After alcohol come the active principles of tea, coffee, and similar drinks: theine, cafeine, theobromine, coumarine (tonka bean), the principle of Peruvian coca.1 This latter substance appears to affect the muscular system especially, while the former have more influence on the nervous system. Messengers, travellers, and workmen have found that by chewing the leaves of the erythroxylum coca they could dispense with any solid or liquid food for one or two days: these leaves allay hunger and thirst, and sustain the strength. This is the reason that the Peruvians deified this tree, leaves of which were afterwards employed by the Incas for money. Ch. Gazeau, however, maintains that this so-called power of fasting is only anæsthesia of the stomach and sophagus, and that the person is autophagus, and in a state of inanition without being aware of it. But as hunger is a universal sensation of the system, it is scarcely possible to maintain this theory, in the face of the well-known instances of nutrition being kept up by cocoa as well as by alcohol. The action of these latter substances cannot be explained by referring it to the presence of nitrogen in their composition and regarding them as azotizing aliments, the plastic aliments of Liebig. Cafeine, theine, etc., contain a large quantity of nitrogen, but their composition closely resembles that of the uric acid, xanthine and hypoxanthine, all of which are excrementitious products or waste from the organism: it thus appears that theine, cafeine, etc., merely pass through the organism, and reappear in the excreta, and this has been proved by experiment. Liebig's extract of meat must also be classed among the economical aliments (aliments d'épargne), if, indeed, this product can be said to have any alimentary utility at all. This extract is now shown to be in no way nutritive. The nitrogenous crystallizable principles which it contains are no more nutritive than theine or cafeine, etc.; the only use of this extract is that of a slight stimulant from the salts which it contains (nearly one-fifth of its weight). In short, Hepp and Müller's experiments (Thèse de Paris, 1871) on animals, seem not only to show the uselessness of this extract as an article of food, but also to ascribe to it a poisonous effect, when taken in large

Ch. Marvaud, "Etude de Physiologie Thérapeutique, Effets Physiologiques, et Thérapeutiques des Aliments d'Epargne ou Antidéperditeurs. Alcool, Café, Thé, Coca, etc." Paris, 1871.

2 Ch. Gazeau, "Nouvelles Recherches Expérimentales sur la Pharmacologie, la Physiologie, et la Thérapeutique de la Coca." Thèse de doctorat, Paris, 1870.

quantities according to Kemmerich the exclusive use of the extract of meat would kill sooner than starvation.

In studying the different phases of the act of digestion, we will take, first, those which are observed in the sub-diaphragmatic part of the canal; next, those of the cavity of the stomach; and, finally, phenomena which take place in the passage through the intestinal tube (large and small intestine).

II. FIRST PART OF THE ACT OF DIGESTION.

THE aliments introduced into the cavity of the mouth are divided by the teeth (mastication), moistened and modified by the saliva (salivation), and then carried into the pharynx, seized by it, and pushed into the stomach by the œsophagus (deglutition).

A. Mastication.

The purpose of mastication is to divide the solid aliments so that they may be more easily attacked by the digestive fluids of the mouth and other parts of the intestinal canal. Meat and nitrogenous substances are more easily digested in the stomach after they have undergone mastication in the mouth, but the operation need not be carried very far in the case of aliments of this kind: thus we observe that the exclusively carnivorous animals have no teeth properly so called, but merely hooks, with which they tear their food into large pieces. Mastication is indispensable, on the contrary, in the case of aliments belonging to the vegetable kingdom; the greater number of nutritive vegetable matters are enclosed in a casing which generally resists the action of the digestive juices the masticating system serves to tear the cells, the envelope of seeds, etc.; prima digestio fit in ore, said the ancients in saying this, they spoke only of mastication, being ignorant of the chemical process which takes place during salivation.

The lower jaw, as it rises and falls, represents a lever, moving round a supposed axis, which, in movements of slight extent, is centred in the condyles; but when the mouth is wide open, the separation of the jaws is greater, and the condyles quit the glenoid cavities, and come further forward. The movement then takes place round an axis crossing the two upright branches of the inferior maxillary at the level of the dental foramen; however little the buccal cavity may be opened, and even in ordinary mastication, the two

movements are combined, as may be proved by placing the finger on the temporo-maxillary articulation: the rotation of the condyle in the cavity, and its forward projection take place at the same time; so that it is difficult and even impossible to decide exactly on a fixed axis around which all the movements of the jaw are made.

In all cases the lower jaw acts as a lever of which the fixed point is behind, in the upright branch of the bone; the point of application of the power, which is represented principally by the masseter and temporal muscles, is in the front edge of this upright branch; the resistance may be found in different points: if an aliment is to be divided, the resistance lies on the level of the incisors, and in this case the lever belongs to the third kind, and the arm of the power is very short in comparison with the arm of resistance (see p. 103, mechanism of the muscles). When the food requires to be ground, the resistance is applied at the level of the molars, and its lever arm becomes shorter, thus giving the advantage to the action of the power, the lever arm of which keeps its original length. Even in the case of a resistance opposed to these latter molars, the fibres of the masseter may be found anterior to the resistance; and the maxillary lever then becomes a lever of the second kind, that which is most favorable to the action of the power (interresisting lever, page 102).

There is also a side movement in the lower jaw, which is restricted in man, but of great extent in the ruminants. It is due to the contraction of the external pterygoid muscle which, by drawing one of the condyles forward, brings it out of the glenoid cavity, while the jaw pivots on the other condyle.

We see thus, that in man mastication is a compound action, resembling both that of the carnivora and the herbivora (ruminants), on account of the compound nature of his food: the carnivora, which only tear their prey, make no upward and downward or sideway movement; thus their condyle turns only on its transverse axis. In the ruminants the sideway inovements are very decided, and for this purpose the condyle is flat and movable in all directions. Another type of condyle is that of the rodents, the anteroposterior diameter of which is of great extent, a glenoid cavity being hollowed out in the same direction. In man, the form of the condyle is intermediate between all these, while the masticatory movements are more varied, and

are combined in a more complex manner than in any other animal.

Beside the action of the jaws in tearing, cutting, and crushing the food, there is also an action of the tongue, lips, and cheeks, which aid mastication by pushing the food between the teeth, and keeping it in place.

Mastication is a voluntary act, and yet it may be said to belong, in some respects, to the class of reflex actions: thus mastication becomes slow, difficult, and even impossible, when there is an insufficiency of saliva, or when the want of food is not felt. There must, then, be here as everywhere, a special peripheral impression, which, being reflected in the nervous centres (the bulb, in mastication), causes the phenomenon of reflex action. Mastication, like walking and many other movements which are, apparently, quite voluntary, is performed, in a great measure, and during most of the time, by means of the mechanism of reflex actions. (See page 45, Physiology of the nervous centres: bulb.)

B. Salivation.

The organs of salivation are not only the salivary glands properly so called, but the whole glandular system spread throughout the cavity of the mouth: such as the molar glands, or glands of the cheeks, the glands of the lips, those of the under surface of the tongue, those of the roof of the mouth, and those of the velum of the palate, which are improperly called mucous glands. All these glands are formed by masses of globules arranged in ramified tubes, opening, sometimes, singly to the outside, and, at others, uniting in a single excretory tube, Steno's duct (parotid), Wharton's duct (sub-maxillary). The saliva is a deliquium, produced by the fusion of the globules of these glands as they fall into decay.

The salivary juice is found to differ slightly in the different glands, but it has one general feature, that of being very watery, and, in this respect, differs greatly from the mucus; it is water, containing scarcely from one to two per cent of solid matter; its reaction is alkaline: when taken from a person in a fasting condition, it is sometimes found to be slightly acid, but this acidity is simply owing to decomposition of the food remaining between the teeth.

The saliva contains an organic nitrogenized (azotic) substance (discovered by Leuchs, 1831); it is not well-defined, but is a peculiar form of albuminous substance called ptya

line (Berzelius) or animal diastase (Mialhe), resembling closely the principle of sprouting barley. This substance has the property of changing starch into glucose. The parotid saliva alone has no power to change starch into sugar (in the horse, and in man); the case is the same with the sub-maxillary gland (the dog): the power of turning substances into sugar thus appears to belong to the complex product of the different salivary glands and of those glands, called mucous, which are so abundant in the buccal cavity. This property does not appear to belong exclusively to the saliva: it is found in nearly all animal substances; the mucous of the bladder, the blood, and the muscular flesh all have it though in a low degree.

The saccharizing property of the saliva is not equally prominent in all animals: man is one of the most favored in this respect, but less so than some of the herbivora, especially the guinea-pig; the saliva of the dog, so often made use of for experiments, is not well adapted for this purpose, possessing the property, as it does, in a much lower degree than many others. In man, this property is developed only with the first appearance of the teeth (Bidder). The ptyaline of the saliva can only be extracted by precipitating it by alcohol, and then redissolving it in water (general process of separation of the albuminoid ferments). In all salivary ptyaline are found peculiar elements of a globular form, called by some authors pyoid globules, and closely resembling the white globules. Leeuwenhoek had already discovered these globular elements, which exhibit decided phenomena of amoeboid movements, and are reproduced by means of fission; these inferior organisms may be compared to ferments, and have a more or less direct part in producing the chemical activity of the saliva; indeed we notice that the more abundant these organisms are, the greater is the saccharizing property of the saliva; thus, in salivation (ptyalism) produced by the use of mercury, Leeuwenhoek's corpuscles are extremely numerous, and the saliva has the property of changing starch into sugar in the highest degree (Rouget).

Ptyaline is a soluble ferment; it partakes of the nature of an albuminoid, but differs a little from other albuminoids in not being precipitated by a heat of 60° (C); this does not, however, imply that it is not destroyed by an increase of temperature (Frerichs, Cohnheim), but the temperature must be raised at least to the boiling point in order to effect