The unabsorbed food, mixed with the various secretions we have mentioned, now passes into the large intestine, where both digestion and absorption go on, though to a less extent.
When food is taken into the mouth it is at once swallowed, unless it is in a solid form. In this case it is chewed into a convenient size for swallowing, for which purpose it is, in addition, mixed with the viscid saliva and juices of the mouth. Many animals can hardly be said to masticate; such are the car nivore (dog, cat, etc.), and they are not provided with grinding teeth.
As a result of mastication, the food is gathered in the form of a round moist bolus on the upper surface of the tongue. It is now ready to be swallowed. In the first place, it is pushed backward by the tongue and seized by muscles, many of which are attached to the hyoid bone.
There is a possibility that during swallowing the food may go the wrong way—i. e., it may pass into the larynx and windpipe. It is prevented from passing into it by the elevation of the larynx which pushes its aperture against and under the back of the tongue, which at the same time is pushed backward. In addition, there is a valve called the epiglottis, which is pushed down over the larynx by the movement just described 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 psalterium (maniplies), so called from the lamellated appearance of its mucous membrane, 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 ferments 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 egg, the chief constituent of meat, the gluten of bread.