302 FUNCTIONS OF THE NERVOUS SYSTEM. 



i 



certainty, owing to the contradictory results obtained by different experi- 

 menters. These contradictions, however, seem partly due to a diversity in 

 the nature of the animals experimented on. It seems to be well established, 

 by the researches of Reid, Valentin, and others, that distinct movements may 

 be excited in the Stomach of the Rabbit, if distended with food, by irritating 

 the Par Vagum soon after the death of the animal; these movements seem to 

 commence from the cardiac orifice, and then to spread themselves in a sort 

 of peristaltic manner along the walls of the stomach; but no such movements 

 can be excited if the stomach be empty. Various experiments upon living 

 animals have led to a similar conclusion ; food taken in shortly before or sub- 

 sequently to its division, having been found to be only dissolved on the sur- 

 face 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 absorp- 

 tion of chyle, might take place; and hence it may be inferred, that no influ- 

 ence 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 

 parts of its oesophagus; and it may, perhaps, be explained by the considera- 

 tion, 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. The muscular walls of the stomach appear to be called 

 into reflex contraction in the act of Vomiting; the mechanism of which will 

 be considered hereafter ( 505). 



388. That the ordinary peristaltic movements of the Intestinal canal, from 

 the stomach to the rectum, may take place without any connection with the 

 nervous system, being due to the direct stimulation of the contact of food, 

 there is now ample evidence ; and though some may yet be found to deny the 

 Hallerian doctrine, that muscular fibre possesses in itself the property of con- 

 tractility, so much additional evidence of its truth has been recently adduced 

 whilst the doctrine itself is so conformable to the analogy supplied by other 

 vital phenomena, that it will be here unhesitatingly adopted. (See Chapter V.) 

 Some Physiologists still suppose, that the peristaltic movements of the ali- 

 mentary canal are due to a sort of reflex action, taking place through the 

 ganglia of the Sympathetic system of nerves, especially, of course, the semi- 

 lunar. This supposition, however, has little or no evidence to support it; for 

 it has been fully proved that the muscular contractions will continue, long 

 after the tube has been separated from its nervous connections through its 

 whole extent ; and the only evidence in its favour is derived from the con- 

 tractions, which may sometimes be induced in parts of the tube which are at 

 rest, when the Sympathetic nerves supplying them are irritated. The ex- 

 periments of Valentin, however, by which the fact that such contractions 

 may be induced (which has been denied by some) is clearly substantiated, 

 also show that the motor influence does not originate in the Sympathetic gan- 

 glia, but in the Spinal Cord. The following are the general results of up- 

 wards of three hundred experiments, so far as they apply to this subject. 

 The pharynx may not only be excited to contraction by irritation of the pha- 

 ryngeal branches of the Par Vagum, or of the roots of the Spinal Accessory, 

 from which their motor power is derived (as will be hereafter explained), but 

 also by stimulating the roots of the first two Cervical nerves ; and the lower 



