Page:The American Cyclopædia (1879) Volume I.djvu/341

ALIMENTARY CANAL 317 appearance, owing to a multitude of little cavi- ties distributed through its substance. These are the cavities originally produced by the bubbles of carbonic acid developed in fermen- tation, and which retain their figure in con- sequence of the stiffening and coagulation of the gluten by the baking process. This is one of the main objects of fermentation; for the spongy texture which the bread thus receives enables it to be more easily masticated and mingled with the saliva and gastric fluids, and thus renders it more healthy and digestible. Cheese is made by coagulating the caseine of milk with rennet, after which the coagulum is compressed, in 'order to free it from the watery, oleaginous, and saline ingredients of the milk ; and when reduced to a sufficiently solid condition, it may be kept for an indefinite time. In many kinds of cheese, however, more or less of the oily ingredients of the milk are retained entangled with the caseine, by which it acquires a richer and stronger flavor. Butter, on the other hand, is simply the oleaginous portion of the milk, separated from the remain- ing constituents. In the natural condition of the milk the butter is in the form of microscopic globules, or spherical masses, of a semi-solid consistency, suspended in a state of minute subdivision in the serous liquid. By the opera- tion of churning, these little globules are made to cohere mechanically together, and gradually the whole of the oleaginous substance is sepa- rated in a distinct pasty mass. It is still fur- ther freed from the accompanying ingredients of the milk by pressure and kneading under water, and is finally obtained as butter in a nearly pure condition. The effect of cooking upon food is twofold. In the first place, it softens and disintegrates the substances which are naturally too hard for digestion, and thus renders them amenable to the digestive opera- tions. This is the effect produced upon many vegetable substances, such as starch grains wherever they may be found, and all substances having a resisting envelope or a tough and solid texture, such as peas, beans, potatoes, turnips, and the like. In animal substances, on the other hand, the most useful effect of cooking appears to be the partial transformation of the albuminoid matters, as in roast meat, by which they acquire a peculiar and agreeable . flavor. There is reason to believe that this flavor, be- sides being pleasant to the palate, is also the indication of a chemical change in the albumi- noid matters, by which they are prepared for digestion and become better fitted to subserve the nutrition of the body. ALIMENTARY CANAL, a tubular passage, ex- isting in man and all the higher animals, com- posed principally of a muscular layer and a mu- cous membrane, extending from the mouth to the anus, and designed for the reception, trans- mission, and digestion of the food or aliment. The cavity of the alimentary canal is continu- ous, anatomically, from its commencement to its termination, forming a hollow passage through which the food is carried in the di- gestive process. Its different parts are, how- ever, partly separated from each other at various points by constrictions and muscular bands, which are alternately closed and opened, to allow, of the temporary retention or onward movement of the alimentary materials. The different portions of the canal are also distin- guished from each other by varieties of form and size, the development of their muscular layers, and the structure of their mucous or lining membrane. Owing to this variety of structure, and the different characters of the se- cretions produced, the action of the alimentary canal upon the food varies in its different parts ; and the process of digestion to which the food is subjected consists of the successive or com- bined operation of the whole. The principal portions into which the canal is thus divided, in the human subject, are known as the mouth, the oesophagus, the stomach, the small intes- tine, and the large intestine. The mouth is the cavity included between the opening of the lips in front and the fauces behind. In it are the teeth, intended for the mastication and com- minution of the food ; the tongue, a muscular and sensitive organ, which subserves both the sense of taste and the proper movement and ad- mixture of the food in mastication ; and a lining membrane which contains mucous glandules destined to supply a viscid secretion form- ing part of the saliva. There are also the pa- rotid, submaxillary, and sublingual glands, sit- uated in the immediate vicinity of the mouth, which pour their secretions into its cavity, and . thus complete the formation and supply of sa- liva, which is mingled with the food in mastica- tion and reduces it to the condition of a soft, pasty mass. Immediately behind the fauces is the pharynx, a short funnel-shaped passage lead- ing directly to the oesophagus. The latter is a nearly straight tube of uniform size, about nine inches long and rather less than one inch in di- ameter. It passes through the neck and pos- terior region of the chest to the upper part of the abdomen, where it terminates in the stom- ach. It has a double layer of transverse and longitudinal muscular fibres, by whose peristal- tic or wave-like contractions the masticated food is rapidly carried from above downward. Its lining membrane is of a simple structure, and produces only a small quantity of mucus, destined by its lubricating qualities to facilitate the passage of the food. The oesophagus, in fact, is simply an organ of transmission, by which the food is transferred from the mouth to the stom- ach, where the more important digestive actions are to begin. The stomach is. a dilatation of the alimentary canal, lying transversely across the upper part of the abdomen. Toward the left side it expands into a wide hemispherical sac or pouch ; toward the right side it becomes narrowed to a smaller diameter, where it united with the upper extremity of the intestine. The orifice by which the stomach communicates with the oesophagus is called the cardia (Gr.