nice distinction of character must be cautiously observed, between sensation and mere irritability: like the higher powers of reason and instinct, they are

"For ever separate, yet for ever near."

The power of voluntary motion in animals necessarily requires corresponding adaptations, even in those organs simply vegetative. Animals cannot, like plants, derive nourishment from the earth by roots; and hence they must contain within themselves a supply of aliment, and carry the reservoir with them. From this circumstance is derived the first trait in the character of animals. They must possess an intestinal canal, from which the nutritive fluid may penetrate, by a species of internal roots, through pores and vessels into all parts of the body. The organization of this cavity, and of the parts connected with it, ought to vary according to the nature of the aliments, and the transformations necessary to supply the juices proper to be absorbed; whilst the atmosphere and the earth have only to present to vegetables the juices already prepared, when they are immediately absorbed.

Animal bodies, having thus to perform more numerous and varied functions than plants, ought to possess a much more complicated organization; and, in consequence of their several parts having the power of changing their position relatively to each other, it becomes necessary that the motion of the fluids should be produced by internal causes, and not be altogether dependent on the external influences of heat and of the atmosphere. This is the reason that animals are endowed with a circulating system, or organs for circulating their fluids, being the second characteristic peculiar to animals. It is not so essential, however, as the digestive system, for it is not found in the more simple species.

The complicated functions of animals require organized systems, which would be superfluous in vegetables; such as, the muscular system for voluntary motion, and the nerves for sensation. It was also necessary that the fluids should be more numerous and varied in animals, and possessed of a more complicated chemical composition than in plants, in order to facilitate the action of these two systematic arrangements. Therefore, another essential element was introduced into the composition of animals, of which plants, excepting some few tribes, are generally deprived; and while plants usually contain only three elements, oxygen, hydrogen, and carbon, animals add to these a fourth, namely, azote or nitrogen. This difference in chemical composition forms the third trait in the character of animals.

Plants derive their nourishment from the soil and atmosphere, and thence obtain water, composed of oxygen and hydrogen; also, carbonic acid, which is a compound of carbon and oxygen; while the atmosphere yields an unlimited supply of air, composed of oxygen and nitrogen [with a slight mixture of carbonic acid]. From these materials, the supplies necessary to preserve their own composition unaltered are obtained; and, while hydrogen and carbon [with a certain portion of oxygen] are retained, they exhale the superfluous oxygen [untainted]. The nitrogen, on the contrary, is [either absorbed in very small quantities, or] altogether rejected. Such is the theory of vegetable composition; in which one of the most essential parts of the process, namely, the exhalation of oxygen, can only be performed by the assistance of light.

When plants are deprived of light, an opposite effect ensues. Instead of giving off oxygen gas, and absorbing carbonic acid, the reverse takes place; and carbonic acid is disengaged, while oxygen is absorbed. The effect of plants upon the air is, therefore, to increase its purity during day-light, but to deteriorate its quality during the darkness of night.

Animals require for their nutriment, directly or indirectly, the same substances which enter into the composition of vegetables, namely, hydrogen, carbon [and a certain portion of oxygen]. But, in addition to these, it is essential, for the preservation of their peculiar constitution, that they accumulate a much larger portion of nitrogen, and disengage any excess of hydrogen, and especially any superfluity of carbon. This is performed by RESPIRATION, or breathing, in which process the oxygen contained in the atmosphere combines with the [excess of] hydrogen and carbon in the blood; with the former of these, it forms watery vapour, and with the latter carbonic acid. The nitrogen, to whatever part of the system it may penetrate, seems chiefly, though not altogether, to remain there.

The quantity of nitrogen retained in the system varies with the seasons, being greater in summer, and less in winter. The degree of variation is different for animals of different species: in some it is very small in quantity, while in others it is equal to their entire bulk.

The effects produced upon the atmosphere by plants and animals, are of an opposite kind; the former decompose water and carbonic acid, while the latter reproduce them. Respiration forms the fourth characteristic of animals, and is the most distinguishing function of the animal frame; namely, that which forms its essential difference from all other beings, and in a manner constitutes it an animal. So important is its influence over the whole body, that we shall presently be able to show, that animals perform the functions of their nature with greater or less perfection, according as their respiration is more or less perfect.

Thus we perceive that animals are distinguished from plants by the following characteristics:— 1st, They are possessed of an intestinal canal; 2dly, Of a circulating system; 3dly, Nitrogen enters largely into their composition; and finally, They are endowed with organs adapted for respiration,

SECT. IV. THE ORGANIC FORMS OF THE ANIMAL BODY, AND THE PRINCIPAL CHEMICAL ELEMENTS OF ITS COMPOSITION.

Cellular Tissue-Membranes-Gelatine-Medullary Substance-Muscular Fibre-Fibrin — Blood— Albumen-Secretion― Nutrition.

A POROUS tissue of network, and at least three chemical elements (carbon, hydrogen, and oxygen), are essential to all living bodies, while a fourth element (nitrogen) may be almost considered peculiar to animals. We shall now proceed to describe the various kinds of meshes, of which the network is composed, and the different combinations into which these four elements are found to enter.

There are three kinds of organized principles, or forms of network; the cellular tissue, the medullary substance, or marrow, and the muscular fibre. To each of these forms is attached a peculiar combination of chemical elements, as well as a particular function.

The cellular substance is composed of an indefinite number of small laminæ, without any apparent arrangement, crossing so as to form very small cells, communicating with each other. It may be compared to a species of sponge, similar in form to the entire body; while all other animal particles either occupy its cells, or traverse its substance. It possesses the property of contracting indefinitely when the causes, which preserved it in a state of extension, are removed. This power retains the body within the limits, and in the form, assigned to it by Nature.

The cellular substance, or tissue, enters into the composition of every part, forming regular series of cells. We find it equally in the brain, the eye, and the nerves, only somewhat finer in its texture than in the bones and muscles. Its cells move with facility, and accommodate themselves to the motions of the body, being moistened, at the points of contact with the adjacent cells, by a liquid, which lubricates them like the synovia or oily fluid of joints, so as to facilitate their motion.

When the cellular substance is compressed into compact plates, it forms lamina of various extent, called membranes. These membranes, when united into cylindrical tubes, more or less ramified, receive the name of vessels. The filiments, called fibres, are entirely composed of cellular substance; and the bones are nothing more than cellular substance, rendered hard by the deposition of earthy particles.

The general matter of which the cellular substance is composed, consists in the proximate principle or combination, called gelatine; the distinguishing character of which is, that it can be dissolved by boiling water, and, upon cooling, takes the form of a tremulous jelly.

Gelatine, when analyzed by Gay-Lussac and Thenard, was found to contain in 100 parts, by weight-carbon, 48; hydrogen, 8; oxygen, 27; and nitrogen, 17; very nearly.

The medullary substance cannot be resolved into any simpler organic structure. It

appears to the eye as a soft whitish pulpy matter, composed of an infinite number of very minute globules. No peculiar motions can be observed in it; but it possesses that most wonderful of all properties, the power of transmitting to the mind the impressions made on the external organs of sense, and of rendering the muscles subservient to the determinations of the will. The brain and spinal marrow are almost entirely composed of medullary substance; and the nerves, which are distributed through all the organs capable of sensation, are, in respect to their composition, nothing but bundles (or fasciculi) of this substance.

The muscular, or fleshy fibre, is composed of a particular kind of filaments, having the peculiar property, during life, of contracting or folding themselves up, when touched or injured by any external body; or when acted upon, through the medium of the nerves, by the will.

The muscles are the immediate organs of voluntary motion, and are composed entirely of bundles of fleshy fibres. All the membranes and vessels, which are required to exercise any compressive force, are armed with these fibres. They are always united intimately with the nervous filaments, or threads; but certain muscles are observed to execute motions altogether independent of the will, especially in the exercise of functions possessed in common with plants. Thus, although the will is frequently the cause of muscular motion, yet its power is neither general nor uniform in its action.

Fleshy fibre has, for the basis of its composition, a particular principle, named fibrin, which is [nearly] insoluble in boiling water, and seems naturally to assume a filamentous arrangement.

It consists of white solid fibres, inodorous and insipid. When analyzed by Gay-Lussac and Thenard, 100 parts were found to contain about 53 parts of carbon, 7 of hydrogen, 20 of oxygen, and 20 of nitrogen.

The nutritive fluid, or blood, when recently extracted from the circulating vessels, may not only be ultimately resolved, for the most part, into the general elements of the animal body, carbon, hydrogen, oxygen, and nitrogen; but it already contains fibrin and gelatine, prepared to contract their substance, and to assume respectively the forms of filaments or of membranes, according to circumstances, whenever a slight repose enables them to exhibit this tendency. In addition to these, the blood contains another proximate principle, called albumen [composed very nearly of 53 parts of carbon, 7 of hydrogen, 24 of oxygen, and 16 of nitrogen]. Its character is to coagulate in boiling water [like the white of eggs, composed almost entirely of albumen]. We also find in the blood nearly all the other elements which enter into the composition of each animal body in small quantities; such as, the lime and phosphorus deposited in the bones of the higher animals; the iron, which seems essential to the colour of the blood and other parts; and the fat, or animal oil, placed in the cellular tissue to render it flexible. In fact, all the solids and fluids of the animal body are composed of chemical elements contained in the blood. It is only by possessing some elements, of which the others are deprived, or by a difference in the proportions in which they combine, that [in general] they can be distinguished. From this it appears that it only requires, for their formation in the body, to abstract the entire, or a part, of one or more elements of the blood; or, in a few cases, to add a foreign element, procured from another source.

Some substances, differing very much in character, seem, however, to possess nearly the same chemical composition; we must therefore consider the peculiar arrangement of the particles as an essential distinction among animal fluids and solids, as well as their composition, and the proportions of their elements.

We might, without impropriety, assign the term secretion to denote the various operations by which the blood nourishes and renovates the solid and fluid parts of the body. But we shall restrict the term to the production of fluids only; while we shall apply the term nutrition to signify the production and deposition of the materials necessary for the growth and maintenance of the solids. To each solid organ, and to every fluid, is assigned that peculiar composition which is suited to its place in the system; and, by the renovating power of the blood, their composition is pre

served during health, and the continual waste repaired. Thus, by affording continual supplies of nutriment, the blood would undergo a perpetual deterioration, were it not restored by the new matter obtained from the digestion of the food; by respiration, which relieves it of the superfluous carbon and hydrogen; by perspiration, and various other means, which deprive it of any excess of other principles.

These continual changes in the chemical composition of the several parts are as essential to the vital action, as the visible motions of the old particles, and the constant influx of new ones: indeed, they seem to be the final object for which the latter motions were designed.

THE muscular fibre is not confined, in its functions, to be merely the organ of voluntary motion. We have shown, that it is one of the most powerful agents employed by Nature, in effecting such necessary motions and transference of particles in the bodies of animals as are possessed by them in common with vegetables. Thus, the muscular fibres of the intestines produce the peristaltic motion, which renders these canals pervious to the aliment; and the muscular fibres of the heart, with the arteries, are the agents in the circulation of the blood; and thus, ultimately, of all the secretions.

The will contracts certain portions of the muscular fibre through the medium of the nerves. Certain other fibres, such as those to which we have just alluded, are independent of the will, and yet are animated by nerves extending through them. We may therefore conclude, from analogy, that these nerves are the causes of their involuntary contraction.

The nerves are composed of several distinct filaments, resembling each other in every respect; and they appear to be formed of the same soft pulpy material, commonly called marrow, or medullary substance, surrounded by a cellular membrane. The filaments are again enveloped in a tube of this membrane, forming a continued nerve, extending from the brain to various parts of the muscles and skin. Yet the functions of the several filaments of the same nerve are very different. One filament is designed for voluntary muscular motion, another for sensation, and a third for involuntary motion. Sir Charles Bell, to whom we owe this remarkable discovery, divides all the nervous filaments of the body into four general systems; namely, of voluntary motion, of sensation, of respiration, and of involuntary motion. The last of these performs the functions of nutrition, growth, and ultimately of decay. Besides these, there are nerves destined to particular functions of sensation; such as sight, smell, and hearing.

When the sensitive filament of a nerve is injured in any part of its course, pain and not motion is the result; and the pain is referred by the animal to that part of the skin where the remote extremity of the filament is distributed into minute fibres. A patient, whose leg has been amputated, will feel a pain, which long-continued habit has taught him to refer to the extremity of the toes; when, in reality, the injury has been inflicted upon that portion of the nervous filament which terminates at the stump.

In the remainder of this section, our author proposes to explain the phenomena of the nerves upon the hypothesis of a nervous fluid, acted upon by certain chemical affinities. We are aware that several, almost insurmountable, objections may be urged against this theory, and indeed against every other which attempts to explain the complicated functions of life. Yet, if an hypothesis correspond pretty accurately with observed facts, it may have its uses, by fixing the phenomena in the memory, provided we always recollect, that it is but an hypothesis, to be modified as knowledge extends. Thus the phenomena of heat are referred to the imponderable fluid caloric; of light, to the vibrations of a highly elastic medium; of eletricity, to the electric fluid ;—none of which can be demonstrated to have a real existence in nature. But, in adopting an hypothesis, we must never forget that it is a temporary, not a final, theory-a motive for seeking further analogies, or, as Dr. Thomas Brown rightly observes, "a reason for making one experiment rather than another."

HYPOTHESIS OF A NERVOUS FLUID.

Every contraction, and, in general, every change in the dimensions of inorganic matter, is occasioned by a change of chemical composition; either by the absolute addition or abstraction of some solid matter, or by the flux or reflux of an imponderable fluid, such as caloric. In this way the most violent convulsions of nature arise, such as explosions, conflagrations, &c.

It is therefore probable that the nerve acts similarly upon the muscular fibre, by means of an imponderable fluid, especially as it has been proved that the impulse is not mechanical.

The medullary matter of the entire nervous system is formed throughout of the same material; and, blood-vessels accompanying all its ramifications, it is thus enabled to exercise, in every part, the functions belonging to its nature.

All the animal fluids being secreted from the blood, there is every reason to infer that the nervous fluid is derived from a similar source, and that the medullary substance is the agent in the secretion. On the other hand, it is certain that the medullary substance is the sole conductor of the nervous fluid; all the other organic elements are non-conductors, and arrest it, as glass opposes the progress of the electric fluid.

All the external causes, capable of producing sensation, or of occasioning contractions in the muscular fibre, are chemical agents, possessing a power of decomposing, such as light, caloric, salts, odorous vapours, &c. It is therefore extremely probable, that these causes act in a chemical manner upon the nervous fluid, by altering its composition; and this view appears to be confirmed by the fact, that the action of the nerves is enfeebled by long continuance, as if the nervous fluid required a supply of new materials to restore its composition, and enable it to undergo a further alteration.

An external organ of sense may be compared to a kind of sieve, which only permits those agents to pass through it, and act upon the nerve, that it is fitted to receive at that place; but it often accumulates the nervous fluid so as greatly to increase its effect. Thus, the tongue has spongy papillæ, which imbibe saline solutions; the ear is furnished with a gelatinous pulp, violently agitated by the 'sonorous vibrations of the air; and the eye is supplied with transparent lenses, which concentrate the rays of light.

Those substances which have obtained the name of irritants, from their power of occasioning contractions in the muscular fibre, probably exercise this action through the medium of the nerves; and they influence them in the same manner as the will does, that is, by affecting the nervous fluid, in the manner necessary to alter the dimensions of the muscular fibre under its influence. Yet the will is not concerned in producing these effects; often the mind is totally unconscious of their action. Even when the muscles are separated from the body, they are susceptible of being irritated, so long as that portion of the nerve which accompanies them retains its power of acting. In this case, the phenomena are totally removed from the influence. of the will. The state of the nervous fluid is altered by muscular irritation, as well as by sensation and voluntary motion: there exists, therefore, the same necessity for restoring its original composition. Irritants occasion those movements and transferences of particles necessary to the functions possessed in common by animals and plants; thus, the aliment stimulates the intestine; the blood irritates the heart. These motions are all performed independent of the influence of the will, and, in general, while health continues, without the consciousness of the animal. To effect these objects, the nerves which produce the motion have, in most cases, an arrangement entirely different from those affected by sensation or controlled by the will.

The nervous functions, by which we mean sensation and muscular irritability, are exercised with more or less vigour upon every point, in proportion as the nervous fluid is more or less abundant there; and as this fluid is produced by secretion, its quantity ought to depend jointly upon the quantity of the medullary matter secreting it, and upon the supplies of blood received by this medullary substance. In animals possessing a circulating system, the blood is distributed to all parts of the body,