CH. XXXIII.] ACTIONS OF GASTRIC JUICE 513 



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) such as Liebig's 

 extract, and peptogens such as 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 

 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. 



Actions of Gastric Juice. 



Gastric juice has the following five actions: 



1. It is antiseptic, owing to the hydrochloric acid present; 

 putrefactive processes do not normally occur in the stomach, and the 

 micro-organisms which produce such processes, many of which are 

 swallowed with the food, are in great measure destroyed, and thus the 

 body is protected from them. 



2. It inverts cane sugar into dextrose and laevulose. This also 

 is due to the acid of the juice, and is frequently assisted by inverting 

 enzymes contained in the vegetable food swallowed. The juice has 

 no action on starch. 



3. It contains lipase, a fat-splitting enzyme. The protein en- 

 velopes of the fat cells are first dissolved by the pepsin-hydrochloric 

 acid, and the solid fats are melted. They are then split in small 

 measure into their constituents, glycerin and fatty acids. This 



2 K 



