384 A TEXTBOOK OF PHYSIOLOGY 



The psychical secretion, or " appetite juice," may be provoked by 

 seeing, hearing, by smelling food, and in human beings by memory alone. 

 The thought of food makes the mouth of a hungry person " water," 

 and the gastric juice flow. " Digestion waits on appetite " is a sound 

 proverb. It is a disputed point as to whether food, by its presence 

 in the stomach, nervously excites a flow of juice. It is now generally 

 held that such is not the case, for introduction of food through an 

 cesophageal fistula into a dog's stomach while it is sleeping excites no 

 flow of juice. The same is true in the case of man. The secretion, 

 excited either by the psychical reflex or by the reflex from the 

 mouth, is abolished by cutting the vagi; this points to these nerves 

 as containing the efferent secretory nerves to the stomach. Division 

 of both vagi may give rise to absence of gastric movement and 

 disorder of digestion; it has been performed below the diaphragm 

 with little ill effect. The existence of the second mechanism of 

 providing juice complicates matters. It is difficult to produce a 

 secretion of gastric juice by stimulating the peripheral end of the 

 vagus nerve, owing to the disturbing action of the nerve upon the 

 heart. About four days after section of the nerve, when the 

 cardio-inhibitory fibres have degenerated, stimulation of the peri- 

 pheral end of the divided vagus excites, after a latent period of 

 three to five minutes, a marked flow of gastric juice. It is difficult, 

 however, to explain the long latent period. 



The Chemical Mechanism. Secretion is called forth by the libera- 

 tion of a hormone " gastrin " and its absorption into the 

 blood-stream. The " gastrin " is stored in the pyloric mucous mem- 

 brane, and is liberated by such bodies as dextrins, maltose, dextrose, 

 peptones ; in fact, the products of digestion of a part of the alimentary 

 tract at higher level than the pylorus. The products, when injected 

 into the blood by themselves, excite little or no secretion. The 

 gastrin is not destroyed by boiling. 



Variation of Composition. It is held by some authorities that 

 the juice obtained by nervous excitation does not vary in quality, 

 whereas that secreted by the chemical mechanism (local changes in 

 the stomach) shows marked variation in the quantity and nature of 

 the juice. Thus, the secretion is said to be greatest in amount with 

 meat, the digestive power greatest with bread. 



100 grammes cut meat = 300 c.c. 



250 milk = 200 c.c. 



Fats are said to increase the amount of pepsin, starch to lessen it. 

 Others doubt this adaptation of the juice to the food eaten. The 

 amount of juice may also vary with the amount and character of the 

 salts in the food, its alkalinity or acidity. Thirst, muscular exercise, 

 and a condition of plethora, markedly influence the quantity of juice 

 secreted. The introduction of food into the ileum and into the rectum 

 excites a flow of juice; at present, it is difficult to say whether by the 

 nervous or chemical mechanism. The effect of food on the juice is 

 given in the following order: 



