374 DIGESTION 



According to Pawlow, the reason why the trypsin is not secreted 

 in the active form is that active trypsin readily destroys the amylo- 

 lytic and lipolytic ferments. In the intestine, where trypsin is 

 rendered active by enterokinase, these ferments are protected from 

 its attack by the proteins of the food and by the bile. Enterokinase 

 is itself immediately destroyed in the presence of free acid (centi- 

 normal hydrochloric acid). 



Having now finished our review of the chemistry of the digestive 

 juices, our next task is to describe what is known as to their secre- 

 tion the nature of the cells by which it is effected and their histo- 

 logical appearance in activity and repose, and the manner in which 

 it is called forth and controlled. 



SECTION IV. THE SECRETION OF THE DIGESTIVE JUICES 

 MICROSCOPICAL CHANGES IN THE GLAND CELLS. 



The digestive glands are formed originally from involutions of the 

 mucous membrane of the alimentary canal, the salivary glands from 

 the ectoderm, the others from the endoderm (Chap. XIX.). Some are 

 simple unbranched tubes, in which there is either no distinction into 

 body and duct, as in Lieberkuhn's crypts in the intestines, or in which 

 one or more of the tubes open into a duct, as in the glands of the f undus 

 of the stomach. Some are branched tubes, several of which may end 

 in a common duct ; such are the glands of the pyloric end of the stomach 

 and the Brunner's glands in the duodenum. In others the main duct 

 ramifies into a more or less complex system of small channels, into each 

 of the ultimate branches of which one or more (usually several) of the 

 secreting tubules or alveoli open. The salivary glands and the pancreas 

 belong to this class of compound tubular or racemose glands, and so 

 does the liver of such animals as the frog. But in the latter organ the 

 typical arrangement is obscured in the higher vertebrates by the pre- 

 dominance of the portal bloodvessels over the system of bile-channels 

 as a groundwork for the grouping of the cells. 



In every secreting gland there is a vascular plexus outside the cells 

 of the gland-tubes, and a system of collecting channels on their inner 

 surface ; and in a certain sense the cells of every gland are arranged 

 with reference to the bloodvessels on the one hand, and the ducts on 

 the other. But in the ordinary racemose gland the blood-supply is 

 mainly required to feed the secretion ; the cells of the alveoli have either 

 no other function than to secrete, or if they have other fxmctions, they 

 are* not such as to entail a great disproportion between the size of the 

 cells and the lumen of the channels into which they pour their products. 

 For both reasons the relation of the grouping of the cells to the duct- 

 system is very obvious, to the blood-system very obscure. In the liver, 

 the conditions arc precisely reversed. We cannot suppose that the 

 manufacture of a quantity of bile less in volume than the secretion of 

 the salivary glands, though doubtless containing far more solids, 

 requires an immense organ like the liver, and a tide of blood like that 

 which passes through the portal vein. And, as we shall see, the liver 

 has other functions, some of them certainly of at least equal importance 

 with the secretion of bile, and one of them evidently requiring from 

 its very nature a bulky organ. Accordingly, both the richness of the 

 blood-supply and the size of the secreting cells are out of proportion to 



