This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
The above cut represents the whole tract of the intestinal canal, not exactly in its natural position, but spread out so as to show the relative proportions ; f, the aesophagus; g, cardia; h, pylorus; i, i, the duodenum, about twelve fingers' breadth in length; k, hepatic duct; I, gall bladder ; m, cystic duct; n, ductus communis, formed by the union of both; o, the opening of this duct into the duodenum; p, pancreatic duct; q, its opening into the duodenum; r, jejunum; s, ilium; these constitute the small intestines, and are about twenty six feet in length, or five times the length of the body; t, termination of ilium in the coecum; u, superior fold of valve of colon ; v, inferior do.; to, coecum ; x, vermiform process; y, y, colon; z, rectum. The coecum, colon, and rectum, form the large intestines, and are about six feet in length ; the coecum being about 4 inches long, and the same in diameter. The arrows show the direction which the food takes in digestion.
12. Hunger and thirst are sensations designed to teach us the necessity of supplying those losses which the system is constantly undergoing by the different secretions and excretions, amounting to several pounds in the course of twenty four hours. The blood first feels the loss, and then the solids, whose particles are continually taken up by the absorbents, and carried into the blood, and thus ejected from the system; and were not these losses supplied by the timely introduction of food, the body would rapidly emaciate, till death closed the scene. Hunger is supposed to be owing to a peculiar affection of the nerves of the stomach; for when the nerve which goes from the brain to the stomach (the par vagum,) is divided, the sensation of hunger is lost, or at least the appetite for food is destroyed.
13. While the food is undergoing the process of mastication, that is, of being divided and ground down by the teeth, it is thoroughly mixed with a quantity of saliva, amounting it is supposed to between eight and ten ounces. The food is thus brought into a condition to be easily swallowed, and readily dissolved by the action of the stomach. It is very important, therefore, that the food should be slowly chewed, and reduced to as fine a state as possible in the mouth, in order that digestion may be easy. Too great rapidity in eating, probably lays the foundation of many cases of indigestion.
14. After the food has been sufficiently masticated, it is carried down the aesophagus into the stomach, first by the contraction of the muscles of the pharynx, and then by a successive contraction of the circular fibres of the gullet itself, from above downwards. That the morsel is not carried down merely by its own weight, is proved by the fact, that a man can swallow with his head downwards.
15. In the stomach, the food is converted into a soft, grey, pulpy mass, called chyme. This process has been called chymosis. It is produced by the motions of the stomach, together with the agency of the secretions, which are thrown out by the gastric vessels. These motions have been called vermicular or worm like; and undulatory, or like wave succeeding wave. The crawling of a worm furnishes a very good illustration of the successive contractions of the muscular fibres of the stomach, commencing, as they do, at the seso phagus, and proceeding onwards to the pylorus, and so back again. These motions of course bring every portion of the contents of the stomach in contact with all parts of its surface ; and so they become intimately mixed with the gastric fluid.
16. The food is thus carried round the interior of the stomach, from one extremity to the other ; and from one to three minutes are employed in each revolution. In the mean time, both orifices are closed; so that the contents cannot escape. During this process, the gastric fluid is secreted in large quantities, and becomes mixed with the food as it passes round. In a short time, the taste, smell, and other sensible properties of the food, are entirely changed. This is produced by the agency of the gastric juice, which subverts the chemical affinities of the food ; and with its elements forms new combinations.
17. The gastric fluid is a clear, transparent fluid, produced by arterial exhalation ; acid to the taste, slightly saline, and free from odour. It possesses the singular property of coagulating albumen; resists the putrefaction of animal matter ; and dissolves nearly every kind of alimentary substance. Its acid properties are owing to the muriatic and acetic acids which it contains. The gastric fluid is found to contain more acid, in proportion as the food is more difficult of digestion. It is only secreted, when food is present in the stomach.
18. The aliment then is converted into chyme, chiefly by the gastric fluid; aided, however, by the motions of the stomach. This is proved by the fact, that this fluid will dissolve alimentary substances out of the stomach: the chyme which is prepared by artificial digestion, presenting the same sensible properties as that which is found in the stomach. Such is the power of the gastric fluid, that it will dissolve bones, not only in the stomach, but out of it. It curdles milk; and for this purpose the rennet, or stomach of the calf, is used by farmers in making cheese. But every thing which is coagulated in the stomach, is dissolved again, in its conversion into chyme.
19. But digestion is a vital and not a chemical process. Though aliment may be reduced to a substance resembling chyme, by the action of gastric fluid out of the body; yet it is destitute of all those peculiar properties which assimilate it to the nature of a living animal. The food may doubtless be brought to a fluid state by a chemical process, and even alimentary principles may in the same way be changed into each other, as starch into sugar and gum; but there is still another power, which may be called vitalizing or organization, by which alimentary substances are brought into such a condition as adapts them for an intimate union with the living body. Such a power is beyond the reach of chemical action.