352 HUMAN PHYSIOLOGY 



Fate of the Digestive Secretions. 



1. Water. Although it is impossible to state accurately 

 the average amount of the various digestive secretions poured 

 into the alimentary canal each day, it must be very consider- 

 able, probably not far short of 3000 ccms., or something 

 considerably more than one-half of the whole volume of the 

 blood. Only a small amount of this is given off in the faeces, 

 and hence the greater part must be re-absorbed. There is 

 thus a constant circulation between the blood and the 

 alimentary canal, or what may be called an entero-hcemal 

 circulation. One portion of this is particularly important. 

 The blood vessels of the intestine pass to the liver, and many 

 substances, when absorbed into the blood stream, are again 

 excreted in the bile and thus are prevented from reaching 

 the general circulation. Among these substances are the 

 salts of the bile acids and their derivatives, many alkaloids 

 such as curarine, and in all probability a set of animal alka- 

 loids called ptomaines formed by putrefactive decomposition 

 of proteids in the gut. If, from disturbances in the functions 

 of the liver, these are allowed to pass through that organ, the 

 feelings of lassitude and discomfort which are associated with 

 intestinal dyspepsia are produced. The liver thus forms a 

 protective barrier to the ingress of certain poisons. 



2. Enzymes. Ptyalin appears to be destroyed in the 

 stomach by the hydrochloric acid. Pepsin is probably 

 partly destroyed in the intestine, but it seems also to be 

 absorbed and excreted in the urine ; for, on the addition of 

 hydrochloric acid, the urine has a peptic action on proteids. 

 Trypsin appears to be destroyed in the alimentary canal ; 

 but the fate of the other pancreatic enzymes and of the 

 enzymes of the succus entericus is unknown. 



3. Bile Constituents. 1. The bile salts are partly re- 

 absorbed from special parts of the small intestine 

 glycocholate of soda being taken up in the jejunum and 

 taurocholate in the ileum. The acids of these salts are also 

 partly broken up. The glycocholic acid yields amido-acetic 

 acid, which is absorbed and passes to the liver to be excreted 

 as urea; while the taurocholic acid yields amido-isethionic 

 acid which goes to the liver and yields urea and probably 



