DIGESTION 123 



prejudice, as lawyers say, we speak of the " vital activity " of 

 the cells when we wish to imply that things happen in a living 

 stomach for which we cannot make provision in a model. Of 

 the many substances which make their appearance as digestion 

 proceeds, some are absorbed, others left in the mixture. 



The mucous membrane shows its power of controlling 

 digestion in yet another way. In the neighbourhood of the 

 pylorus its structure is unlike that which it presents elsewhere. 

 The gastric glands are short, and tend to branch. Their lining 

 cells are all of the same kind. Over the greater part of the inner 

 wall of the stomach the tubes are long. They do not branch. 

 The cells which line them are of two kinds : small cubical 

 cells (the term refers to their form as seen in section), similar 

 to those of the pyloric glands ; large oval cells, placed with 

 their longest axes in the same direction as the axis of 

 the gland-tube. These oval cells do not project into the bore 

 or lumen of the tube, but are displaced from it by the cubical 

 cells. They rest on the investing, or basement, membrane. 

 All parts of the gastric mucous membrane secrete pepsin, 

 although the pyloric portion produces very little ; the area 

 which contains oval cells alone secretes hydrochloric acid. 

 If a short time after a meal an extract is made from some of 

 the mucous membrane near the pylorus, by pounding it with 

 salt-solution and sand to break up its cells, this extract, when 

 filtered and injected into the blood, stimulates the glands of the 

 cardiac end of the stomach. Under its influence they pour 

 out both pepsin and hydrochloric acid. The extract contains a 

 substance which acts as a chemical messenger. It is a repre- 

 sentative of a class of bodies which play a most important part 

 in co-ordinating the activities of the various organs. Hitherto 

 physiologists have concerned themselves with the visible or 

 " external " secretions of glands. They have shown how the 

 production of these secretions is controlled by the nervous 

 system. Recently they have discovered that another set of 

 influences has to be taken into consideration. Glands, and 

 possibly all other tissues, take from the blood the materials 

 out of which they make their characteristic secretions, or, if 

 they do not discharge secretions, the substances which they 

 require for the building of their own structures, and return 

 to the blood " internal secretions " which act as stimuli to other 



