PHYSIOLOGICAL 343 



This process of simplification which goes on in the alimentary canal 

 is carried out by the action of the digestive juices secreted into the 

 canal by the gland-cells which line it at many points or by associated 

 glands; and the most important constituents of these juices are the 

 enzymes. Enzymes (which are discussed in the chapter on the Cell) 

 are colloidal organic substances of unknown nature which catalyse — 

 that is to say, hasten by their presence — reactions which otherwise 

 would proceed too slowly to be effective. The saliva, for example, 

 though primarily a lubricating fluid, contains an enzyme (ptyalin) 

 which splits up the large molecule of boiled starch. 



The digestive juices of the stomach contain a large amount of 

 hydrochloric acid, which is secreted by certain cells of the mucous 

 lining. This acid greatly aids the action of the enzymes, as well as 

 being of itself a powerful antiseptic. There are probably two distinct 

 enzymes of importance in the stomach; one of which (rennin) 

 attacks the protein of milk, causing clotting, while the other (pepsin) 

 commences the long work of splitting up proteins in general, by 

 forming simpler and more soluble compounds (metaproteins and 

 proteoses) . 



From the stomach the partially digested food passes into the 

 duodenum, where yet other digestive juices begin to work. The walls 

 of the duodenum itself secrete a fluid which has only a weak digestive 

 action, but which is strongly alkaline in reaction and aids the 

 powerfully digestive pancreatic juice. Various substances may also 

 be cast out of the body by way of the duodenal secretion, especially 

 if the kidneys are not working well, but these are always liable to 

 be reabsorbed by the intestine. 



Closely applied to the outside of the duodenum is a compact 

 gland, the pancreas, whose secretion enters the gut by two or more 

 ducts. This secretion is also alkaline, so that the hydrochloric acid 

 from the stomach is more than neutralised. There are also present 

 several important enzymes; one of these (steapsin) attacks fats, 

 splitting them into glycerol and fatty acids, and another attacks 

 starch (amylopsin), forming from it the sugar maltose. Very impor- 

 tant also is the substance trypsinogen, which is itself inactive, but 

 which is speedily converted by an enzyme (enterokinase) into the 

 enzyme trypsin, which digests the proteins already simplified by the 

 pepsin of the stomach. Unlike pepsin, trypsin acts best in alkaline 

 medium, such as is found in the gut at and below this point. There are 

 present also, here as in the secretion of the duodenum itself, enzymes 

 which convert the more complex sugars (maltose, lactose, cane sugar, 

 with twelve carbon atoms) into simpler ones (glucose, CeHjzOe). 



Close to the point where one of the pancreatic ducts enters the 

 duodenum is the opening of the duct by which the bile, formed in 

 the liver, enters the gut. The bile is a complex fluid, containing three 

 main types of substance. There are in the first place pigments 



