Page:Collier's New Encyclopedia v. 03.djvu/417

DIGESTION 363 DIGESTION and by muscular fibers, which act upon it for that especial purpose. The walls of the stomach and intestines are, like the gullet, provided with muscular fiber. An external layer passes in the length of the gut, and within this is a circular layer. These muscles contract slowly on stimulation, and are outside the domain of voluntary action. During the diges- tion they contract peristaltically, urging the food toward the rectum. In many animals, such as the sheep, ox, and camel, the stomach consists of several cavities communicating with one another. In the ox and sheep the cardiac and the pyloric portions are each sub- divided into two compartments. The cardiac part consists of a very dilated cavity, the paunch (rumen), into which the food is passed as soon as swallowed. In addition there is a smaller part, the reticulum (honeycomb) so called from the folds of lining mucous membrane which intersects, forming a reticulum. Thepyloric half is divided into two parts. The psaltcrium (maniplies), so called from the lamellated appearance of its mucous membi'ane, communicates with the last division, the rennet stomach (abomasum). Fluid passes either into the first, second, or third parts of the stomach, and thence on into the fourth. Solid matter, such as grass, roots, etc., passes either into the paunch or retic- ulum. This is mixed with the saliva swallowed with it, and in addition it is mixed with juices formed by the mucous membrane of these cavities. When the animal has finished feeding, it lies down and rumination commences. Due in part to the contraction of the abdominal mus- cles and diaphragm, the food is propelled in the form of rounded pellets from the paunch and reticulum up into the mouth. The pellets are there thoroughly masti- cated, and are returned in a pulpy con- dition to the stomach. Now, however, the food passes into the psalterium, and into the rennet stomach. The most essential change which food undergoes in digestion is one of solution. Albumen, starch, fat, and other food- stuffs are insoluble in the circulating fluids of the body, and injected into the blood they would block up the smaller blood-vessels. During digestion these pass either into nearly allied chemical substances which are readily dissolved in water, or in the case of fat partly into a soluble soap and partly into a state of microscopically minute subdivision. The digestion of food is brought about by the action of the saliva, the gas- tric, pancreatic, intestinal and other juices. These are mixed with the finely divided food by the movements of the alimentary canal. The digestive juices are in all cases secreted by the micro- scopic cells which line the various glands opening into the digestive system. The digestive fennents are not whole cells, they are the products of cells. When food is passed into the stomach, secretion occurs. This too may result from mechanical irritation, as when through an opening (fistula) the mucous membrane is brushed with a feather. In all cases the stomach, pale before, be- comes suffused with blood, and the gas- tric juice is poured out. The flow of the intestinal juice, the pancreatic juice, and bile all follow the stimulation of the mu- cous membrane, and in all cases the blood-vessels enlarge so as to give the cells a good supply of food, though, as we have seen, they themselves actually pass into a condition of activity as a re- sult of the influence of special secreting nerves. Foods belong to four classes: (1) Proteids — albumens, globulins, etc.; e. g., the white of eg^, the chief constituent of meat, the gluten of bread. (2) Carbohydrates — starches, sugars, gums; e. g., potato-starch, cane or grape sugar. (3) Fats and oils; e. g., suet, marrow, olive oil. (4) Minerals; e. g., water, table salt, iron, phosphates. Some few substances are absorbed without being digested at all; they do not need to be. Such are water and the minerals, though even many of these un- dergo some change. Grape-sugar is ab- sorbed and probably proteids too are of- ten absorbed to some extent at least. Fat is profoundly modified during digestion, though not as the result of any digest- ing ferment. The saliva, of which about 30 ounces are secreted during the 24 hours, contains a ferment termed ptya- lin, which is capable of turning a starch into a soluble sugar called grape-sugar, or, according to other observers, into another soluble sugar termed maltose. When the food has reached the stomach and the acid gastric juice has mixed with it, the saliva is unable to act and is probably killed. Any digested starch is subsequently converted into sugar when the food reaches the small intes- tines by the panci*eatic juice. When the food reaches the stomach it causes a reflex secretion of gastric juice. This is but slowly produced when insipid heavy food, such as coagulated white of egg, boiled meat, sago, etc., is eaten, but flows readily when soups, broths, and fluids containing salts and extractions in abundance are taken. The gastric juic- . several pounds of which are secreted daily, is acid in reaction containing free hydrochloric acid. In addition lactic and butyric acids are formed during the progress of digestion.