52 PHYSIOLOGY. 



in vomiting. When the sphincter is paralysed by division of the 

 pneumogastric nerve, the food regurgitates into the oesophagus. 



Chymijicalion. — As soon as the bolus has entered the stomach it 

 is subjected to several agencies, all of which are more or less con- 

 cerned in effecting its solution. It is exposed in the first place, to the 

 movements of the stomach, which have for their object to produce the 

 thorough intermixture of the gastric juice with the alimentary mass. 

 The fibres of the muscular coat of the stomach are so arranged, as 

 to shorten its diameter in every direction ; by the alternate contrac- 

 tion and relaxation of these bands, a great variety of motion is 

 induced. This contraction is due to the stimulus of the food; and 

 when the aliment is difficult of digestion, the muscular coat is pro- 

 portionately stimulated. These movements are also increased by the 

 action of the respiratory muscles. The contraction of the muscular 

 fibres extends also to those of the two orifices of the stomach so as 

 to prevent the escape of the food. This is particularly the case as 

 regards the pyloric orifice in the first period of digestion. 



The bolus, in the next place, is exposed to the action of the gastric 

 juice^ a pure colourless and slightly viscid fluid, having a distinctly 

 acid reaction, which has been observed to distil from the surface of 

 the mucous membrane and mingle with the food. The exudation of 

 this fluid is always excited by the contact of any foreign substance, 

 but it is never present in the organ when empty, the sole contents 

 being then a little viscid mucus. According to the analysis of Pro- 

 fessor Dunglison, the gastric juice contains free muriatic and acetic 

 acids, phosphates and muriates of potassa, soda, magnesia, and lime. 

 According to the experiments of Blondlot, its acid reaction is due to 

 the presence of the super-phosphate of lime, while Professor Thomson 

 and MM. Bernard and Barreswil attribute it to the presence of 

 lactic acid, the existence of which in the healthy stomach has been 

 positively denied by Liebig. 



The gastric juice is secreted through cell agency, by follicles of a 

 tubular shape, resting upon the sub-mucous tissue, having their open 

 ends towards the cavity of the stomach. * 



But diluted acids, of themselves, have no power in chymi Tying 

 alimentary substances, although their presence in the gastric fluid is 

 essential to its action. The active agent is an organic compound 

 obtained from the mucous membrane of the stomach, to which the 

 name of pepsine has been given. It is a proteine compound in a 

 state of change, and it seems to act in precisely the same manner as 

 Ihc diastase does in the conversion of starch into sugar. In so doing, 

 it acts as a sort q^ ferment^ having the power of exciting change in 

 another substance, in which it does not itself participate. Pepsine 

 undergoes no change itself, and forms no combinations with the sub- 

 stances on which it acts, but merely disposes them to solution in 



