402 OF FOOD, AND THE DIGESTIVE PROCESS. 



approximated sides, the uvula is applied like a valve. A sort of inclined plane, 

 directed obliquely downwards and backwards, is thus formed; and the morsel 

 slides along it into the pharynx, which is brought up to receive it. Some of 

 these acts may be performed voluntarily; but the combination of the whole is 

 automatic. The third stage of the process, the propulsion of the food down the 

 oesophagus, then commences. This is accomplished, in the upper part, by means 

 of the constrictors of the pharynx; and in the lower, by the muscular coat of 

 the oesophagus itself. When the morsels are small, and are mixed with much 

 fluid, the undulating movements from above downwards succeed each other very 

 rapidly; this may be well observed in Horses whilst drinking; large morsels, 

 however, are frequently some time in making their way down. Each portion 

 of food and drink is included in the contractile walls, which are closely applied 

 to it during the whole of its transit. The gurgling sound, which is observed 

 when drink is poured down the throat of a person in articulo mortis, is due to 

 the want of this contraction. The whole of the third stage is completely in- 

 voluntary. At the point where the oesophagus enters the stomach, the "cardiac 

 orifice" of the latter, there is a sort of sphincter, which is usually closed, but 

 which opens when sufficient pressure is made on it by accumulated food, closing 

 again when this has passed, so as to retain it in the stomach. 



426. The purely automatic nature of the act of Deglutition is shown by the 

 fact that no attempts on our own part will succeed in performing it really 

 voluntarily. In order to excite it, we must supply some stimulus to the fauces. 

 A very small particle of solid matter, or a little fluid (saliva, for instance), or 

 the contact of the back of the tongue itself, will be sufficient; but without 

 either of these, we cannot swallow at will. Nor can we restrain the tendency, 

 when it is thus excited by a stimulus ; every one knows how irresistible it is, 

 when the fauces are touched in any unusual manner ; and it is equally beyond 

 the direct control of the will, in the ordinary process of eating voluntary as 

 we commonly regard this. The only mode in which the will can influence it, 

 is by regulating the approach of the stimulus necessary to excite it ; thus, we 

 voluntarily bring a morsel of food, or a little fluid, into contact with the surface 

 of the fauces, and an act of deglutition is then involuntarily excited; or we 

 may voluntarily keep all stimulus at a distance, and no effort of the will can 

 then induce the action. Moreover, this action is performed, like that of respi- 

 ration, when the power of the will is suspended, as in profound sleep, or in 

 apoplexy affecting only the brain; and it does not seem to be at all affected by 

 the entire removal of the brain, in an animal that can sustain the shock of the 

 operation; being readily excitable, on stimulating the fauces, so long as the 

 nervous structure retains its functions. This has been experimentally proved 

 by Dr. M. Hall ; and it harmonizes with the natural experiment sometimes 

 brought under our notice in the case of an anencephalous infant, in which the 

 power of swallowing seems as vigorous as in the perfect one. But, if the nerv- 

 ous circle be destroyed, either by division of the trunks, or by injury of any 

 kind to the portion of the nervous centres connected with them, the action can 

 no longer be performed; and thus we see that, when the effects of apoplexy are 

 extending themselves from the brain to the spinal cord, whilst the respiration 

 becomes stertorous, the power of deglutition is lost, and then respiration also 

 speedily ceases. 



427. Our knowledge of the nerves specially concerned in this action is prin- 

 cipally due to the very careful and well-conducted experiments of Dr. J. Reid. 1 

 The distribution of the Glosso-pharyngeal evidently points it out as in some 

 way connected with it ; but this, when carefully examined, discloses the im- 



1 "Edinb. Med. and Surg. Journ.," vol. xlix. ; and "Physiological, Anatomical, and 

 Pathological Researches," CHAP. iv. 



