156 MANUAL OF PHYSIOLOGY. 



Mode of Secretion. The gastric juice is not secreted in 

 large quantity when the stomach is empty, but only when the 

 mucous membrane is irritated with some chemical or mechanical 

 stimulus. The swallowing of alkaline saliva acts as a gentle 

 stimulus and causes secretion, so that the surface of the stomach 

 becomes acid. When the lining membrane of the stomach is 

 mechanically stimulated through a fistula it becomes red, and 

 drops of secretion appear at the point of stimulation, but the 

 amount of secretion thus produced is very scanty when compared 

 with that called forth by chemical irritants. 



Thus, ether, alcohol, and pungent condiments produce copious 

 secretion. Weak alkaline solutions also cause secretion, but the 

 most perfect form of stimulant seems to be the normal one, 

 namely, a consistent mass of food saturated with the alkaline 

 saliva. 



In all probability the secretion of the gastric juice is under the 

 control of a special nerve mechanism, and the way in which the 

 state of activity follows stimulation of the part seems to point to 

 its being a simple reflex act. However, the nervous connections 

 (vagi and splanchnics) between the stomach and central nervous 

 system may all be severed without any marked effect on the secre- 

 tion other than that which would naturally follow the changes 

 in the amount of blood supply, which, of course, is profoundly 

 altered by cutting the vasomotor nerves the splanchnics. 

 Whether this be so or not, there must be some connection with 

 the nerve centres, for sudden emotions check the secretions, and 

 the sensations caused by the sight or smell of food give rise to 

 gastric secretion. 



It has been suggested that Meissner's submucous ganglionic 

 network may act as a reflex centre and regulate the secretion. 

 But since reflection from local ganglionic centres has not yet been 

 definitely demonstrated, we are hardly entitled to assume that it 

 occurs here, and since the stimulus comes into close contiguity 

 with the secreting cells, it seems quite as probable that these 

 elements are excited to activity by direct stimulation of their 

 protoplasm. 



As in the salivary glands, so in the gastric tubes, the cells show 



