through the arteries, by means of their irritability and the action of the heart. If these arteries be irritated in an unusual degree, they act more forcibly and propel a greater quantity of blood; the nervous fluid becoming more abundant, increases the local sensibility; and, reacting upon the irritability of the arteries, carries their mutual action to a high degree. This is called nervous excitement, or orgasm; when it becomes painful and permanent, it is termed inflammation.

This mutual influence of the nerves and muscular fibres, whether in the intestinal or arterial systems, is the true source of those involuntary actions, common both to plants and animals.

Each internal organ is susceptible of irritation only from its peculiar irritant, to which it is in a manner especially adapted, just as an external sense can be affected only by its particular objects. Thus mercury irritates the salivary glands, and cantharides the vesica. These agents have been called specifics.

As the nervous system is continuous and of uniform structure, local irritations, and frequently repeated sensations, fatigue it throughout the whole extent; so that any function, when excessively exercised, may enfeeble all the others. Thus, too much food impedes the action of the intellectual powers, and long protracted study impairs the powers of digestion.

An excessive local irritation may affect the whole body, just as if all the vital energies were concentrated upon one single point. But a second irritation, in another place, will diminish the first, or, as it has been called, determine the first into another part; such is the effect of blisters, laxatives, and other counter-irritants.

We have thus shown, in the above brief sketch, that it is possible to account for all the phenomena of physical life; if we merely assume hypothetically the existence of a nervous fluid, possessed of certain properties, which are deduced from generalizing the phenomena of the vital system.

SECT. VI. The organs oF ANIMAL BODIES, THEIR APPROPRIATE FUNCTIONS, WITH THEIR VARIOUS DEGREES OF COMPLICATION.

Sensation-Touch-Taste-Smell-Sight-Hearing-Muscular Sense-Head-Brain- Voluntary Motion-Muscles-Bones-Tendons-Ligaments—Nutrition—Stomach-Gastric Juice-Chyme -Chyle-Lacteals-Arteries Veins - Lymphatics-Respiration-Lungs-Gills-TracheoCapillary Vessels-Secretory Glands-Generation.

AFTER having considered the organic elements of the animal body, the chemical elements of its composition, and the active forces which prevail in it, nothing now remains to complete a general view of the animal system, excepting a summary account of the several functions of which life is composed, with a description of their appropriate organs.

The functions of the animal body may be divided into two classes-the animal functions, which are peculiar to animals-and the vital or vegetative functions, common to animals and plants. The former comprise sensation and voluntary motion, the latter nutrition and generation.

It

We shall commence with SENSATION, which resides in the nervous system. The sense of touch is the most extensively diffused of all the external senses. is seated in the skin, a membrane enveloping the entire body, and traversed in every part by nerves. Their extreme fibres are expanded at the surface of the skin into minute papillæ, or small projecting filaments, where they are protected by the outer skin, and by other insensible coverings, such as hair or scales.

The degree of perfection in which different animals possess this sense varies considerably; but its exercise, in a high degree, is always accompanied by certain conditions. The organ must be supplied with numerous nerves and papillæ, under a very fine cuticle; with a soft cellular substance, like a cushion: and with a hard resisting base. It must also be endowed with a considerable degree of flexibility, as a close contact with the surfaces of bodies is indispensable. Most

ceros.

animals are possessed of some particular organ, in which the sense of touch is developed in a high degree. In the hand of man, and particularly at the extremities of the fingers, we find all the necessary requisites of this sense, combined in their most perfect form. The proboscis, or trunk, of the elephant seems to rank next to the human hand; and, among the higher orders of animals, either the snout or the lip is endowed with much sensibility. This quality is particularly observable in the nose of the tapir and of the hog, in the lips of the mole, and in the upper lip of the rhinoThe seal, and animals of the cat kind, such as the lion and tiger, have whiskers, possessed, near their roots, of considerable delicacy, which renders them of important use to these animals as feelers. Certain species of monkeys have delicate prehensile tails, which they use with surprising agility. In birds, the nerves of touch seem chiefly developed in the feet and toes, and most of the aquatic species are endowed with bills of considerable feeling. Serpents use their slender tongues as instruments of touch; and the great flexibility of their bodies renders them well adapted for the exercise of this sense. The snouts of fishes have some nicety; but, with this exception, these animals seem nearly destitute of delicate sensation. Insects feel chiefly by means of their antennæ ; and the several tribes of annelida, actiniæ, and polypi, by their tentacula. Several animals are covered with a dense integument in many of their parts, which are thus wholly unfitted for this sense. The thick hides of the elephant and rhinoceros, the feathers of birds, the scales, horny coverings, and shells of the lower animals, are evidently inconsistent with the necessary conditions of touch. Bats are enabled to fly in the darkest places, by the extreme acuteness of their tactual

nerves.

Taste and smell are merely more delicate modifications of this sense, for the exercise of which the membranes of the tongue and nostrils are specially organized.

In most of the lower animals the sense of taste is very imperfect, or it is altogether wanting. The tongue of man is supplied with numerous papillæ, of a conical form and spongy texture, projecting in a manner visible even to the naked eye. Taste seems in him to attain its most perfect state; and he not only enjoys the natural varieties of an omnivorous animal, but also a number of acquired tastes, which other species are wholly denied. The tongues of birds, of reptiles, and of fishes, are often covered with a hard and horny cuticle, which renders them altogether unfit for the delicate exercise of the sense. Many animals swallow their food without mastication; and they must be thereby effectually deprived of the enjoyments of taste, as a certain degree of contact between the food and the organ is essentially necessary for its exercise.

The sense of smell resides in an organ rendered susceptible by the extreme delicacy and extent of its ever humid surface.

Very minute particles of an odoriferous substance are darted forth in every direction, and are received upon the extensive and complex membrane, which lines the internal parts of the nasal cavity. Matter is thus perceived, when in a state of great subdivision, with a degree of acuteness far surpassing any of the other senses. The extreme minuteness of these particles may be inferred from the fact, that musk, and many other substances, will exhale odour for several years, and yet no loss in their weight can be detected, even by the most delicate balances. Carnivorous animals, in general, possess a more acute sense of smell than those living upon vegetable food; and the structure of their nasal cavities is consequently much more intricate. This power was obviously given to facilitate the discovery of their food. In man the sense of smell seems best adapted for vegetable effluvia. A dog, though surpassing him in detecting the most minute effluvium of another animal, will derive no pleasure from the finest vegetable odours. M. Audubon is of opinion that birds of prey are not endowed with an acute sense of smell. The degree in which this sense is enjoyed by the lower tribes of animals has not yet been completely determined, but it is observed to exist in bees and snails.

The beauty of the eye, and the unbounded sphere which it exposes to observation, give to the sense of sight a decided pre-eminence. Light, when emitted from the sun or any luminous body, strikes upon the external covering of the eyeball. By means of the crystalline lens, it is then refracted or bent from its original direction to a focal point, from which the rays of light are again distributed on the expanded extremity of the optic nerve, prepared to receive them. The size of the eyes in quadrupeds, and the intensity of their vision, bear a constant relation to the nature of their food. Herbivorous animals, such as the elephant and the rhinoceros, have very small eyes in comparison with their entire bulk. The eyes of the whale, when viewed singly, are very large; but they seem disproportionately small, if we contrast them with the enormous mass of the entire body. But quadrupeds and birds feeding on flesh, require powers of vision of very great intensity. In these animals we accordingly find the organ large, and highly developed, so as commonly to impart a peculiar expression of forocity to their countenances. The animals which are the objects of pursuit are frequently supplied with acute vision, thereby enabling them to escape or avoid danger; and this is particularly exemplified in the squirrel, the rat, the deer, and the hare. Ani

mals which burrow under ground, as the mole and the shrew-mouse, have, in general, exceedingly small eyes; while in some they have been found nearly wanting, as in the blind rat (Mus typhlus, Linn.). The cat, the lemur, and other animals which pursue their prey during the night, are peculiarly adapted, by the construction of their eyes, for acutely perceiving objects, when illuminated by a very small quantity of light. The eyes of reptiles and fish are accommodated to the medium in which they reside. The chameleon can move one eye with rapidity, and in various directions, while the other remains fixed. Reptiles residing generally in the water, also fish, and the cetacea, such as the dolphin and seal, have their eyes covered with a dense skin, and the lens is more convex than in other animals. The arachnides, or spiders, possess generally eight eyes, arranged upon the upper part of the head in a symmetrical form; and there are not less than twenty-eight in the common millepede (Julus terrestris). The insect tribes enjoy great variety and intensity in their visual organs; but the precise limits of this sense among the lowest animals in the scale of creation is not yet clearly ascertained.

The organ of hearing is excited by vibrations or undulations of air, of water, or of some solid medium, recurring at intervals, with different degrees of frequency. These impulses are received upon the tympanum or ear-drum; thence they are communicated to the acoustic nerve, and are finally transmitted to the brain. When the vibrations are not performed in equal times, or do not occur more frequently than seven or eight in a second, there is heard merely a noise. But when they rise much above this velocity, a very low or grave musical note is first heard. By an increase of velocity, the note becomes higher or more acute, and the ear is finally capable of perceiving sounds resulting from 31,000 impulses in a second. There is a regular gradation among animals in the perfection of the organ of hearing, but none of them can rival the delicacy with which the practised ear in man perceives minute changes of tone, alterations in the quality or expression of sound, and varieties in its intensity and loudness. Feeble and timid quadrupeds generally have their ears directed backwards, to warn them of approaching danger; while, in the predaceous tribes, the ears are placed forwards, to aid in discovering their prey. Animals, though seldom susceptible of musical notes, sometimes exhibit an aversion for the low or grave sounds. This is remarkably the case with the lion. In bats, the sense of hearing is surprisingly acute.

L'organe de la génération est doué d'un sixième sens, qui est dans sa peau intérieure.

Perhaps a greater claim to the right of being termed the sixth sense may be established in favour of that feeling of resistance, or muscular sense, by which we perceive the degree of force exercised by particular muscles. The mouth and lips of a new-born animal are directed by this sense to their proper function; and the adult would be in danger of a fall while engaged in walking, leaping, or other active exercises, if he were for a moment unconscious of the present state of the muscles appropriated to those actions. Shooting, bounding, and taking aim, presuppose a consciousness of the degree of muscular exertion sufficient to produce a certain effect; and instances are not wanting of its surprising accuracy. Thus, the Indian fresh-water fish called the Chatodon rostratus, will hit an insect with a small drop of water at a distance of several feet, and the encumbered insect speedily falls an easy prey. When the elephant is annoyed by flies, he will discharge a large quantity of water upon the part attacked, with sufficient accuracy and force to dislodge them. The deadly spring of the lion and tiger exhibits the instantaneous result of the muscular sense in its most tremendous form. Dr. Yellowley mentions the case of a woman who was afflicted with the disease called anesthesia, where the muscular sense of her hands was lost, although the muscular power remained. On turning her eyes aside, she used to drop glasses, plates, &c., which were held in safety as long as another sense supplied the place of the lost one.

The stomach and intestines are possessed of certain peculiar sensations which declare the state of these viscera; and indeed every part of the body is susceptible of sensations, more or less painful, when affected by accident or disease.

Pain teaches an animal to avoid hurtful objects, and is wisely given as a safeguard to his frame; accordingly, its seat is mostly at the surface. The deep parts of the body have but little sensation, as it would there be only a useless encumbrance. The animal is continually warned, by uneasy sensations, to change his posture frequently, to avoid high degrees of heat, and in general to accommodate his frame to surrounding circumstances.

Many animals are defective both as to ears and nostrils, several are destitute of eyes, and some are reduced solely to the sense of touch, which is never wanting. In the higher animals, impressions made upon the external organs of sense are transmitted by the nerves to the brain and spinal marrow, which form the central masses of the nervous system. The elevation of an animal in the scale of creation may [frequently] be determined by the volume of its brain, and the degree in which

the power of sensation is concentrated there. Animals of a lower grade have the medullary masses much dispersed; and in the more simple genera, all trace of nerves seems to be lost in the general substance of the body. That part which contains the brain and principal organs of sense is called the head.

We now proceed to consider the second animal function—namely VOLUNTARY MOTION.

When the animal wills to move, in consequence of a sensation upon an external organ, or any other cause, the motion is transmitted to the muscles by means of the

nerves.

This power of originating motion, residing in the nervous system of living animals, is one of the most wonderful properties of their nature. Every machine, however complicated or varied in its structure, can only be set in motion by some external power already existing in nature, or produced by art, whether it be the expansive force of steam, the descent of weights, the action of running water, or the recoil of a spring. No perpetual motion can ever be preserved by any arrangement of the parts of a machine among themselves; they must rest ultimately upon a prime mover. But the exquisite arrangement of the animal frame surpasses, in this respect, the highest mechanical skill. The mind wills the muscle contracts. How much soever we may desire to unravel the mystery, the process is inexplicable, and seems for ever removed beyond the reach of human ingenuity. The only fact hitherto ascertained is, that if the nerve be separated, seriously injured, or even tightly compressed, the motion of the muscle will not follow the volition of the mind.

The muscles are bundles of fleshy fibres, by the contraction of which the animal body performs all its motions. The extension and lengthening of the limbs are equally the result of muscular contraction with their bending and drawing in. They are arranged in number and in direction to suit the motions which each animal is destined to perform; and when it becomes necessary to execute these motions with vigour, the muscles are inserted upon hard parts, which are so articulated, one over the other, as to constitute them so many levers. These parts, in the vertebrated animals, are called bones. They are situate internally, and are formed of a gelatinous mass [of cellular substance, the pores of which are] penetrated by particles of phosphate of lime. In some of the lower tribes of animals, such as the Mollusca, the Crustacea, and the Insects, these hard parts are external, and composed either of calcareous or of horny substances, called shells, crusts, or scales, all of which are secreted between the skin, and the epidermis or cuticle.

A considerable difference is found between the chemical composition of the bones belonging to the higher orders of animals and the external coverings of crustaceous animals. Human bones, when analyzed by Berzelius, were found to contain in 100 parts nearly as follows: of animal matters (being chiefly gelatine, cartilage, and marrow), 34 parts; of phosphate of lime, 51 parts; of carbonate of lime, 11 parts; of fluate of lime, 2 parts; of phosphate of magnesia, 1 part; and of soda, muriate of soda and water, 1 part. Here the principal ingredient is phosphate of lime; but in the hard parts of crustaceous animals, such as crabs and lobsters, the carbonate of lime is considerably in excess. The shells of the mollusca, such as muscles and oysters, are almost entirely formed of the carbonate of lime. On the contrary, the horny coverings of insects contain a very minute portion of earthy matter, and are mostly composed of animal substances. The same proximate elements enter into the composition of horns, nails, and hoofs, being gelatine, with a membranous substance, resembling the white of eggs boiled hard. The scales of fish are composed of layers of membrane alternately with those of phosphate of lime, which arrangement is the cause of their brilliancy; but the scales of serpents contain no phosphate of lime, and very much resemble, in their constitution, the horny coverings of insects.

The fleshy fibres are inserted upon the hard parts, by means of other fibres of a gelatinous nature, called tendons, which seem to be a continuation of the first.

These tendons exercise the same office as straps or ropes in ordinary machinery, when it is required to transfer motion from one part to another. By this means a moving power can be exercised, in a spot where its immediate presence would be highly inconvenient. Thus, the hand is moved by tendons communicating with muscles, fixed at a considerable distance upon the arm; and the velocity and delicacy of its movements are not obstructed by their presence. Often these tendons are strapped down by cross cords, and pass along grooves in the bones, or through a pulley formed by a ligament. By these mechanical contrivances, the direction in which the muscular power acts may be changed; the forces of different muscles are compounded, and altered in intensity; and the velocity of the resulting motions modified according as circumstances may require. This

arrangement also permits the accumulation of force upon one point; for a great number of muscular fibres are employed to contract one tendon, in the same manner several horses may be employed to draw the same rope.

The peculiar shape observable in the articulated surfaces of the hard parts confine the motions of the tendons within certain limits, and they are still further restrained by cords or envelops, usually called ligaments, attached to the sides of the articulations. Animals become enabled to execute the innumerable motions involved in the exercise of walking or leaping, flying or swimming, according as the bony and muscular appendages are adapted for these various motions; and also, according to the relative forms and proportions which the limbs, in consequence, bear among themselves.

NUTRITION, which we shall now explain, forms the first of the vegetative functions.

The muscular fibres connected with digestion and circulation are not influenced by the will, but, on the contrary, as we have already explained, their principal arrangements and subdivisions appear to be specially intended to render the animal completely unconscious of their exercise. It is only when the mind is disturbed by violent passions, or paroxysms, that its influence is extended beyond the ordinary limits, and that it agitates these functions common to vegetable life. Sometimes, when the organs are diseased, their exercise is accompanied by sensation; but, in ordinary cases, digestion and circulation are performed without the consciousness of the animal.

The aliment is first masticated, that is, minutely divided by the jaws and teeth, or sucked in, when taken by the animal in a liquid form. It is then swallowed entirely by the muscular action of the back parts of the mouth and throat, and deposited in the first portions of the alimentary canal, which are usually expanded into one or more stomachs, where the food is penetrated and dissolved by corrosive juices.

This gastric juice possesses the very remarkable property of dissolving most animal and vegetable matters, when deprived of life, and some mineral substances. It more especially acts upon such as yield nutriment to the animal, and are adapted to its general habits and formation. When recently procured from the stomach of a healthy animal, it appears as a clear mucilaginous fluid, slightly salt to the taste. Substances, when undergoing fermentation or putrefaction, are immediately checked in their action by the gastric juice, and are formed by its corrosive influence into a new fluid, possessed of entirely different properties, called chyme. But most mineral substances are indigestible. Certain tribes of savages, as the Otomacs, will, however, swallow daily large quantities of earth to allay the cravings of hunger. But this substance does not appear to be dissolved; it merely acts mechanically in distending the stomach.

The higher region of the alimentary canal is occupied by the stomach, which receives the food conveyed to it through the oesophagus or gullet. The form and structure of the stomach bear a constant relation to the nature of the food. In herbivorous animals, it is composed of a complicated system of reservoirs, where, by a slow and intricate process, the small quantity of nutriment contained in vegetable matter is abstracted and conveyed into the system. In carnivorous animals, the stomach is comparatively simple; and a supply of abundant nourishment is readily procured from animal food.

After passing through the stomach, the food is received into the remaining part of the canal, where it is acted upon by other juices destined to complete its preparation. The chyme formed in the stomach having passed into the intestine, comes in contact with the bile and the pancreatic juice. An immediate change takes place. The chyme acquires the yellow colour and bitter taste of bile, and at length divides into two portions; the one, a white tenacious liquid called chyle, and the other, a yellow pulp.

The coats of the intestinal canal are supplied with pores, which imbibe that portion of the alimentary mass adapted for the nutrition of the body [being the chyle, while the useless residue is finally conveyed away and ejected.

The canal in which this first function of nutrition is performed, appears to be a continuation of the skin, and it is composed, in a similar manner, of laminæ. Even the surrounding fibres are analogous to those adhering to the internal surface of the skin, and called the fleshy pannicle. A mucous secretion takes place throughout this canal, which seems to have some connexion with the perspiration from the surface