374 PRINCIPLES OF GENERAL PHYSIOLOGY 



no doubt that the glucose is, in great part, absorbed from the alimentary canal 

 before the bacteria are able to complete its destruction. 



Absorption of Sugars. It is a remarkable fact that no absorption of digestive 

 products takes place in the stomach. The chief absorption is done in the small 

 intestine. 



Digestion and Absorption of Proteins. The saliva contains no enzyme which 

 acts on proteins. But in the stomach they are acted upon by a powerful 

 hydrolysing enzyme, pepsin, which acts only in acid solution. Hydrochloric acid 

 is secreted by glands in the wall of the stomach, and is of very wide distribution, 

 being found even in the selachian fishes. It appears that, under usual conditions, 

 pepsin does not carry the hydrolysis beyond the stage of the higher polypeptides, 

 known as peptones. These are not absorbed, but passed on to the duodenum to be 

 acted on further. The acid of the stomach has also a function as an antiseptic ; 

 not a very powerful one, however, since we know that certain bacteria, for example 

 the Bulgarian lactic acid bacillus, can be introduced into the intestine by way of 

 the mouth. Acid, in any case, is a very unfavourable medium for the growth of 

 bacteria, and hydrochloric acid in the concentration of that in the sto*mach kills a 

 large number. We saw that the first stage in the fate of bacteria in the food 

 vacuoles of amoeba, or phagocytes, takes place in an acid reaction ; but no digestive 

 process commences until the reaction changes to an alkaline one. The acid 

 reaction is associated with the killing of the organisms taken as food. 



As the acid contents of the stomach are allowed to pass in small portions at a 

 time into the duodenum, pancreatic juice is poured out by the mechanism already 

 described (page 344), and further hydrolysis of the peptones formed in the 

 stomach, together with that of any unattacked protein, is brought about by trypsin, 

 in an alkaline solution. This alkalinity is not so great as was thought at one time, 

 and as that of the pancreatic juice as it leaves the duct might lead one to suppose. 

 There is considerable neutralisation by the acid of the stomach contents when they 

 mix with the pancreatic juice. In fact, according to Michaelis and Davidsohn 

 (1911), the optimal hydrogen ion concentration for trypsin is lO^ 8 normal, which 

 is only just faintly alkaline to phenolphthalein. 



We have already seen that the pancreatic juice does not contain active trypsin, 

 but only its zyraogen, and that it is necessary for it to be acted upon by another 

 enzyme, enterokinase, produced by the cells of the small intestine, for conversion 

 into active enzyme. As shown by Mellanby and Woolley (1912), this process of 

 activation has a remarkable time course. It starts slowly and becomes more and 

 more rapid as it proceeds. Whether it has the typical S-shape of the curve of 

 autocatalysis is difficult to make out, but it continues to accelerate in rate until 

 the reaction is practically complete. No satisfactory explanation has yet been 

 given of this phenomenon. According to Vernon (1913), it is due to the produc- 

 tion of an unstable substance, which itself acts as an activator, but is rapidly 

 destroyed. Ordinary trypsin, in fact, does not activate trypsinogen. 



It is evident that some kind of autocatalysis takes place in the activation of 

 trypsin. It may be that some product is formed by the action of enterokinase, 

 which product has the property of increasing the activity of the enterokinase. 

 Vernon (.Biochemical Journal, 8, p. 528) holds that the trypsin, as it is set free, 

 acts upon some precursor with the formation of another enzyme, "deuterase," 

 which itself activates trypsinogen, independently of enterokinase. 



Bernard (1856, p. 513) refers to the fact that pancreatic juice is much more active when mixed 

 with the contents of the duodenum. In fact, it does not appear that he had found it, as 

 secreted, to act on proteins to any perceptible extent. His attention was thus dimud 

 chiefly to its action on fats and on starch. The facts serve to show Bernard's experimental 

 skill, since it is evident that he had obtained pure juice without contamination. 



Trypsin hydrolyses proteins to amino acids. There are, however, as seems 

 probable, some of the simpler di- or tri peptides which are not attacked very 

 rapidly by it. These are hydrolysed by the erepsin of the succus entericus, an 

 enzyme which does not act on proteins themselves, or rather only on caseinogen 

 and fibrin, and on these only slowly, but converts peptones and other polypeptides 

 into their component amino-acids. It was discovered by Cohnheim (1906). 



