The term digestion is applied to all those processes by which the food substances are converted into a convenient form for absorption. Digestion is carried on in the alimentary canal. This, regarded simply, is a hollow muscular tube richly supplied with blood vessels, having its mucous surface bathed with juices yielded by glands, and lined by epithelium from which these glands are developed and which along a great part of its length is possessed of the special property of absorbing nutritive matter. The food is exposed to the digestive juices at particular regions in its passage along the muscular tube, and the chemical changes which it undergoes are brought about by the action of ferments contained in these juices. The structure of the alimentary canal as well as the ferments provided in the secretions differ in various animals according to the nature of the food. A diversity of structure is shown at the very commencement of the digestive tube by the teeth. Carnivorous animals have long canines for securing and tearing their food. The herbi-vora are provided with more molars, having a broad grinding surface suitable for the prolonged chewing which vegetable foods require to bring them to a semi-fluid consistence and to separate the fibre so that the digestive secretions may thoroughly permeate the mass. Animals feeding on insects have upper and lower incisors which accurately fit one another. In man the teeth are of the omnivorous type and are suited both for cutting and tearing and for grinding. The importance of the thorough mastication of the food is shown by the frequency with which indigestion follows when the food is eaten too rapidly or the teeth are lost.
The food, in the process of chewing, is brought into intimate relation with the saliva, produced from the parotid, sub-maxillary and sub-lingual glands and from numerous small glands in the mucous membrane of the mouth. The salivary glands are of especial historical interest, for their ducts were the first channels from pure secretory digestive glands to be discovered. That of the sub-maxillary gland was found by Wirsung in 1642, but its significance was first recognized by Wharton ten years later and described in a lecture before the Royal College of Physicians. The mixture of saliva from the different glands is an alkaline fluid containing 1/2-1 per cent of solids, consisting of mucin, a little protein and inorganic salts and a ferment ptyalin belonging to the class of amylases, which have the power of effecting the conversion of starch to sugar. The uses of saliva are mechanical and chemical. The solid food is mixed with it in mastication and made into a semi-fluid mass which can easily be swallowed. In the whale, whose food and pharynx do not need to be lubricated by a secretion, the salivary glands are rudimentary. The saliva, by keeping the mouth moist, is also a great aid to articulation.
The time which the food stays in the mouth is so short in man that the chemical action of the saliva cannot be at all complete, in spite of the fact that the action of ptyalin upon boiled starch is a rapid one. We shall see, however, that salivary digestion proceeds in the stomach for a considerable time after the food has been swallowed.
The secretion of saliva is brought about reflexly by food, and this before any of it is put into the mouth. The sight, the smell, and even the thought of food is sufficient to excite a flow, provided that the individual be hungry. Mechanical stimulation of the mucous membrane of the mouth has no effect, but food, and other substances with a strong taste, produce a copious secretion. Of the four primitive tastes, acids are the most effective, then salts, then bitters and then sugar. Pawlow has found in the dog that the nature of the saliva differs with that of the food - meat, for instance, calls forth a juice which is rich in mucin; the dog does not thoroughly masticate meat but simply lubricates it with this mucinous fluid and swallows it. If some dry material be put into the mouth, the saliva produced is watery and contains but little mucin. These differences are due to the fact that the different salivary glands produce different kinds of secretion, that from the sub-maxillary gland, containing much mucin, being poured out when meat is taken into the mouth, whilst when a plentiful supply of fluid is required to dilute some dry material, the watery secretion of the parotid gland is provided. Malloizel reports that the psychical flow of juice which follows the sight of food also varies with the nature of the food, a secretion rich in mucin being poured into the mouth of a hungry dog at the sight of meat.
The act of deglutition is begun by an ordered contraction of the muscles of the tongue, the soft palate and the pharynx, which surround the mass of food or of fluid and force it into the oesophagus. No other route is open, for the naso-pharynx and the larynx are both shut off by special muscular arrangements. The food is then grasped by the muscular wall of the gullet and passed down into the stomach. In the case of fluid the expulsion into the cardiac end of the stomach can be heard with the stethoscope over the lower dorsal spine, or the epigastrium, and by this means it was ascertained that the course down the oesophagus occupies about 6 seconds. This has been confirmed by watching the passage of a bismuth mixture with the X-rays, the average time between the commencement of swallowing and the expulsion into the stomach having been found by Hertz to be between 5 and 6 seconds, though there is considerable variation in this time both between different individuals, and in the same person. The transit is much quicker in the upper part of the oesophagus, the muscle of which is striated, and slower below where it is unstriated, about half the time being taken up in going through the cardiac orifice. Hertz found that a solid dry mass, such as a cachet of bismuth carbonate, was passed down very slowly indeed, unless well lubricated with saliva, and on one occasion took as long as 15 minutes to reach the cardia, the subject of the experiment being, however, under the impression that it had been successfully swallowed, and quite unconscious of its presence in the oesophagus. Such a delay does not take place with ordinary masticated food.