FIG 44.

there are some characters, supposed to be commonly associated with its pathogenic properties, which can be easily recognized, and should be familiar to a student of Physiology. The want of brilliant limpidity must be regarded with suspicion. Any kind of smell, disagreeable or not, indicates impurity. The reduction (loss of color) of permanganate of potash, when added in small quantity to acidified water, indicates the probable presence of organic matter. A high percentage of chlorides is often associated with sewage contamination.

Salts.-Great varieties of salts are taken into the system, of which chloride of sodium forms the largest proportion. These have,

Section of Pea, showing starch and
aleurone granules imbedded in the
protoplasm of the cells. (After Sachs.)
a. Aleurone granules.
st. Starch granules.
i. Intercellular spaces.

no doubt, very important functions to perform, in entering into combination with the various tissues, and also, probably, in aiding the chemical changes of parts of which they do not form a normal constituent. They help to render certain substances soluble, and stimulate the cells of certain glands to more active secretion, e. g., the kidney excretes more urea when there is an abundant supply of common salt in the food.

CHAPTER VI.

THE MECHANISM OF DIGESTION.

The acts of digestion may be divided into mechanical and chemical processes. Under the mechanical processes come the

mixture of the food.

may be said to be the

arrangements for the subdivision, onward movement and general The chief objects of the chemical changes change from the insoluble to the soluble form of certain kinds of food stuffs (starch and proteids) and the finer subdivision of others, such as fats, which do not dissolve in the intestinal secretions or in the juices of the body.

Attention has already been called to the fact that there are different kinds of contracting textures, and that they are capable of different kinds of motion, some slow and steady, some rhythmical, some sharp, short and sudden. It must also be remembered that the more energetic and sudden the motions are, the more marked becomes the differentiation of the tissue. Thus the active, quick-contracting skeletal muscles and the rhythmically acting heart are made up of tissue which is very distinct in structure and in mode of action from that of the contracting cells composed of ordinary protoplasm, while in the slowly moving internal organs we meet tissue elements which in different animals show many stages of gradation between simple, undifferentiated protoplasm and the special striated muscle tissue.

It is necessary that in the first stages of alimentation the motions should be quick and energetic; so the mouth, pharynx and upper part of the oesophagus are supplied with striated muscle tissue, which differs in function and structure from that of the rest of the alimentary canal. In the stomach and intestines slower and more gradual kinds of motion are required, and here we find a good example of non-striated muscle tissue.

Around the extremity of the rectum is a band of smooth muscle, which remains in a condition of continuous or tonic contraction.

For further details concerning the muscle tissue the student must turn to the Chapter (XXIV) on that subject. Here, how

ever, it may not be out of place to describe briefly the special character of the muscles found in the wall of the digestive tube and their general arrangement.

MASTICATION.-In man, the introduction of food into the mouth is generally accomplished by artificial means, so that the biting teeth (incisors) and the tearing teeth (canines) (Fig. 46) are comparatively little used for obtaining a suitable morsel of food. In the mouth, the essential act of chewing or mastication is accomplished by means of the motions of the lower jaw, the tongue and the cheeks. This process of breaking up the

solid parts of the food ought to be continued until all hard substances are ground into a soft pulp.

Structure of the Teeth.The exposed part of the teeth is covered by a dense substance of flinty hardness called enamel, which is developed from the epithelium, and consists of hexagonal prisms set on end, which are really modified epithelial cells, but only contain about 2 per cent. of animal matter (Fig. 47). The bulk of the tooth is made up of dentine, a substance like bone in composition, pierced by numerous fine canals dentine tubules

which radiate toward the surface, from the pulp cavity,

in the centre of the tooth. Filaments of protoplasm run in the dentine tubules from the tooth cells, which line the pulp cavity and preside over the nutrition of the tooth. The cavity contains vessels, nerves, etc., which enter at the root of the tooth, which

is enclosed in a kind of modified bone tissue called crusta petrosa.

The two rows of grinding teeth, composed of molars and premolars, of the lower jaw are made to rub against the corresponding teeth in the fixed upper jaw by the combined vertical and horizontal movements induced by the action of the powerful muscles of mastication, the temporal muscles, together with the masseters and internal pterygoids, all tending by their contraction to elevate the lower jaw and bring the teeth forcibly together. This action is opposed by the digastric, the genio- and mylo-hyoid muscles, which by their combined force depress the jaw and separate the teeth. The horizontal movements are in the main accomplished by the external pterygoid muscles, which, acting together, pull the lower jaw forward so as to make the lower teeth protrude beyond the upper. In this action they are opposed by the digastric and hyoid muscles. One external pterygoid on either side acting alone, advances that side of the lower jaw only, and thereby causes the lower teeth to incline toward the opposite side in a lateral direction. The two muscles acting alternately cause a horizontal motion from side to side. Thus, while the lower teeth are pressed firmly against the upper ones, they are at the same time made to glide over them, either from side to side or backward and forward. By these movements the bruised food is soon pushed from between the teeth, and passes toward either the tongue or cheek. The morsel is soon replaced between the teeth by the action of the tongue on the one hand and the buccinator muscle in the cheek on the other.

While the process of mastication is going on, the food becomes thoroughly moistened with the fluid secreted within the mouth.

DEGLUTITION.—The next step is swallowing. When the food is sufficiently triturated and moistened, it is collected together by means of the tongue and placed upon.the upper surface of that organ, which becomes concave and presses or rolls the soft pulp against the hard palate so as to shape it into an oblong mass or