DIGESTION. 105 



stance, mucinogen, which produces mucin. The gastric glands in 

 both situations are surrounded by a fine connective tissue, which sup- 

 ports blood-vessels, nerves, and lymphatics. 



Changes in the Cells During Secretion. During the periods of 

 rest and secretory activity the cells of the glands undergo changes in 

 structure which are supposed to be connected with the production 

 of the pepsin and hydrochloric acid. During rest, the protoplasm 

 of the central cells becomes filled with granular matter; during the 

 time of secretion this disappears, presumably passing into the lumen 

 of the tubule, and as a result the protoplasm becomes clear and 

 hyalin in appearance. The granular material is probably the mother 

 substance, pepsinogen, which, inactive in itself, yields the active 

 ferment, pepsin. The parietal cells during digestion increase in 

 size, but do not become granular. It is at this period that they 

 secrete the hydrochloric acid. After digestion they rapidly diminish 

 in size and return to their former condition. The pyloric glands 

 secrete pepsin only. 



Mechanism of Secretion. In the intervals of digestion, the mucous 

 membrane of the stomach is covered with a layer of mucus. As 

 soon as the food passes from the esophagus into the stomach, the 

 blood-vessels dilate, the circulation becomes more active, and Jihe 

 membrane assumes a bright red appearance. Coincidentally, small 

 drops of gastric juice begin to exude from the glands, which as they 

 increase in number, run together and trickle down the sides of the 

 stomach. This pouring out of fluid continues during the presence of 

 food in the stomach. 



The secretion of gastric juice is a reflex act, taking place through 

 the central nervous system and called forth in response to the stimulus 

 of food in the stomach. That the central nervous system also directly 

 influences the production of the secretion is shown by the fact that 

 emotion, such as fear or anger, will arrest or vitiate the normal secre- 

 tion. The reflex nature of the process can be shown by experimenta- 

 tion upon the pneumogastric nerve. If during digestion, when the 

 peristaltic movements are active and the gastric mucous membrane 

 is flushed and covered with gastric juice, the pneumogastric nerves 

 are divided on both sides, the mucous membrane becomes pale, the 

 secretion is arrested, and the peristaltic movements become less 

 marked. Stimulation of the peripheral end produces no constant 

 effects ; stimulation of the central end, however, is at once fol- 



