150 FUNCTIONS OF THE NERVOUS SYSTEM. 



upon the lower part of the alimentary canal. It has been already stated, that 

 the motor function of the Par Vagum appe'ars to terminate at different points 

 in different animals ; and this may in part explain the great variety in the 

 results obtained by different experimenters, in regard to the effect of section 

 of the par vagum upon the function of digestion. Valentin agrees with Dr. 

 Reid in stating, that distinct movements of the stomach may be excited in the 

 rabbit by irritation of the par vagum ; and he adds, as a precaution, that the 

 experiment should be performed very soon after death, as the irritability of the 

 stomach is soon lost ; and that the stimulation of the nerve should not be per- 

 formed too high up, but rather in the lower part of the neck, or in the thorax. 

 Various experiments upon living animals have led to the belief, that the motions 

 of the muscular parietes of the stomach, which perform a very important part 

 in chymification, are due to the influence of this nerve ; food taken in shortly 

 before or subsequently to its division, having been found to be only dissolved 

 on the surface of the mass, where it was in contact with the mucous membrane. 

 But these experiments have been made for the most part upon Herbivorous 

 animals, such as horses, asses, and rabbits, whose food is bulky and difficult 

 of solution, requiring to be constantly changed in its position, so that every 

 part of it may be successively brought to the exterior. On the other hand, 

 Dr. Reid found, in his experiments upon Dogs, that, after the first shock of 

 the operation had gone off, solution of the food in the stomach, and absorption 

 of chyle, might take place ; and hence it may be inferred, that no influence of 

 this nerve upon the muscular parietes of the stomach is essential to digestion 

 in that species. This conclusion harmonizes well, therefore, with the fact 

 already stated respecting the absence of such influence in the lower part of its 

 oesophagus ; and it may, perhaps, be explained by the consideration, that the 

 natural food of the dog is much less bulky and more easy of solution than that 

 of the animals already named ; so that there is not so much need of the peculiar 

 movement which is in them so important an aid to the process of reduction. 



199. In regard to the functions of the afferent portion of the gastric branches 

 of the Par Vagum, there has also been considerable difference of opinion ; some 

 physiologists maintaining that it is by impressions on them alone that the sense 

 of Hunger or satiety is occasioned ; whilst others deny that it has any power 

 of transmitting such impressions, and maintain that they do not originate in 

 the stomach at all. Dr. Reid has arrived at the conclusion, from his numerous 

 experiments, that the par vagum is the channel through which the mind 

 becomes cognisant of the condition of the stomach ; but that it is not the sole 

 excitor of the sense of hunger. Animals which have sustained section of the 

 nerve on both sides will eagerly take food, if they have not received too great 

 a shock from the operation ; but they seem to experience no feeling of satiety 

 when the stomach is loaded. This inference is confirmed by Valentin, who 

 mentions that puppies after the operation will take three times, and even more, 

 the same quantity of milk as uninjured individuals of the same age ; so that the 

 abdomen is greatly distended. The act of Vomiting has been now sufficiently 

 shown to be excitable through the par vagum; an impression propagated 

 through which to the Medulla Oblongata excites to contraction a considerable 

 number of muscles. But, as in the case of hunger, although the sense of nausea 

 and the tendency to vomit may be excited by various irritating causes operating 

 through this nerve only, it may be produced also through other channels. 

 Thus severe vomiting has been excited by the injection of a solution of tartar 

 emetic or of emetin into the blood-vessels ; a fact of which it has been pro- 

 posed to take advantage in extreme cases of narcotic poisoning, when the nervous 

 system has become so torpid, that emetics administered in the ordinary manner 

 are of no avail (See 300). 



200. That the ordinary peristaltic movements of the intestinal canal, from 



