Digestion

Digestion. The food which is taken into the mouth consists of substances of varied composition, which are classified as follows: Proteids, fats, sugars, starches, mineral salts, and water.

Proteids are substances containing the elements carbon, oxygen, nitrogen, hydrogen, with a little phosphorus and sulphur. Most proteid substances are non-diffusible, i.e. they will not pass through animal membranes. (The particular group of proteids called peptones are, however, an exception to this rule.) The myosin of muscle, the albumin of white of egg, the gluten of bread, and the casein of cheese are examples of proteid substances.

Fats contain the elements carbon, hydrogen, and oxygen, the two last-named elements not being in the same proportions in which they are found in the case of water (H₂O). Fats, chemically considered, are compounds of glycerine with a fatty acid; they do not readily pass through animal membranes, but are convertible, under certain conditions, into soluble soaps, which do so readily.

The sugars and starches are often grouped together, and called carbohydrates. A carbohydrate is a substance containing carbon, hydrogen, and oxygen, the hydrogen and oxygen being present in the same proportions in which they are found in water. The sugars are diffusible substances, the starches are non-diffusible.

The structure of the wall of the alimentary canal presents certain peculiarities in different situations, but, generally speaking, the outer tunic or tunics are composed of unstriped muscle, while internal to these is the mucous membrane, and between the two is what is called the submucous coat.

On the internal aspect of the mucous membrane there is always found (from the oesophagus to the anus) a single layer of columnar epithelial [Epithelium] cells. In the mucous membrane ramify innumerable capillaries, and diffusible substances readily pass from the digestive tract into these capillaries. Moreover, the mucous membrane is beset with glands, the secretions of which pass into the alimentary canal and mix with the food contained therein.

The bulk of the food undergoes absorption in the alimentary canal - that is to say, it finds its way into the capillaries which ramify in the mucous membrane. Only about one-tenth of the dry solid matter ultimately remains unabsorbed. The process of absorption is facilitated by the conversion of the non-diffusible constituents of the food into diffusible substances, this conversion being brought about by the various digestive juices secreted by the glands and poured from them into the alimentary canal, where they mix with and act upon the food.

Thus, by the action of these secretions, or digestive juices, non-diffusible substances in the food are converted into diffusible substances. These diffusible substances find their way into the network of capillary blood-vessels which exists in all parts of the mucous membrane, and so into the portal system of blood-vessels, whence they are conveyed by the portal vein to the liver, and so reach the general circulation. The absorption of nutrient materials from the alimentary canal is not entirely a question of diffusion through an animal membrane - that is to say, it is not purely a physical process. To this matter, however, it will be necessary to return.

It will now be convenient to consider the various parts of the alimentary canal, to allude very briefly to their structural peculiarities, and to discuss the several changes in the food brought about by the secretions proper to each.

In the mouth the food undergoes the process of mastication; that is to say, the portions of food are broken up by the teeth and mixed with the secretions of the glands opening into the mouth until a "bolus," or pulpy mass, is produced, which is passed backwards by the tongue into the grasp of the muscles of the pharynx, which contract upon it and force it downwards into the oesophagus.

The secretions of the mouth comprise the mucus yielded by the buccal glands and the several salivary secretions. The latter are produced by the salivary glands (q.v.), parotid, submaxillary, and sublingual, which are arranged in pairs, so that there are six salivary glands in all. These glands elaborate the saliva, and discharge it through their ducts into the mouth, where it is mixed with the food.

The saliva is a very watery secretion (994 parts of water in 1000 parts); it contains, however, a substance ptyalin, which possesses the property of converting starch into sugar. Only a small portion of the starchy elements of the food, however, are converted into sugar, in the mouth.

The bolus of food passes down the cesophagus (q.v.), or gullet, and reaches the stomach (q.v.).

In the stomach two kinds of glands are present. The one kind (cardiac or peptic glands) is the more numerous, and is found throughout the whole mucous membrane of the stomach, excepting the portion near the pylorus. The other kind (pyloric glands) occurs in the neighbourhood of the pyloric orifice of the stomach. The food remains for some time (ten minutes to two hours or more) in the stomach, and becomes thoroughly mixed with the secretion of the glands of the stomach, which is called gastric juice. The muscular walls of the stomach contract, and cause the contained food to be thrown into movement and to be broken up and dissolved in the gastric secretions. The pyloric orifice is closed by a band of muscular fibres which surround it - the sphincter of the pylorus, as it is called. From time to time this sphincter relaxes and allows the contents of the stomach to escape into the small intestine. The material which thus escapes is of the consistence of thin gruel, and is called chyme. The gastric juice is mainly derived from the cardiac or peptic glands. It is acid in reaction (the acidity being due to free hydrochloric acid), and contains a substance called pepsin. Pepsin possesses the property of converting non-diffusible proteids into diffusible peptones. The peptone formed partly diffuses through the mucous membrane of the stomach and reaches the capillary network of the gastric mucous membrane.

Thus of the different kinds of food stuffs the proteids are those which are chiefly acted upon in the stomach. Starchy foods are unaffected. Sugars may be to a certain extent absorbed. Fats are broken up, the particles of fat being reduced to a fine state of subdivision, but they are not made diffusible. To the result of this process of breaking up of fatty substances the term emulsion is applied, and the fats are said to be emulsified.

At the commencement of the small intestine two important secretions are mingled with the food, namely, the bile and the pancreatic juice. The bile (q.v.) is alkaline, and the further processes of digestion which occur throughout the small intestine are carried on in an alkaline and not as in the stomach in an acid medium.

Moreover, the bile possesses the property of emulsifying fats.

The functions of the pancreatic juice are very important. In the first place it converts starches into sugars; secondly, it acts upon proteids, converting them into peptones (and more than this it decomposes further some of the peptone with the formation of bodies known as leucin and tyrosin); thirdly, it emulsifies and saponifies fats. The pancreatic juice thus acts upon the three kinds of non-diffusible food stuffs (viz. proteids, starches, and fats), converts them into diffusible material, and so facilitates their absorption.

The structure of the small intestine may be briefly alluded to; the mucous membrane is thrown into folds called valvulm conniventes; these folds secure the thorough admixture of the secretions with the liquid intestinal contents, and, moreover, offer a largely increased extent of mucous surface for secretion and absorption. The glands of the small intestine are of three kinds - Brunner's glands in the duodenum, the crypts of Lieberkuhn, which occur throughout both large and small intestines, and the lymphoid glands. Little is known concerning the secretion of the two former kinds of glands. The lymphoid structures either occur singly (glanelulce solitaries) or in patches (ylandulce agminatec). The latter are also called Peyer's patches; they are limited to the lower six feet of the small intestine.

A remarkable and characteristic element of structure in the small intestine is the villus. Each villus is a projection of mucous membrane J;, inch or thereabouts in length, containing capillaries, unstriped muscular fibres, adenoid tissue, and a central lymph channel or lacteal. The villi being closely set, give a velvety appearance to the surface of the intestinal mucous membrane as seen with the naked eye. The figure shows the appearance presented by a thin section through the small intestine of a dog when viewed under a magnifying power of some 60 diameters, ths projections being the villi. The richness of the blood supply to these villi is seen in the adjoining figure, which shows the appearance presented by a similar section through the small intestine of the mouse, in which the capillaries have been filled with injection material, in order to make their presence and arrangement obvious. The central lacteal (not shown in either figure) serves for the conveyance of fatty matter. Each central lacteal is, in fact, in direct communication by a system of lymph channels with the thoracic duct, and this duct receives the contents of the several lacteals, and passing upwards in front of the vertebral column through the abdomen and thorax, ultimately empties itself into the junction of two large veins in the neck. The material which thus finds its way from the villus through the lacteal into the thoracic duct, and which finally is discharged into the veins of the neck and so reaches the general circulation, is called chyle.

Chyle is a milky fluid which contains fat in a finely divided state. The exact way in which these fatty particles find their way into the lacteal is not known. The villus, like all the rest of the mucous membrane of the small intestine, is lined by columnar epithelium, and hence the particles of the chyle, in order to reach the central lymph vessel of the villus, must somehow pass this wall of epithelium.

From what has been said it will be seen that the elements of the food which pass into the blood and serve for the nutrition of the body pursue one of two courses. Either they are taken up directly by the capillaries of the portal system and conveyed by the portal vein to the liver, after passing through which they reach the general circulation; or they are taken up by the lacteals and conveyed by the thoracic duct into the venous system and so reach the general circulation. Diffusible peptones, sugars, and saponified fats pursue the former course, while the main constituents which enter the lacteals are, as already stated, fats which have been reduced to a fine state of subdivision.

The intestinal contents during their passage through the large intestine do not undergo changes of importance comparable to those which occur in the small intestine. In fact the main alteration is the acquirement of a semi-solid consistency.

All the assimilable substances having been taken up by the processes of absorption, the waste or non-utilisable elements of the food leave the body as fasces.