THE DIGESTIVE SYSTEM 3OI 



The nerves chiefly concerned in the first stage of deglutition are 

 the fifth cranial nerve to the muscles which close the jaws and the 

 mylohyoid, and the twelfth nerve to the muscles of the tongue. The 

 afferent nerves connected with the reflex act are the second division 

 of the fifth, the glosso-pharyngeal, and the branches of the superior 

 laryngeal nerve to the pharynx. The efferent nerves are those which 

 form the pharyngeal and cesophageal plexuses, the ninth, tenth, and 

 eleventh cranial nerves. The reflex centre is in the medulla oblongata, 

 and it appears to consist of a series of centres ; for if the oesophagus be 

 cut across, stimulation of the afferent nerves will be followed by an 

 orderly wave of contraction, just as in the intact oesophagus. 



The inhibition of respiration which accompanies swallowing depends 

 on stimulation of the glosso-pharyngeal nerve ; if the latter is divided 

 and the central portion is excited electrically, the respiratory move- 

 ments are arrested for a period corresponding with that of a normal 

 act of deglutition, that is, for five or six seconds. 



SECTION III. 

 THE (STOMACH AND ITS FUNCTIONS. 



The stomach forms a dilated portion of the digestive tube capable 

 of storing considerable quantities of nutritive material, and it thus 

 obviates the necessity of taking food at inconveniently frequent intervals. 

 The food remains in the stomach for some hours, and during this period 

 it is acted upon by the gastric juice, so that, when it afterwards comes 

 under the influence of the more potent digestive juices found in the 

 small intestine, the hydrolysis of the protein constituents is already 

 well advanced, some of the fats have been acted upon, and, as has 

 already been described, the saliva has effected a conversion of starch 

 into dextrin and maltose. 



The Composition of Gastric Juice. Gastric juice may be obtained 

 for analysis by producing a permanent gastric fistula in an animal. 

 Pawlow's method is to make an incision in the stomach, separating it 

 into a larger and a smaller portion. The larger portion is stitched 

 up and remains in continuity with the digestive tract. The smaller 

 portion is kept separate from the larger by a layer of mucous membrane, 

 and is made to open on the surface of the body (fig. 119). 1 It is found 

 by experiment that the juice secreted by the small stomach has the 

 same composition as that produced by the large stomach, and also that 

 it is secreted in the same proportional amount when the available extent 



1 We are indebted to the kindness of C. Griffin & Co., Ltd., for permission to 

 use these diagrams. s 



