THE FUNCTIONS OF THE DIGESTIVE ORGANS. 513 



an invertase, a' maltase, and, in mammals, a lactase, have been found. 

 Finally there is erepsin, already described, which has no direct action 

 upon native proteins, with the exception of casein, but does act upon 

 their hydrolytic decomposition products, the peptones. 



The function of the mucous membrane of the small intestine is by no 

 means restricted to the production of the intestinal juice. We shall 

 soon see that substances are secreted by it which are of far-reaching 

 importance for the functions of the pancreas and its ferment, trypsin. 



The secretions produced by the Brunner and Lieberkuhn cells are un- 

 important compared with that of two more important .accessory glands, 

 the liver and pancreas. To be sure, this is not necessarily true of the 

 physiological functions themselves, which in no case are to be judged 

 entirely from the standpoint of quantity, but rather from that of quality. 

 Particularly the more recent investigations have taught us that no matter 

 how insignificant the function of any organ may appear, it must not be 

 disregarded. All sorts of different processes are closely related to one 

 another and influence each other reciprocally. Whether the particular 

 link in the chain of the total processes is long or short is immaterial. 

 We may well imagine that the secretion produced by the glands of Brunner 

 is in many cases highly significant for the digestion of proteins. The 

 pancreatic juice is not able, or at least only imperfectly, to attack con- 

 nective tissue, for example, while pepsin in the presence of hydrochloric 

 acid quickly accomplishes this. Now we know that when fat is present in 

 the food the secretion of the gastric juice becomes considerably diminished, 

 and it is very probable that in such cases the secretion from these intestinal 

 glands is of great assistance. 



One of the above. organs, the liver, constantly gives up a peculiar secre- 

 tion, the bile, which is continually passing through the bile-duct into the 

 intestine. It should be mentioned at once that the formation of the bile 

 is continuous, although the amount secreted varies. It continues during 

 fasting, though in diminished amount. After eating, the secretion in- 

 creases in amount; and, in fact, it has been found that the extent of the 

 secretion depends, in part, upon the nature of the food. We shall soon 

 come back to these relations. 1 



The bile, as it reaches the intestine, represents a mixture of the secre- 

 tions of the liver-cells, the glands of the gall-bladder and the biliary pass- 

 ages. The latter yield mucous chiefly. The bile reacts alkaline to litmus. 

 Its color varies in different animals and likewise in different individuals 

 of the same species. In man the fresh bile is usually a golden yellow, 

 but sometimes it has a greenish hue. It contains, besides salts, mucin 

 and water, its own specific substances. These are the bile-acids, which 

 are combined with alkali, and the bile-pigments. There are also constitu- 



1 Barbera: Bull, della szienz. med. di Bologna (7) 9, (1898). 



