75. M. Majendie says, we are informed that mastication and insalivation are carried sufficiently far by the degree of resistance and savour of the food; besides, the sides of the mouth being endowed with tact, and the tongue with a real sense of touch, they are very capable of appreciating the physical changes which the food undergoes. Though deglutition is very simple in appearance, it is nevertheless the most complicated of all the muscular actions that serve for digestion. It is produced by the contraction of a great number of muscles, and requires the concurrence of many important organs. It has been divided into three periods: in the first, the food passes from the mouth to the pharynx; in the second, it passes the opening of the glottis, and that of the nasal canals, and arrives at the oesophagus; in the third, it passes through this tube, and enters the stomach. The progress of the alimentary bolus is facilitated by mucus, which is pressed out of the follicles over which it passes. Its passage through the oesophagus appears to be comparatively slow, and it sometimes stops for several seconds: every person must be convinced of this fact from his own sensations; and where the bolus has been very large, its passage has been accompanied with vivid pain, occasioned by the distention of the nervous filaments which surround the pectoral portion of the canal; thus then is the morsel passed on by the successive contractions of the muscular fibres of the canal, and not by its own weight, as we might be apt to consider: we should also remember that during the feeding of graminivorous animals it is propelled upwards into the stomach, and even man, as a matter of experiment, has often swallowed liquids, while standing on his head.

76. The sensations excited upon the oesophagus by pressure, laceration, etc. are analogous to those of the skin on similar occasions; but this mode of sensibility would appear to terminate at the cardia; for the stomach, in a healthy state, does not experience any sensation on the contact of food.

77. When the aliment is introduced into the sto-, mach, it appears to remain there but a short period before it undergoes a change; but this varies according to its nature, and other circumstances. It has been stated (13) that, although the stomach is a single bag, it may be considered, with respect to its functions, as divisible into two distinct cavities, the one termed the pyloric, the other the splenic extremity1; and these portions are, during the activity of the stomach, separated from each other by a peculiar muscular contraction. These chambers evidently appear to perform different offices in the process of digestion. The splenic portion would seem to separate from the food the superfluous quantity of water, and then to transmit it to the pyloric division, where it undergoes the first great alimentary change, or is converted into chyme: during this operation both orifices of the stomach are closed. It has been stated that, as the food accumulates within the cardiac chamber, the stomach becomes prominent in the epigastrium, the abdomen is distended, and the neighbouring viscera are com-pressed. Under such circumstances it is evident that the reflux of food along the oesophagus would readily take place, had not some provision been established to prevent it; it has been accordingly observed that the muscular fibres of the tube fall into frequent contractions, at such a period, and which more especially occur during the act of inspiration, when the pressure upon the abdomen is the greatest.

In certain morbid states these muscular efforts are too feeble to accomplish their object, and a species of rumination takes place, which greatly infests the dyspeptic sufferer. I shall not consume the time of the reader by relating the numerous theories of putrefaction, concoction, fermentation, and trituration, which have been suggested by physiologists of different ages, to account for the changes which the food undergoes: it will be sufficient to state, that this question is at length determined, and that the solvent energy of the peculiar liquid, which has been already described (30) under the appellation of gastric juice, together with the motions of the stomach, alone produce that change upon the aliment which we have next to consider.

1 In the horse, the mucous membrane of the two extremities of the stomach has a striking difference of structure: the horse being a graminivorous animal, this arrangement may, in some degree, perhaps, answer the purpose of the more complicated stomachs of the ruminantia.

78. It is not easy to define the exact nature of chyme; but authors agree in considering it a homogeneous paste, greyish, of a sweetish taste, slightly acid, and retaining some of the properties of the food. M. Majendie has lately examined the subject with greater precision, and it follows from his experiments, that there are as many species of chyme as there are varieties of food; if, at least, we may judge by colour, consistence, and sensible qualities.