HUMAN PHYSIOLOGY. 



HUMAN PHYSIOLOGY. VI. 



ORGANS AND PROCESS OF DIGESTION (<xmtmf). 



THK firat step in the passage of the food through the alimentary 

 canal in, of course, its reception into tin- mouth. The gnuiping 

 of food by the mouth is, undoubtedly, in the adult a Toluiitary 

 act ; but it take* place in many instances quite independently 

 of the will. Thus the lower animals by inatinct seize their food 

 with the lips and convey it into the mouth, and the infant, even 

 in norno oases where the brain ban boon absent, led by the same 

 ,'ui<l", firmly grasps the nipple with it* lips when it is placed 

 between them. The food then, having entered the mouth, 

 in firat subjected to the process of triturating or grinding by 

 th. which is called mastication. As will be at onco 

 .vi.l.-nt. ;ii. extent to which this process needs to bo carried 

 depends entirely on the kind of food on which the animal lives. 

 In the herbivore, feeding on the tough fibres of grass and roots 

 ami hunl tfrain, mastication requires to bo very perfectly per- 

 lonm -1 : imioed, in the ruminants one ordeal is not sufficient, 

 HO the food, after being swallowed, is brought back to undergo 

 further reduction by the teeth. The carnivore go to the other 

 extreme ; their animal food being just of that kind which is 

 most easily acted upon by the gastric juice, ia simply torn or 

 out through by their sharply-pointed teeth, and then, after being 

 slightly lubricated by the saliva, ia bolted whole. Man, feeding 

 on both animal and vegetable food, needs to steer a middle 

 course. He does not require a second stomach like the rumi- 

 nant, nor can he with impunity simply bolt his food like tho 

 camivora. To ensure perfect and speedy digestion, a considerable 

 amount of mastication is necessary, and if this, whether from 

 habit or imperfection of the teeth, is not obtained, indigestion 

 and dyspepsia, with all their attendant horrors, are sure, sooner 

 or later, to supervene. By mastication the food is reduced to 

 a pulp, and intimately mixed with the saliva and the mucus of 

 the mouth. By this means it is brought into that condition in 

 which it can be most easily and perfectly acted upon by tho 

 various secretions with which it afterwards comes in contact. 

 The saliva is a transparent, viscid, watery fluid, of an alkaline 

 reaction, containing about one per cent, of solid matter, the 

 bulk of which ia a peculiar nitrogenous substance called ptyalino 

 a close analogue in the animal kingdom of the vegetable 

 body diastase. When the parts are at rest, the secretion of 

 saliva ia small, but is very largely increased by tho introduction 

 of food into the mouth, or even into the stomach, by the mental 

 impressions produced by the sight or smell, or by the description 

 or thought of food ; the latter class of causes give rise to tho 

 common expression of " making one's mouth water." When 

 auch incitements are provided, the quantity secreted during tho 

 twenty-four hours has been very variously estimated, some 

 authorities placing it as high as two to throe pounds. The duties 

 it performs are of two kinds the first mechanical, in which it 

 id assisted by the ordinary mucus of the mouth, to moisten tho 

 passages of the mouth and throat, and to lubricate the food ; 

 the second, and more important, chemical. It effects no change 

 in tho albuminous or fatty elements, but confines its action 

 entirely to the starchy constituents of tho food ; these it changes 

 into dextrine or glucose, and by so doing renders them soluble. 

 The bolus of food, mixed with and lubricated by the saliva, is 

 conveyed by the tongue to the back of the mouth, there to 

 undergo the second stage the act of swallowing. This func- 

 tion, when analysed, is found to be rather complex, and may 

 oe more easily understood if it ia considered in throo parts. 

 Ia tho first, the morsel of food is carried by the tongue to tho 

 back part of the month ; this is an entirely voluntary act per- 

 formed by the muscles of the tongue and cheeks. The second 

 part is when tho food passes through the arches of tho 

 palate into the pharynx ; in order to effect this, which is 

 the most complicated part of the act of swallowing, it ia ne- 

 cessary that the food should avoid the posterior orifices of 

 the nose and the larynx or windpipe. This is provided for 

 in the following manner : As soon as the food passes into the 

 pharynx, the curtain of the soft palate is raised so as to shut off 

 the posterior nares, the larynx is drawn up, and its cover, tho 

 epiglottis, ia pressed tightly down over it. The third and hist 

 part is when the food, leaving the pharynx, enters the oeso- 

 phagus, and is grasped by its muscular walls and pressed down 

 into the stomach. This is, of course, an involuntary act, and 

 quite beyond control. When the food reaches the stomach, it 



128-x.F. 



come* under the effect* of two net* of influences the gMtrio 

 juioe and the movement* of the miwoalar walls of that organ. 

 By the Utter the food is kept in constant motion, in a kind of 

 double current, so that it is reduced before it leaves the stomach 

 to a condition much nearer that of fluid than it had when it 

 entered it, and every atom is brought into immediate contact 

 with the gMtrio juioe, the nature of which we must now con- 

 sider. As usually obtained, the gastric juioe is a pal* yellowish 

 transparent fluid, of a saltish acid taste, without smell. It 

 readily mixes with wine, water, or spirit, has a very stiuns, 

 power of coagulating albumen, and is remarkably antiseptic, 

 retarding for a considerable time the putrefaction of meat to 

 whi.-li it is applied. It contain* from two to three per osnt. of 

 solid matter, one-half of which is a nitrogenous body called 

 pepsine. It also contains some free acids, the nature of which 

 is Htill undecided, but most probably they are hydrochloric and 

 lactic acids : at least these are the ones most constantly pruant 

 when the juice is examined. When the stomach is empty, DO 

 gastric juice is secreted, and the surfaces are kept moist by a 

 small quantity of ordinary mucus ; but directly food is intro- 

 duced, the peptic glands commence secreting the acid juice, 

 which trickles down over the walls of the ttomacb and mingles 

 with its contents. The quantity produced during the twenty- 

 four hours has been very variously estimated, but it is generally 

 assumed that it ranges from ten to twenty pints. The nature 

 of the gastric juice, and the conditions under which it was 

 secreted, were very imperfectly understood until a compara- 

 tively recent date, when owing to the circumstance of a man 

 receiving a shot-wound through the walls of the stomach, which 

 in healing left a fiatulous communication with the exterior, Dr. 

 Beaumont was enabled to withdraw some of the gastric juioe as 

 soon as it was secreted, and also to perform a series of experi- 

 ments, showing what circumstances were favourable to and what 

 hindered and retarded the secretion. The results of these inveu- 

 tigations may bo shortly stated. It was found that though any 

 mechanical irritation produced a flow of gastric juice, it was not 

 nearly so effectual an excitant as the presence of food. Pepper, 

 salt, and other soluble stimulants, as well as alkalies, increased 

 the secretion, whilst acids had a contrary effect. Cold water or 

 ice, in small quantities, at first checked but eventually increased 

 it, whilst the continued presence of cold very much diminished 

 or quite prevented it. Some interesting information was also 

 obtained as to the relative time various substances took to 

 digest. This was ascertained in this manner : After the man 

 hod taken a meal, a certain time was allowed to elapse, and 

 then the contents of his stomach were examined, and the speed 

 and perfectnesa of digestion estimated. From the results of 

 these observations, a table was formed, which, amongst others, 

 showed the following facts : At the head of the list of quickly- 

 digested substances were rice and tripe ; these were completely 

 chymified in an hour. Eggs, salmon, trout, apples, and venison 

 took an hour and a half ; tapioca, barley, milk, liver, and fish, 

 required two hours. Turkey, lamb, potatoes, pig, two hours and 

 a half ; beef and mutton, three hours ; and veal required a still 

 longer period. Animal substances, as a general rule, required 

 less time than vegetable. 



As another result of these observations, it may be laid down 

 that the quantity of food taken into the stomach should be 

 sufficient to fill, but not unduly to distend it. For as the quantity 

 of gastric juice poured out is not in proportion entirely to the 

 amount of food taken, but varies according to the necessities of 

 tho system, those persons who are in the habit of overloading 

 the stomach do not get the food digested as soon as the more 

 moderate eater ; tho gastric juice secreted being only sufficient 

 to digest a certain portion of the mass, the remainder becomes 

 an obstruction, preventing the walls of the stomach from per- 

 forming their duty of reducing the food to pulp, and thus 

 hindering digestion, lays the foundation of permanent derange- 

 ment of that function. The time that elapses between meals 

 should be sufficient to allow the stomach fairly to perform 

 its duties, and for the chyme to escape into the intestines ; 

 this, of course, varies with the character of the food, but may 

 be generally taken to be from four to five hours. Exercise 

 taken either before or after a meal powerfully modifies the 

 rapidity of digestion ; if moderate and gentle, it is found to be 

 beneficial ; but violent or exoessivt* exertion is hurtful, and 

 retards the process vory considerably. The mind should be 

 tranquil, neither much excited nor depressed ; the latter coa- 



