486 THE GASTRIC JUICE [CH. XXXI. 



Indeed, if the animal is hungry and shown the meat and not 

 allowed to swallow it, the effect is as great. The following striking 

 experiment also shows the importance of the psychical element. Two 

 dogs were taken, and a weighed amount of proteid introduced into the 

 main stomach in each without their knowledge ; one was then sham 

 fed on meat, and one and a half hours later the amount of proteid 

 digested by this dog was five times greater than that which was 

 digested by the other. 



In the meat, however, it is not the proteid which acts most 

 strongly as the stimulus ; egg-white, for instance, is not a stronger 

 stimulus than water, but extract of meat is a powerful stimulus ; 

 what the exact extractives are that act in this way is not yet known, 

 and Herzen has since shown that dextrin acts even more powerfully. 

 Herzen distinguishes between succagogues (juice-drivers) like Liebig's 

 extract, and peptogens like dextrin, which produce not only an 

 increased flow, but a juice rich in pepsin-hydrochloric acid. The 

 products of proteolysis are also peptogenic, so that when once 

 digestion has started, a stimulus for more secretion is provided. 



If the vagi are cut (below the origin of the recurrent laryngeal to 

 avoid paralysis of the larynx), and then sham feeding is performed 



I with meat, no secretion is obtained ; the vagi therefore contain the 

 secretory fibres. The experiment of stimulating the peripheral end 

 of the cut nerve confirmed this hypothesis. The nerve was cut in 

 the neck four or five days before it was stimulated; in this time 

 degeneration of the cardio-inhibitory fibres took place, so that 

 stoppage of the heart did not occur when the nerve was stimulated ; 

 under these circumstances a secretion was obtained with a long 

 latency ; the latency is explained by the presence of secreto-inhibitory 

 fibres. Atropine abolishes the action of the vagus. In other animals 

 the spinal cord was cut at the level of the first cervical nerve, and the 

 animal kept alive by artificial respiration ; the vagus nerve was then 

 cut, and its peripheral end stimulated ; an abundant secretion usually 

 followed. Division of the cord renders an anaesthetic unnecessary, 

 and also prevents the afferent impulses set up by the operation passing 

 to the vagal centres, and thus exciting the inhibitory impulses which 

 pass down the vagus, and tend to prevent secretion under ordinary 

 circumstances. 



Pawlow thinks that the sympathetic also contains some secretory 

 fibres, but this has not yet been proved. 



Action of Gastric Juice. 



The principal action of the gastric juice consists in converting the 

 proteids of the food into the diffusible peptones. In the case of milk 

 this is preceded by the curdling due to rennet (see p. 462). 



