PHYSIOLOGICAL CHEMISTRY 407 



produced by the former. The intestinal bacteria are no doubt very 

 important as digestive agents. Thus, without their aid, cellulose 

 cannot be digested, so that, for herbivora, bacteria are indispensable, 

 at least after they have ceased to live on the mother's milk. 

 Carnivorous animals could probably live without them. It has, for 

 example, been shown that if guinea pig foetuses be excised from 

 the uterus just before full term under antiseptic precautions, and 

 kept in a chamber aspirated with sterile air, and fed on sterile milk, 

 they thrive, and if after some time the intestinal contents be examined, 

 the latter will be found free of bacteria. A repetition of this experi- 

 ment on chickens (fed on grain) has given quite a different result, 

 for although they took abundance of sterile food, yet they died as 

 soon as if no food had been taken. 



The bacteria live in symbiosis with the host in whose intestine 

 they grow that is to say, both they and their host are benefited by 

 their presence in the intestine. 



The Faeces. These are composed of the following substances : 



1. Substances which have escaped digestion, e.g. pieces of vegetables 

 (cellulose, etc.), muscle fibres, elastic tissue, casein, fat, nuclein, haematin, 

 etc. 



2. Remains of juices secreted into the intestines, e.g. mucin, traces 

 of bile salts and pigments, inorganic salts (alkaline earths), epithelial 

 cells, and cholesterol. 



3. Products of digestion, e.g. aromatic bodies (indol and skatol), 

 fat acids, methane, ammonia, etc. 



4. Micro organisms. The faeces contain a certain amount of 

 nitrogen, which probably comes from the various secretions rather 

 than from undigested foods (about 1 grin. N. per diem on ordinary 

 diet). The amount of faeces varies very much with the nature of 

 the diet, being about 170 gms. in 24 hours on a mixed diet and 

 400-500 gms. on a vegetable diet. 



CHAPTER XVI. 

 THE CHEMISTRY OF URINE. 



No portion of Biochemistry is of so much practical importance to the 

 medical practitioner as the chemistry of the urine. It is in the urine 

 that the waste products of protein metabolism are chiefly excreted. 

 Urea, uric acid, and creatinin are almost entirely derived from protein, 

 and an estimation of these products in the urine yields valuable 



