HUMAN PHYSIOLOGY. 



with the pharynx), is driven into it by the action 

 of certain muscles, which almost surround the 

 pharynx, and are termed its constrictor muscles. 

 All voluntary action ceases as soon as the food is 

 pressed backwards by the tongue into the pharynx. 

 It is impossible to recall the pellet, and it is 

 necessarily carried on (without even our cogni- 

 sance) into the stomach. This involuntary mech- 

 anism is called a reflex action. All reflex 

 actions require a stimulus to call the parts into 

 action. The stimulus in this case is the contact 

 of the food with the back of the tongue and 

 throat. It will be found on experiment that the 

 reader cannot perform the action of swallowing if 

 nothing, not even saliva, is in his mouth. 



5. DIGESTION IN THE STOMACH. 



The whole of the alimentary canal below the 

 diaphragm, or muscular partition which sepa- 

 rates the cavity of the chest from that of the 

 abdomen or belly, possesses the following points 

 in common, in relation to structure. The stomach, 

 the small intestine, and the large intestine, are all 

 lined by mucous membrane, have a muscular coat 

 consisting of two sets of distinct fibres namely, 

 circular fibres which surround the tube or viscus 

 after the manner of a series of rings, and longi- 

 tudinal fibres running in the same direction as the 

 intestine itself and are invested with a smooth, 

 glossy, serous membrane, which, while it retains 

 the viscera in their proper position, also permits 

 their necessary movement with a minimum of 

 friction. 



The human stomach is an elongated curved 

 pouch, lying immediately below the diaphragm. 

 It is very dilatable and contractile, and its func- 

 tion is to retain the food until it is duly acted 

 upon and dissolved by the gastric juice, which is 

 secreted by glands lying in its inner coat, and 

 then to transmit it, in a semi-fluid state, into the 

 first part of the small intestine, called the duo- 

 denum. Its average capacity is about five 

 pints. 



The mucous membrane, or lining coat of the 

 stomach, is thick and soft, and lies in irregular 

 folds, in consequence of the contraction of the 

 muscular coat, unless when the organ is distended 

 with food. On opening the stomach, and stretch- 

 ing it so as to remove the appearance of folds, we 

 perceive numerous very shallow pits or depressions. 

 The rest of the thickness is chiefly made up of 

 minute tubes, running vertically towards the surface 

 of the stomach, and secreting the gastric juice from 

 the blood in the capillaries or minute blood-vessels 

 which abound in the mucous membrane These 

 tubes are lined half-way down with epithelial 

 cells, and the bottom is during digestion filled 

 with molecular matter and a few cells. Other 

 tubular glands are also found in the stomach, 

 which are believed to secrete mucus. 



When food is introduced into the stomach, it 

 is subjected to three actions first, to heat, the 

 temperature of the stomach being, during diges- 

 tion, about 99 F.) ; second, to a slow movement 

 round and round, so as to bring the food into con- 

 tact with the lining ; and, third, to the chemical 

 action of a special fluid the gastric juice. 



The food on entering the stomach first passes 

 into the cardiac end, thence along the greater 

 curvature from left to right to the pyloric end, and 



from thence along the lesser curvature from right 

 to left. 



The changes in the mucous membrane are 

 The inner surface of the healthy fasting stomach 

 is of a paler pink than after the introduction of 

 food, which causes the exudation of a pure, colour- 

 less, viscid fluid, having a well-marked acid reac- 

 tion. This fluid, which is the gastric juice, collects 

 in drops, which trickle down the walls, and mix 

 with the food. Its two essential elements are i. 

 A free acid, which in some cases seems to be 

 hydrochloric alone, and in others a mixture of 

 hydrochloric and lactic acids ; and 2. An organic 

 matter called pepsine, which is highly nitrogenous, 

 and allied to the albuminates. 



The uses of this fluid are not only to dissolve but 

 also to modify the nitrogenous elements of the food 

 (such as albumen, fibrine, caseine, and, in short, 

 all animal food except fat, and the albuminous 

 principles of vegetable food), converting them into 

 new substances, termed peptones, which, although 

 they coincide in their chemical composition, and 

 in many of their physical properties, with the 

 substances from which they are derived, differ 

 essentially from them in their more ready 

 solubility in water, and in various chemical 

 relations. 



The gastric juice exerts no action on the 

 fats and the carbon-hydrates (sugar, starch). If 

 the fats or starches are in cells, the walls of which 

 are formed of albumen, the walls are dissolved, 

 and the contents set free. 



The process of gastric digestion is slow. Accord- 

 ing to Beaumont's researches on Alexis St Martin, 

 a man who had an injury which left a permanent 

 opening into his stomach, guarded by a little valve 

 of mucous membrane, the mean time required for 

 the digestion of ordinary animal food, such as 

 butcher-meat, fowl, and game, was from two hours 

 and three-quarters to four hours. 



What becomes of the matters that are thor- 

 oughly dissolved in the stomach ? The albumin- 

 ates, &c. which are converted into peptones, are 

 for the most part taken up by the blood-vessels, 

 and by another set of vessels called the lacteals. 

 The rapidity with which aqueous solutions of 

 iodine of potassium, the alkaline carbonates, 

 lactates, citrates, &c. pass into the blood, and 

 thence into the urine, saliva, &c. shews that the 

 absorption of fluids must take place very shortly 

 after they are swallowed ; and there is little doubt 

 that the blood-vessels (capillaries) of the stomach 

 constitute the principal channel through which 

 they pass out of the intestinal tract into the 

 blood. 



There can be no doubt that the stomach is 

 admirably adapted for the digestion of the food 

 introduced into it, because it has been shewn by 

 numerous experiments that digestion will go on 

 in gastric juice out of the stomach, but that it 

 requires three or four times longer a period than 

 when performed by the stomach itself. In the 

 stomach, in most individuals, rice and tripe are 

 digested in one hour; eggs, salmon, and venison 

 in one and a half hours; tapioca, liver, fish, in 

 two hours ; lamb, pork, and turkey in two and a 

 half hours ; beef, mutton, and fowl, three and a 

 half hours; and veal in four hours. There are, 

 however, considerable differences in various indi- 

 viduals, or even in the same individual at different 

 times. 



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