variety of pathological states causes 

 marked increases in the protein level, largely as serum proteins. This 

 change is due either to modifications in permeability or to interference 

 with reabsorption. At present a preferred explanation cannot be 

 chosen. 



Small amounts of other compounds also occur in normal urine. 

 Some of these include the water-soluble vitamins, peptide hormones, 

 organic sulfates, glycosides, and steroids. Many of the mineral ma- 

 terials discussed elsewhere also are excreted in the urine. 



Feces 



As formed in the normal intestinal tract, feces consist primarily of 

 water, undigested fragments of food, indigestible materials, bile pig- 

 ments, enzymes, leucocytes, bacteria, certain mineral materials, and 

 mucus and cellular debris from the wall of the intestine. The mixture 

 leaving the small intestine is rather fluid, a state favoring enzyme ac- 

 tion and the intestinal absorption of nutrients. In the caecum and 

 colon, much of the water is absorbed, reducing the total weight to a 

 half or a third of the original value. 



The indigestible materials are rather limited in amount since pro- 

 teins, lipides, and most carbohydrates are efficiently hydrolyzed and 

 the components absorbed. In ruminants and other herbivores like 

 horses, much of even the cellulose is digested. However, lignin and 

 some cellulose are excreted by these animals and by the omnivores 

 man, rat, and pig. Carnivores normally ingest very little cellulose. 

 Other materials, like hair, cartilage, soil, seeds, and unchewed pieces 

 of comparatively hard and insoluble foodstuffs, make up a varying 

 percentage of the bulk. 



The other major component of feces consists of bacteria, which 

 may represent a fourth of the total dry weight. A variety of organisms 



