necessary in a physiological point of view; and shall next detail what has been called the mechanism of the function, or the mode in which it is effected. In many cases, it will happen, that some external agent is concerned,-as light in vision; sound in audition; odours in olfaction; tastes in gustation. The properties of these agents will, in all instances, be detailed in a brief manner.

The difficulty of observing actions, that are carried on by the very molecules of which the organs are composed, has given rise to many hypothetical speculations, some of which are sufficiently ingenious; others too fanciful to be indulged for a moment; and, as might be expected, the number of these fantasies generally bears a direct proportion to the difficulty and obscurity of the subject. It will not be proper to pass over the most prominent of these, but they will not be dwelt upon; whilst the results of direct observation and experiment will be fully detailed; and where differences exist amongst observers, such differences will be reconciled, where practicable.

The functions, executed by different organs of the body, can be deduced by direct observation; although the minute and molecular action, by which they are accomplished in the very tissue of the organ, may not admit of detection. We see blood proceeding to the liver, and the vessels that convey it ramifying in the texture of that viscus, and becoming so minute as to escape detection even when the eye is aided by a powerful microscope. We find, again, other canals in the organ becoming perceptible, gradually augmenting in size, and ultimately terminating in a larger duct, which opens into the small intestine. If we examine each of these orders of vessels in its most minute appreciable ramifications, we discover, in the one, always blood; and, in the other, always a very different fluid-bile. We are hence led to the conclusion, that in the intimate tissue of the liver, and in some part communicating directly or indirectly with both these orders of vessels, bile is separated from the blood; or that the liver is the organ of the biliary secretion. On the other hand, functions exist, which cannot be so demonstratively referred to a special organ. We have every reason for believing that the brain is the exclusive organ of the mental and moral manifestations; but, as few opportunities occur for seeing it in action; and as the operation is too molecular to admit of direct. observation when we do see it, we are compelled to connect the organ and function by a process of reasoning only; yet, we shall find, that the results at which we arrive in this manner are often by no means the least satisfactory.

The forces which preside over the various functions are either general-that is, physical or chemical; or special-that is, organic or vital. Some of the organs afford us examples of purely physical instruments. We have in the eye, an eye-glass of admirable construction; in the organ of voice, an instrument of music; in the ear, one of acoustics: the circulation is carried on through an ingenious hydraulic apparatus: and station and progression involve various laws of mechanics. In many of the functions, again, we have examples of chemical agency, whilst all in which innervation is concerned are incapable of being explained on any physical or chemical principle; and we are constrained to esteem them vital.

BOOK I.

NUTRITIVE FUNCTIONS.

THE human body, from the moment of its formation to the cessation of existence, is undergoing constant decay and renovation-decomposition and composition:-so that at no two periods can it be said to have exactly the same constituents. The class of functions about to engage attention embraces those that are concerned in effecting such changes. They are seven in number;-digestion, by which the food, received into the stomach, undergoes, in that organ and in the intestines, such conversion as fits it for the separation of its nutritious and excrementitious portions; absorption, by which this nutritious portion, as well as other matters, is conveyed into the mass of blood;1 respiration, by which the products of absorption and venous blood are converted into arterial blood; circulation, by which the vital fluid is distributed to every part of the system; nutrition, by which the intimate changes of composition and decomposition are accomplished; calorification, by which the system is enabled to resist the effects of greatly elevated or depressed atmospheric temperature, and to exist in the burning regions within the tropics, or amidst the arctic snows; and secretion, by which various fluids and solids are separated from the blood;-some to serve useful purposes in the animal economy; others to be rejected from the body.

CHAPTER I.

OF DIGESTION.

THE food, necessary for animal nutrition, is rarely found in such a condition as to be adapted for absorption. It has, therefore, to be subjected to various actions in the digestive organs; the object of which is to enable the nutritive matter to be separated from it. These actions constitute the function of digestion; in the investigation of which we shall commence with a brief description of the organs concerned in it. These are numerous, and of a somewhat complicated

nature.

1. ANATOMY OF THE DIGESTIVE ORGANS.

The human digestive organs consist of a long canal, varying considerably in its dimensions in different parts, and communicating ex

1 M. Robin, under Digestion, appears to include both these acts. "La digestion est cette fonction qui introduit par endosmose les matériaux, et satisfait à l'acte chimique de composition ou assimilation nutritive." Béraud, Manuel de Physiologie, p. 54, Paris,

ternally by two outlets,--the mouth and anus. It is usually divided into four chief portions-the mouth, pharynx, oesophagus, stomach, and intestines. These we shall describe in succession.

1. The mouth is the first cavity of the digestive tube, and that into which the food is immediately received, and subjected to the action

Fig. 2.

Diagram of the Stomach and Intestines to show their course.

1. Stomach. 2. Esophagus. 3. Left, and 4. Right

end of stomach. 5, 6. Duodenum. 7. Convolutions miform appendix. 11. Ascending; 12. Transverse;

of jejunum. 8. Those of ileum. 9. Cæcum. 10. Ver

and 13. Descending colon. 14. Commencement of sigmoid flexure. 15. Rectum.

of the organs of mastication and insalivation. Above and below, it is circumscribed by the jaws, and laterally by the cheeks;-anteriorly by the lips and their aperture, constituting the mouth proper; and, posteriorly, it communicates with the next portion of the tube,--the pharynx. It is invested by a mucous exhalant membrane, which is largely supplied with follicles; and into it the ducts from the different salivary glands pour their secretion.

In all animals furnished with distinct digestive organs, means exist for comminuting the food, and enabling the stomach to act with greater facility upon it. These consist, for the most part, as in man, of the jaws, the teeth fixed into the jaws, and muscles by which the jaws are moved.

The jaws chiefly determine the shape and dimensions of the mouth; the upper forming an essential part of the face, and moving only with the head; the lower, on the contrary, possessing great mobility. Each of the jaws has a prominent edge, forming a semicircle, in which the teeth are implanted. This edge is called the alveolar arch.

The teeth are small organs, of a density superior to bone; and covered externally by a hard substance called enamel. By many, they have been regarded as bone; but they differ from it in many essential respects, although they resemble it in hardness and chemical composition. At another opportunity we shall inquire into their origin, structure, and developement. We may merely remark, at present, that by many they are looked upon as analogous to the corneous substances, which develope themselves in the tissue of the skin. De Blainville assimilates them to the hair; and believes, that they are

primarily developed in the substance of the membrane lining the mouth; and that their enclosure in the substance of the alveolar arches of the jaws occurs subsequently.

The number of the teeth is sixteen in each jaw. These are divided into classes, according to their shape and use. There are, in each jaw, four incisores; two cuspidati or canine teeth; four bicuspidati; and six molares or grinders. Each tooth has three parts:--the crown, neck, and fang or root;-the first being the part above the gum; the second that embraced by the gum; and the third, that contained in the alveolus or socket. The crown varies in the different classes. In the incisors, it is wedge-shaped; in the canine, conical; and in the molar, cubical. In all, it is of extreme hardness, but in time wears away by the constant friction to which it is exposed. The incisor and canine teeth have only one root; the molares of the lower jaw, two; and the upper, three. In all cases, they are of a conical shape, the base of the cone corresponding to the corona, and the apex to the bottom of the alveolus. The alveolar margin of the jaws is covered by a thick, fibrous, resisting substance, called gum. It surrounds accurately the inferior part of the crown of the tooth, adheres to it strongly, and thus adds to the solidity of the junction of the teeth with the jaws. It is capable of sustaining considerable pressure without inconvenience.But we shall have to return to the subject of the teeth hereafter.

The articulation of the lower jaw is of such a nature as to admit of depression and elevation; of horizontal motion forwards, backwards, and laterally; and of a semi-rotation upon one of its condyles. The muscles that move it may be thrown into two classes :-elevators and depressors. These, by a combination of their contraction, can produce every intermediate movement between elevation and depression. The raisers or levator muscles of the jaw extend from the cranium and upper jaw to the lower. They are four in number on each side,-the temporal and masseter,

Fig. 3.

which are entirely concerned in the function; the external pterygoid, which, whilst it raises the jaw, carries it at the same time forward, and to one side; and the internal pterygoid, which, according as it unites its action with the temporal or with the external pterygoid, is an elevator of the jaw or a lateral motor. The depressors may be divided into immediate and mediate, according as they are, or are not, attached to the lower jaw itself. There are only three of the former class: 1, the digastricus, the anterior fasciculus of which, or that which passes from the os hyoides to the lower jaw, depresses the latter; 2, the geniohyoideus; and 3, the mylo-hyoideus, all of which concur in the formation of the floor of the mouth. The indirect or mediate depressors are all

those, that are situate between the trunk and the lower jaw, without being directly attached to the latter;--as the thyro-hyoideus, the sternothyrouleus, and the omo-hyoideus; the names of which indicate their origin and insertion. These, in the aggregate, form a muscular chain, which, when it makes the trunk its fixed point, depresses the lower jaw. The arrangement of the elevators and depressors is such, that the former predominate over the latter; and hence during sleep the jaws continue applied to each other, and the mouth is consequently closed.

The human organs of mastication hold an intermediate place between those of the carnivorous and herbivorous animal. In the carnivorous animal, which has to seize hold of, and retain its prey between its teeth, the jaws have considerable strength; and the movement of elevation is all that is practicable; or, at least, that can be effected to any extent. This is dependent upon organization. The condyle is broader from side to side, which prevents motion in that direction: the glenoid

Fig. 4.

cavity is very deep, so that the head of the jaw-bone cannot pass out of it; and it is, moreover, fixed in its place by two eminences before and behind. The muscular apparatus is also so arranged as to admit of energetic action on the part of the muscles that raise the jaw; but of scarcely any in a horizontal direction. The deep impressions in the regions of the temporal and masseter muscles indicate the large size of these muscles in the purely carnivorous animal; whilst the pterygoid muscles are extremely small. The teeth, too, are characteristic; the molares being comparatively small, at the same time that they are much more pointed. On the other hand, the cuspidati are remarkably large, and the incisors, in general, acuminated.

The herbivorous animal has an arrangement the reverse of this. The condyle or head of the lower jaw is rounded; and can, therefore, be moved in all directions; and as easily horizontally as up and down. The glenoid cavity is shallow, and yields the same facilities. The articulation, which is very close in the carnivorous animal, is here quite loose. The levator muscles are much more feeble; the temporal fossa is less deep; the zygomatic arch less convex; and the zygomatic fossa less extensive. On the other hand, the pterygoid fossa is ample and the muscles of the same name are largely developed. The molares are large and broad; and their magnitude is so great as to require, that the jaw should be much elongated in order to make room for them.

The joint of the lower jaw has, in man, solidity enough for the jaws