Vomiting Eructations.— Eructations are frequently caused by anti-peristalsis and by a movement of this kind, food is brought back into the mouth for further in the ruminants (sheep, oxen, etc.). The act of vomiting is a reflex nervous act. It can be excited by stimulating the branches of the vagus nerve, as when indigestible and irri tative food is taken, or emetics are adminis tered.' By tickling the back of the throat with a feather, the glossopharyngeal nerve is stimu lated and vomiting may readily be produced. It is of frequent occurrence in irritations of the uterine nerves in pregnancy, of the nerves of the liver biliary apparatus and kidneys during the passage of a hepatic or renal stone, or in deed when irritation of any sensory nerves takes place. Vomiting may be caused by chemic sub stances acting directly on the vomiting centre in the medulla. Apomorphin acts in this man ner. Irritating substances acting on the afferent nerves in the stomach can cause vomiting. Mustard acts in this manner.
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 subdivided 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 addi tion there is a smaller part, the reticulum (honeycomb), so called from the folds of lin ing mucous membrane which reduplicate, form ing a reticulum. The pyloric half is divided into two parts. The psalterium (manyplies), 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, third part of the stomach, and thence on into the fourth. Solid matter, such as grass, roots, etc., passes either into the paunch or reticulum. 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, lies down and rumination commences. Due in part to the contraction of the abdominal muscles 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 masticated, and are re-, turned in a pulpy condition to the stomach. Now the food passes into the psalterium, and into the rennet stomach. Hence the consistency of the food determines into which part of the stomach it passes.
In the bird some interesting modifications in the structure of the alimentary canal are seen. The gullet at about the middle of its course is provided with a pouch or crop. Into this the
food passes, and is bathed by a secretion formed by its glands. It is then propelled onm ward into a dilated cavity, the proventriculus,' and is acted on by digestive juices. Thence it passes into the gizzard. This cavity is provided with muscular walls of enormous thickness in the case of birds that are vegetable It is lined by thick and corneous epithelium, and in its interior are generally found pieces of stone, chalk, etc. The gizzard is a powerful mill, which grinds the food into a soft pulp, upon 'which digestive juices can readily See Brans. • The most essential change which food under goes in digestion is one of solution. This is effected by the chemic breaking down the large molecule of proteid carbohydrate and fat into smaller molecules. Some physiologic chemists claim that the complex food molecule is broken down to its very elements and completely.
The protein fat and carbohydrate of the food, if they were injected as such into the circulation, would not serve as food, but would be excreted as foreign matter — the proteid and carbohydrate by the kidneys, the fat by the intestines. Albumen, starch, fat, and other foodstuffs are insoluble in the circulating fluids. 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. It is not sufficient, however, to have a soluble foodstuff in order that it may be absorbed and used by the body. Cane (table) sugar is solu ble in water, but it is of no use as cane-sugar to the body. If injected into the blood vessels it is at once excreted by the kidneys. During digestion it is converted into glucose, but in a form which can be used by the economy.
The digestion of food is brought about by the action of the saliva, the gastric, 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 microscopic cells which line the various glands opening into the digestive system. All digestive juices exert their efforts on the foodstuffs by means of chemic agents known as enzymes or ferments. These are not formed as active enzymes, but always as inactive pro enzymes, incapable of showing their specific digestive effect until they are made active by an activator. For in stance pepsin is not secreted as such, but as pepsinogen. The pepsinogen is activated into pepsin by the hydrochloric acid (HC1) of the gastric juice.