816 



DIGESTION 



Movements of Food in the Alimentary Canal. 

 When food is taken into the mouth it is at once 

 swallowed, unless it is in the solid form. In this 

 case it is chewed or masticated ; the use of which 

 is to divide the solid fragments taken into a con- 

 venient size for swallowing, for which purpose it," in 

 addition, is mixed with the viscid saliva and juices 

 of the mouth. The chewed food, moreover, is more 

 easily digested, inasmuch as the gastric and other 

 juices can act more readily upon finely divided 

 than upon larger masses of food. Many animals 

 can hardly be said to masticate ; such are the 

 carnivora (dog, cat, &c. ), and they are not pro- 

 vided with grinding teeth. In most animals living 

 on vegetable food, which frequently consists of 

 hard grains, roots, and fibres, large flat grinding 

 molars are found. In these animals, not only is 

 the food finely divided in the mouth, but the food, 

 largely consisting of starch, is partially digested by 

 the saliva. In mastication, the head is firmly fixed 

 by the powerful muscles of the neck, while the 

 lower jaw is moved upon the upper. The lower 

 jaw is approximated to the upper by powerful 

 muscles ( the temporal, masseter, and internal ptery- 

 goids), which pass upwards, and are attached to 

 the side of the head and face. Their contraction 

 may be felt by placing the hand in front of the ear 

 and voluntarily contracting the jaws. The lower 

 jaw is depressed by muscles which pass down the 

 front of the neck. Most of these spring from the 

 hyoid bone, which may be felt deep in the tissues 

 of the neck above the 'Adam's apple.' Rotating 

 movements and those of protrusion and retraction 

 of the jaw are produced chiefly by the action of the 

 pterygoid muscles. It is obvious that during masti- 

 cation the food would naturally tend to escape from 

 between the grinding surfaces of the teeth, and 

 would collect within the mouth and outside the 

 gums. This is prevented, however, by the muscles 

 placed in the substance of the cheeks ( buccinators ) 

 and lips (orbicularis oris). These keep the cheek 

 and lip walls closely opposed to the outside of the 

 teeth. On this account food will only escape from 

 between the grinders into the interior of the mouth, 

 from which it is collected and pushed back between 

 the teeth by the muscular tongue. 



The very complex muscular movements just 

 described result from very perfectly co-ordinated 

 nervous impulses, which pass from the brain to the 

 muscles, and cause and regulate their contractions. 

 One can masticate ' at will' that is to say, one can 

 consciously cause the muscles to contract. In 

 order that the mastication may be effective, how- 

 ever, it is necessary not only to know the sizes, but 

 also to be aware of the ever-changing positions of 

 the particles of food. This is effected by sensory 

 nerves, which pass to the brain from the mucous 

 membrane of the mouth. Although mastication is 

 frequently voluntary, yet, like most other volun- 

 tary actions frequently performed, it can be per- 

 formed reflexly. In this case the sensory impulses 

 pass from the mucous membrane to the brain, and 

 initiate appropriate motor impulses which pass to 

 the muscles, without exciting attention and special 

 volition in their passage. 



As a result of mastication, the food is gathered 

 in the form of a round moist bolus on the upper 

 surface of the tongue (see fig. 1). It is now ready to 

 be swallowed. In the first place, it is pushed back- 

 wards by the tongue, and seized by muscles, many 

 of which are attached to the hyoid bone, which can 

 be felt to move during their contraction. According 

 to the most recent investigation, the bolus is pro- 

 pelled with great rapidity through the pharynx and 

 gullet into the stomach. If the finger be placed 

 upon the hyoid bone, or Adam's apple, and the ear 

 placed against the upper third of the back of a 

 patient, the movement of the hyoid bone indicat- 



ing the commencement of swallowing is almost co- 

 incident with the passage of food down the gullet, 

 which produces a very audible sound. Following 

 the propulsion of the food downwards, there is 

 a wave of contraction, which, commencing in the 

 pharynx, travels downwards through the gullet. 

 This, however, is comparatively slow. It is seen 

 then that swallowing is not due to the falling of 

 liquids down the throat. A horse drinks ' up 

 hill,' and the jugglers, or indeed any one, can 

 drink or swallow with the head vertically down- 

 wards. 



When the food has reached the back of the 

 mouth, the rest of the act of swallowing occurs 

 irrespective of the action of the will. The nerves, 

 which commence in the mucous membrane of the 

 pharynx and gullet (sensory branches of vagus), 

 carry impressions to the brain, which disengage of 

 themselves the appropriate muscular movements 

 without necessarily involving either the will or con- 

 sciousness. Thus, in alcoholic stupor, or fainting 

 from drowning, in both of which conditions con- 

 sciousness is suspended, food and liquid placed at 

 the back of the mouth are at once swallowed. 



There is a possibility that during swallowing 

 the food may go the wrong way i.e. it may pass 

 into the larynx and windpipe. It will be seen 

 from the diagram that the food in its passage to 

 the gullet must actually pass over the aperture of 

 the larynx. It is prevented from passing into it 

 by the elevation of the larynx (this can be felt by 

 the hand placed on the tliroat), which pushes its 

 aperture against and under the back of the tongue, 

 which at the same time is pushed backwards. In 

 addition, there is a valve called the epiglottis, 

 which is pushed down over the larynx by the move- 

 ment just described, and by muscular fibres, which 

 act upon it for that especial purpose. If the 

 epiglottis be destroyed, as by ulceration, gun-shot 

 wounds, &c., it is necessary for the patient to have 

 the food he takes carried over the aperture of the 

 larynx into the gullet by a feeding-tube. It is 

 obvious that one cannot speak with the larynx 

 shut, and with the larynx open we cannot safely 

 swallow. Food is prevented from passing into the 

 nose by the elevation of the soft palate which meets 

 the constricting pharynx, and shuts off the cavity 

 of the nose like a valve ( fig. 1 ). 



The walls of the stomach and intestine are, like 

 the gullet, provided with muscular fibre. An ex- 

 ternal layer passes in the length of the gut, and 

 within this is a circular layer. These muscles, 

 unlike the muscles of the limbs, contract slowly on 

 stimulation, and they are outside the domain of 

 voluntary action. During digestion they contract 

 peristaltically, urging the food towards the rectum. 

 The peristaltic waves may begin in any part of the 

 gut and pass slowly downwards, followed at vary- 

 ing intervals by other waves. It is probable that 

 what is called antiperistaltic waves may occasion- 

 ally occur, tending to bring the food back towards 

 the mouth, for bilious matter is frequently vomited, 

 the bile having in all probability passed upwards 

 into the stomach by aritiperistalsis from the duo- 

 denum. 



Eructations are frequently caused by antiperis- 

 talsis, and by a movement of this kind food is 

 brought back into the mouth for further chewing 

 in the ruminants ( sheep, oxen, &c. ). The peristalsis 

 is particularly active during digestion, and is pro- 

 duced in great part by the food stimulating the 

 mucous membrane. If a portion of the intestine or 

 the stomach be removed from the body, peristalsis 

 may continue or may be produced artificially, espe- 

 cially by irritating the mucous membrane. In the 

 body the canal is under the influence of additional 

 nerves (vagus), through which fibres the digestive 

 processes are chiefly regulated among themselves. 



