Digestion
2 of the 7 encyclopedias on this shelf carry an entry for Digestion. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
the change which food undergoes in order to prepare it for the nutrition of the animal frame. It is carried on in the higher animals in the digestive system. In some of the lowest forms of animal life (amœbæ) which have no special organs, particles of food are drawn into the body and digested. In higher organisms there is a simple pouch which leads inward from the center of the cluster of tentacles; into this fish and other food are drawn and digested, while the undigested parts are afterward voided through the same aperture by which they entered. In still higher organisms, man himself included, this simple pouch is changed into a complex and greatly elongated tube, which is provided with one ( the mouth) by which food , and another aperture ( the anus) which undigested matter leaves body. The mouth in most animals is a C d b i j e k HUMAN ALIMENTARY CANAL g. Pancreatic duct a. Esophagus h. Ascending colon b. Stomach i. Transverse colon c. Cardiac orifice d. Pylorus j. Descending colon k. Rectum e. Small intestine f. Biliary duct provided with hard tissues-teeth, beaks for the subdivision of food before it is swallowed. Vegetable feeders, eating tough grains, roots and fibers, have large molar or grinding teeth, while the carnivora have these same teeth modified so as to present a cutting edge, with which and their pointed canines meat is torn and cut into pieces, which are then swallowed. Below, the cavity of the mouth passes into the gullet or œsophagus, and in front of this tube runs the windpipe. Food will pass through the pharynx, or the interior of the throat, into the gullet; and air, during respiration, passes through the pharynx on into the larynx larynx and windpipe; a valve called the epiglottis partly closes the aperture of the larynx. The gullet or œsophagus is a long tube passing from the pharynx to the stomach. Its mucous coat is loaded with very large glands which secrete a quantity of very viscid mucus. The stomach itself is a greatly dilated part of the digestive system. It may be said to consist of two parts, even in the human subject; a more complex arrangement is found in many animals, such as the ruminants. The large dilated portion into which the gullet opens is termed cardiac, and the opening the cardiac or œsophalageal opening. The w! ole is lined with mucous membrane, which, in the empty stomach, is thrown into projecting folds or rugæ, but these folds are effaced when the organ is distended with food. In the membrane are innumerable glands which secrete the digestive juices of the stomach. The gastric juice is acid, and the chief acid secreted is hydrochloric acid. The substance called pepsin which is necessary for digestion, is secreted by the whole of the glands. The food now called the chyme passes into the small intestine, a tube about 20 feet long. This tube, besides the muscular and mucous coats, possesses an external coat of loose fibrous tissue, covered by a single layer of flat cells. This coat is prolonged into, and helps to form the mesentery, a membrane connecting the intestine with the abdominal walls. This membrane is called the peritoneum. The small intestine is somewhat arbitrarily divided into three portions-the upper (duodenum), the middle (jejunum), and the lower (ileum). The mucous coat contains glands very like the pyloric glands of the stomach, called Lieberkühn's follicles. They secrete the intestinal juice. In the duodenum one finds in addition highly branded glands called Brunner's. In both the mucous and submucous coats, and generally involving both layers, are found masses of tissue-lymphoid-similar to that found in a lymphatic gland. Their function is probably connected with the blood and the blood corpuscles. Collections of these solitary glands, forming oblong patches about two inches long, are called Peyer's patches. In addition to the follicles of Lieberkühn and the glands of Brunner, there are two very important glandular structures, the liver and the pancreas, which pour their digestive juices into the small intestine. The bile, which is the secretion of the liver, is formed continually by that organ, but the amount thus formed is influenced by the kind and quantity of food taken. The bile is to be looked upon not only as a digestive juice, but as a drain or channel of excretion, whereby effete and useless matter is removed from the body. The pancreas is very similar in structure to a salivary gland. land. It secretes the pancreatic juice which pours with the bile into the digestive system. The mucous membrane, of the small intestine contains, in addition to the structures already mentioned, little projections called villi. These are important absorbents. This property they share with the whole of the digestive system through any part of which, and especially through the walls of the stomach and small intestine, digested matter passes into the numerous blood-capillaries which form everywhere a dense network. The villi are peculiar, for each one contains in addition to blood-vessels a small lymphvessel or lacteal. Nearly all the fat absorbed by the digestive system is taken up by the little cells of the villi, and passes on into the lacteals and thence to the blood. 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 carnivora (dog, cat, etc.), and they are not provided ed 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 digestion 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 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 addition there is a smaller part, the reticulum (honeycomb) so called from the folds of lining mucous membrane which intersects, forming a reticulum. The pyloric half is divided into two parts. The psalterium (maniplies), so called from the lan lamellated appearance of mucous membrane, communicates with the last vision, themeunicat 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 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, it 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 returned in a pulpy condition 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 foodstuffs 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 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. The digestive ferments are not whole cells, they are the products of cells. When food is passed into the stomach, This too may result secretion occurs. from mechanical irritation, as when through an opening (fistula) the mucous membrane is brushed with a feather. In all cases the stomach, pale before, becomes suffused with blood, and the gastric juice is poured out. The flow of the intestinal juice, the pancreatic juice, and bile all follow the stimulation of the mucous 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 result of the influence of special secreting Foods belong to four classes: nerves. (1) Proteids- albumens, globulins, etc.; e. g., the white of egg, 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 undergo some change. Grape-sugar is abcha Is too are ofsorbed and probably proteids ten absorbed to some extent at least. Fat is profoundly modified during digestion, though not as the result of any digesting ferment. The saliva, of which about 30 ounces are secreted during the 24 hours, contains a ferment termed ptyalin, 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 intestines by the pancreatic 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 juice, 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. Within the small intestines most of the food undigested by the stomach is rendered fit for absorption. This takes place through the issue of the mucous membrane; much of the sugar and peptones find their way into capillary bloodvessels. Absorbed products and notably fat globules, pass into the lacteals, and thence into the blood, circulating through the veins at the root of the neck.
Aiton's Encyclopedia (1910)
. See Alimentary Canal ; Food