306 ESSENTIALS OF PHYSIOLOGY. 



obtained by the production of secretion as a result of stimulation of 

 the vagus. The nerve is divided in the neck, and, four days later, 

 when the cardio-inhibitory fibres have degenerated, the distal end of 

 the nerve is stimulated by a tetanising electric current. Five minutes 

 after the commencement of stimulation, the gastric juice begins to 

 flow. 



The efferent nerves concerned in the secretion are therefore the two 

 vagi ; the afferent nerves are normally the branches of the fifth and 

 glosso-pharyngeal to the mucous membrane of the mouth, but stimula- 

 tion of other sensory nerves, such as those of sight and smell, may also 

 excite the secretion. The production of gastric juice through the 

 latter mechanism is spoken of as " psychic " secretion ; and the juice 

 formed is described as "appetite juice," since the sight of food in a 

 hungry animal gives rise to its secretion. 



The Chemical Factor in the Secretion of Gastric Juice. In a dog 

 provided with a gastric fistula of the kind described above, a tistulous 

 opening is made into the main stomach, and the two vagi of the animal 

 are divided so as to preclude the possibility of reflex secretion through 

 these nerves. It is then found that the introduction of meat into the 

 main stomach is followed in from twenty to forty-five minutes by a*flow 

 of gastric juice. The same effect is produced by Liebig's extract of 

 meat, certain preparations of peptone, or semi-digested bread, but not 

 by pure proteose or peptone, or by bread, starch, or white of egg. 

 Mechanical stimulation, or the introduction of a mechanical irritant, 

 such as sand, is also without effect. The substances which stimulate 

 secretion are spoken of as secretogogues, and Edkins has shown that 

 they produce their effect by exciting the cells of the gastric mucous 

 membrane to produce a hormone. If the pyloric mucous membrane 

 of the stomach is boiled with water or dilute acid, a decoction is obtained, 

 which, when injected into the blood stream, excites the secretion of 

 gastric juice. Extracts of the mucous membrane of the body of the 

 stomach have no such effect. As the result of his experiments, Edkins 

 concludes that the partially digested food products excite the formation 

 of a hormone in the cells of the mucous membrane of the pyloric 

 portion of the stomach ; it is called gastric secretin or gastrin, and 

 is absorbed into the blood stream, and is carried in the course of the 

 circulation to the glands of the stomach, stimulating them to produce 

 their secretion. 



The group of hormones, to which gastrin belongs, possess certain 

 definite properties. They are substances of relatively low molecular 

 weight, and are easily diffusible. Each exercises a specific function 

 in exciting the activity of a particular organ or tissue, and, when its 



