FOOD AND DIGESTION 373 



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 considerable, 

 probably not far short of 3000 c.cms., or something consider- 

 ably 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 alkaloids 

 called ptomaines formed by putrefactive decomposition of 

 proteins 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 proteins. 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 Hie salts are partly reabsorbed 

 from special parts of the small intestine sodium glycocholate 

 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 sulphuric acid. The 



