The passage of the food into the nostrils is prevented by the interposition of a moveable fleshy curtain or valve hanging down from the palate, and visible at the back part of the mouth; this, in the act of swallowing, is stretched backwards so as to extend to the back part of the throat, and thus entirely shut up the opening into the nostrils. The passage into the windpipe, again, is protected by a cartilaginous lid or covering called _epiglottis_ (from επι, _epi_, upon, and γλωττις, _glottis_, the tongue), which projects backwards from the root of the tongue, and conducts the morsel over the _glottis_ or opening of the windpipe. The epiglottis, however, is greatly assisted in this operation by that rising upwards and forwards of the gullet and windpipe to meet the morsel, of which we are conscious, and which can be felt by the hand in the act of swallowing, and the effect of which is in some degree to hide the glottis under the backward projection of the root of the tongue, and allow the morsel to drop past it into the gullet. Once fairly in the gullet, the course of the food into the stomach is easy enough. The gullet is simply a round tube, made up of two rows of muscular or fleshy fibres, the one longitudinal and the other transverse and circular, with a soft moist lining membrane to facilitate the transmission of its contents. When the morsel is introduced, its upper part contracts involuntarily, and pushes the mass downwards; the portion now reached contracts in its turn, and propels it farther; and so on in succession till it arrives at the stomach. _Deglutition_ or _swallowing_ is thus a more complicated operation than at first sight it appears to be. On looking at any person eating, one is apt to think that the morsel passes along the gullet into the stomach by its own weight; but we speedily perceive the error, when we recollect that, in the horse and the cow for example, the mouth is on a level with the ground when feeding, or drinking, and that the morsel or water is consequently propelled upwards into the stomach against its own gravity. It is well known also, and often made a matter of public exhibition, that a man can swallow even liquids when standing on the crown of his head, with the natural position of the stomach reversed. Deglutition is easier and quicker when the appetite is keen, and the alimentary bolus or morsel is moist and properly softened. It is slow and difficult when the morsel is dry and mealy, and the appetite nauseated. In vomiting, the action of the muscular fibres is _inverted_, or proceeds from the lower end of the gullet towards the mouth; and hence the object is carried upwards instead of downwards, as in the natural order. CHAPTER IV. ORGANS OF DIGESTION--THE STOMACH--THE GASTRIC JUICE.