When food is taken into the mouth it is at once swallowed, unless it is in a solid form. In this case it is chewed into a convenient size for swallowing, for which purpose it is, in addition, mixed with the viscid saliva and juices of the mouth. Many animals can hardly be said to masticate; such are the carnivora (dog, cat, etc.), and they are not provided with grinding teeth. In most animals living on vege table food large flat grinding molars are found. In these animals, not only is the food finely divided in the mouth but the food, largely con sisting of starch, is partially digested by the saliva. During mastication the food would nat urally tend to escape from between the grind ing surfaces of the teeth, and would collect within the mouth and outside the gums. This is prevented by the muscles placed in the sub stance of the cheeks and lips.
Physiology of Swallowing.-.-As a result of mastication, the food is gathered in the form of a round moist bolus on the upper surface of the tongue. It is now ready to be swallowed. In the first place, it is pushed backward by the tongue and seized by mus cles, many of which are attached to the hyoid bone. According to the most recent in vestigation, the bolus is propelled with great rapidity through the pharynx and gullet into the stomach. Following the propulsion of the food downward there is a wave of contraction, which, commencing in the pharynx travels com paratively slowly downward through the gullet. It is evident that swallowing is not due to the falling of food down the throat. A horse drinks "up-hill,B and the jugglers, or indeed anyone, can drink or swallow with the head vertically downward. When the food has reached the back of the mouth, swallowing occurs irrespectively of the action of the will.
There is a possibility that during swallowing the food may go the wrong way — that is, it may pass into the larynx and windpipe. It is prevented from passing into it by the eleva tion of the larynx which pushes its aperture against and under the back of the tongue, which at the same time is pushed backward.
The act of deglutition or swallowing has been studied by Magendie, Kronecker and Meltzer and by Cannon. The bolus is moved from the anterior end of the tongue along the hard palate to the anterior pillars of the fauces. By the stimulation of the sensory nerves ending in the mucous membrane at this part, a reflex action is produced which results in a forcible contraction of the mylohyoid and hyoglossal muscles. By this the bolus is pushed backward into the oesophagus.
In order that food shall only escape back ward, the pharyngeal cavity must be separated from the mouth, nose and larynx. .
s accomplished as follows: The opening into the mouth is closed by the posterior part of the tongue being pressed against the hard palate and against the neigh boring anterior pillars of the fauces. The nasal cavity is separated from the pharynx by the elevating of the soft palate (by means of the levator palati mollis), by the arching of the posterior walls of the pharynx (by means of the superior constrictors of the pharynx) and by the meeting of the two posterior pillars of the fauces at the median line. The opening into the larynx is closed by the elevation of the larynx by means of the mylohyoid and the geniohyoid and the digastric, to such an extent that it can be covered by the roof of the tongue and the epiglottis.
The first act of deglutition here described may occur with so much force that by means of it the food reaches the stomach. According to Kronecker and Meltzer this is accomplished by contraction of the mylohyoid muscle alone. This is especially true for liquids and soft foods. Only solid and dry foods remain in the pharynx or upper part of the esophagus till the second step in deglutition carries them downward.
The' second part of deglutition consists of a peristaltic movement, that is, a constriction of the oesophagus beginning at the top and traveling downward. The pharynx first con stricts by means of the constrictors, then the oesophagus by means of the constriction of the circular muscles. Thus the parts of the pharynx and oesophagus successively con stricted push the bolus toward the stomach.
The propagation of the contraction from one part of the oesophagus to another does not take place by direct conduction in the muscles, but is dependent upon the central nervous sys tem. After the oesophagus has been cut across, the wave of contraction is set up in the lower part when it has ceased in the upper segment.
The nerve centres which govern the proc esses of chewing, sucking and swallowing lie in the medulla oblongata.
The walls of the stomach and intestines are, like the gullet, provided with muscular fibre. An external layer passes in the direction of the length of the gut, and within this is a circular layer. These muscles contract slowly on stimu lation, and are outside the domain of voluntary action. During the digestion they contract peristaltically, moving the food toward the rec tum. The peristaltic waves may begin in any part of the gut and pass slowly downward fol lowed at varying intervals by other waves.