of the skin; for, when the latter is suppressed, the former becomes more abundant. The skin exercises a power of absorption very much resembling that possessed by the intestines.

The whole length of the intestinal canal is much greater in herbivorous than in carnivorous animals.

It is only in the very lowest tribes of animals that the same orifice is applied to the double purpose of receiving fresh supplies of aliment, and of ejecting the substances unfitted for nutrition. Their intestines assume the appearance of a sack with only one entrance. But in a far greater number of animals, having the intestinal canal supplied with two orifices, the nutritive juice [or chyle] is absorbed through the coats of the intestines, and immediately diffused [by the lacteals] through all the pores of the body. This arrangement appears to belong to the entire class of insects. If we commence from the arachnides [or spiders] and the worms, and then examine all animals higher in the scale of creation, it will be found that the nutritive fluid circulates through a system of cylindrical vessels; and that it only supplies the several parts requiring nourishment by means of their ramified extremities [or lacteals], through which the nutriment is deposited in the places requiring sustenance. These vessels, which distribute the nutritive fluid or blood to all parts of the body receive the name of arteries. Those, on the contrary, are called veins, which restore the blood to the centre of the circulating system. This motion of the nutritive fluid is sometimes performed simply in one circle; often there are two circular motions, and even three, if we include that of the vena-porto [which collects the blood of the intestines, and conveys it to the liver]. The velocity of its motion is frequently assisted by certain fleshy organs called hearts, which are placed at some one centre of circulation, often at both.

In the vertebrated and red-blooded animals, the nutritive fluid, or chyle, leaves the intestines either white or transparent; and is conveyed into the venous system, by means of particular vessels called lacteals, where it mixes with the blood. Other vessels similar to the lacteals, and composing with them one arrangement, called the lymphatic system, convey into the venous system those nutritive particles which have either escaped the lacteals, or have been absorbed through the cuticle or outer skin. Before the blood is fitted to renovate the substance of the several parts of the body, it must receive, from the surrounding element, through the medium of respiration, that modification which we have already noticed. One part of the vessels belonging to those animals, which possess a circulating system, is destined to convey the blood to certain organs, where it is distributed over a large extent of surface, in order that the action of the surrounding element may be the more energetic. When the animal is adapted for breathing the air, this organ is hollow, and called lungs. But when the animal only breathes [the air dissolved] in water, the organ projects, and is called branchia, or gills. Certain organs of motion are always arranged so as to draw the surrounding element either within or upon the organ of respiration.

In animals which do not possess a circulating system, the air penetrates into every part of the body, through elastic vessels called trachea; or else water acts upon them, either by penetrating, in a similar manner, through vessels, or simply by being absorbed through the surface of the skin.

In Man, respiration is performed by means of the pressure and elastic force of the air, which rushes into the lungs, where a vacuum would otherwise have been formed by the elevation of the ribs, and the depression of the diaphragm. Muscular force then expels the air, after the necessary purification of the blood existing in the lungs has been performed; and the same actions are again repeated. The blood, which was of a dark purple colour, while slowly travelling from all parts of the body to the heart, has no sooner been purified by yielding its excess of carbonic acid to the surrounding air, and by absorbing oxygen, than its colour changes into a bright vermilion.

In Birds, it was necessary to combine lungs of small bulk with an extensive aeration of the blood; and, accordingly, the blood not only passes into the lungs, but through them into capacious air cells; from which, by the action of the chest, it is again expelled. The lungs thus act twice upon the same portion of air.

The change of the tadpole into the frog is accompanied by extraordinary alterations in its respira

tory organs, which will be more fully explained hereafter. In the first, or tadpole state, the organs are branchial, in the frog they are pulmonary. The arrangements are striking and singular. All respiration must be either aquatic or atmospheric. In the former case, the respiration is said to be cutaneous or branchial, according as it is performed through the skin or through gills. On the other hand, atmospheric respiration may be either tracheal or pulmonary, according as it is performed through the air-tubes called trachea, or by means of lungs.

After the blood has been purified by respiration, it is fitted to restore the composition of all parts of the body, and to execute the function of nutrition properly so called. The wonderful property, possessed by the blood, of decomposing itself so as to leave precisely, at each point, those particular kinds of particles which are there most wanted, constitutes the mysterious essence of vegetative life. We lose all traces of the secret process by which the restoration of the solids is performed, after having arrived at the ramified extremities of the arterial canals. But in the preparation of fluids we are able to trace appropriate organs, at once varied and complicated. Sometimes the minute extremities of the vessels are simply distributed over extended surfaces, from which the liquid exudes; and sometimes the liquid runs from the bottom of minute cavities. But the more general arrangement is, that the extremities of the arteries, before changing into veins, form particular vessels called capillary, which produce the requisite fluid at the exact point of union between these two kinds of vessels. The blood-vessels, by interlacing with the capillary vessels which we have just described, form certain bodies called conglomerate or secretory glands.

With all animals destitute of a circulation, and especially with insects, the nutritive fluid bathes the solid parts of the body; and each of them imbibes those particles necessary for its sustenance. If it become requisite that any particular fluid should be secreted, capillary vessels, adapted for this purpose, and floating in the nutritive fluid, imbibe, through their pores, the elements necessary for the composition of the fluid to be secreted.

It is thus that the blood continually renovates all the component parts of the body, and repairs the incessant loss of its particles, resulting necessarily from the continued exercise of the vital functions. The general idea which we are able to form of this process is sufficiently distinct, although the details of the operations performed at each particular point are involved in obscurity, from our ignorance of the precise chemical composition of each part, and our consequent inability to determine the exact conditions necessary for their reproduction.

In addition to the secretory glands necessary for performing a part in the internal economy of the system [such as the liver and the pancreas], there are others which secrete fluids destined to be rejected, either as being superfluous, or for some purpose useful to the animal. Of the latter we may mention the black fluid secreted by the Cuttle fish [with which, when pursued, he obscures the water to cover his retreat], and the purple matter of several Mollusca.

The function of GENERATION is involved in much greater obscurity and difficulty than that of simple secretion; and this difficulty attaches chiefly to the production of the germ. We have already explained the insuperable difficulties attending the pre-existence of germs; yet, if once we assume their existence, no particular difficulty remains attached to generation [which is not equally applicable to ordinary secretion]. While the germ adheres to the mother, it is nourished as if it formed a part of her own body; but when the germ detaches itself, it possesses a distinct life of its own, essentially similar to that of an adult animal.

The form of the germ, in its passage through the several progressive states of development, successively termed the embryo, the fœtus, and, finally, the new-born animal, never exactly resembles that of the parent; and the difference is often so very great that the change has received the name of metamorphosis. Thus, no person could ever anticipate that the caterpillar would finally be transformed into the butterfly, until he had either observed or been informed of the fact.

These remarkable changes are not peculiar to insects, for all living beings are more or less metamorphosed during the period of their growth; that is to say, they lose certain parts altogether, and develop others which were formerly less considerable. Thus, the antennæ, the wings, and all the parts of the butterfly, were concealed

under the skin of the caterpillar; and, when the insect cast off its skin, the jaws, the feet, and other organs, which belong not to the butterfly, ceased to form a part of its body. Again, the feet of the frog are enclosed within the skin of the tadpole; and the tadpole, in order to become a frog, loses its tail, mouth, and gills or branchiæ.

Even the infant, before its birth, at that period, and during its progress to maturity, undergoes several metamorphoses. In the earlier periods of development, the embryo corresponds, in some of its parts, with certain of the lower animals. At first, it seems destitute of a neck, and the heart is situate in the place where a neck afterwards appears, an arrangement which is found to exist permanently in fish. There is also a striking resemblance between the lower extremity of the vertebral column in the embryo, and the tail of the fish. About the end of the fifth month, it is covered all over with a yellowish white silk, like the down of a young duck, which entirely disappears in six or seven weeks. The limbs are formed under the skin, and reaching it, gradually shoot out into their permanent position; yet, even when fully developed in other respects, the shoulders and thighs are still concealed under the skin. In this respect, the embryo resembles the horse and other animals, which have the shoulders and thighs permanently enveloped under a thick covering of muscle. The fingers, when first formed, are surrounded by a skin, which entirely covers them, like the mitten-gloves used for an infant. This covering is gradually absorbed, when it takes the form of a duck's web, and finally disappears. M. Tieddeman and M. Serres, have shown that the brain of the fœtus, in the highest class of animals, assumes in succession the various forms which belong to fishes, reptiles, and birds, before it acquires those additions and modifications which are peculiar to the mammalia. "If you examine the brain of the mammalia," says M. Serres, "at an early stage of uterine life, you perceive the cerebral hemispheres consolidated, as in fish, in two vesicles isolated one from the other; at a later period, you see them affect the configuration of the cerebral hemispheres of reptiles; still later again, they present you with the forms of those of birds; finally, they acquire, at the era of birth, and sometimes later, the permanent forms which the adult mammalia present."

As the infant grows towards manhood it loses, at a certain age, the thymus gland; by degrees it acquires hair, teeth, and beard; the relative size of its organs changes; the body increases at a much greater rate than the head, and the head more rapidly than the internal part of the ear.

Le lieu où les germes se montrent, l'assemblage de ces germes se nomme l'ovaire; le canal, par où les germes une fois détachés se rendent au dehors, l'oviductus; la cavité où ils sont obligés, dans plusieurs espèces, de séjourner un temps plus ou moins long avant de naître, la matrice ou l'utérus; l'orifice extérieur par lequel ils sortent, la vulve. Quand il y a des sexes, le sexe mâle est celui qui féconde; le sexe femelle celui dans lequel les germes paraissent. La liqueur fécondante se nomme sperme; les glandes qui la séparent du sang, testicules; et, quand il faut qu'elle soit introduite dans le corps de la femelle, l'organe qui l'y porte s'appelle verge.

SECT. VII.-A BRIEF NOTICE OF THE INTELLECTUAL FUNCTIONS OF ANIMALS.

Mind-Matter-Sensation-Illusions-Perception-Memory-Association of Ideas-Abstraction— Judgment- Faculties of Man and other animals compared-Instinct-Connexion between the Brain and Intellectual Faculties.

We have already explained, when treating of the nervous system. that before the mind can perceive an object, an impression must be made upon an organ of sense, either immediately, or through some material medium; and that this impression must be transmitted through the nerves to the brain.

But the manner in which sensation, and its consequent perception, are produced, is a mystery impenetrable to the human understanding; and, since philosophy is unable to prove the existence of matter, it is only hazarding a gratuitous hypothesis to attempt to explain mind by materialism [or by analogies borrowed from the qualities of matter. The consideration of the Physiology of the Human Mind, of

* First truths do not admit of proof; they are assumed. We cannot prove the existence of mind, but we are conscious of its existence; and we cannot prove the existence of matter, for we perceive it.

Metaphysics, forms the subject of another science]. But it is the province of the naturalist to ascertain the conditions of the body attendant on sensation,―to trace the extreme gradations of intellect in all living beings, to investigate the precise point of perfection attainable by each animal,-and, finally, to ascertain whether there be not certain modifications of the intellectual powers, occasioned by the peculiar organization of each species, or by the momentary state of each individual body.

It has been already explained, that, to enable the mind to perceive, there must be an uninterrupted communication of nerves between the external organ of sense, and the central masses of the nervous system. The mind is, therefore, conscious only of some impression made upon these central masses. It follows, then, that the mind may be conscious of real sensations, without any corresponding affection of the external organ; and these may be produced either in the nervous chain of communication, or in the central masses themselves. This is the origin of dreams and visions, and of several casual sensations.

The various kinds of spectral illusions proceed from impressions, which being made on the retina, are thence communicated to the brain, and are referred by the mind to an object of actual existence. When the eye or the head receives a sudden blow, a bright flash of light shoots from the eyeball. In the act of sneezing, gleams of light are emitted from each eye, both during the inhalation of the air, and during its subsequent protrusion; and in blowing air violently through the nostrils, two patches of light appear above the axis of the eye and in front of it, while other two luminous spots unite into one, and appear as it were about the point of the nose, when the eyeballs are directed to it. In a state of indisposition, the phosphorescence of the retina appears in new and more alarming forms. When the stomach is under a temporary derangement, accompanied by headache, the pressure of the blood-vessels upon the retina shows itself, in total darkness, by a faint blue light floating before the eye, varying in its shape, and passing away at one side. The blue light increases in intensity becomes green and then yellow, and sometimes rises to red; all these colours being frequently seen at once; or the mass of light shades off into darkness. When we consider the variety of distinct forms which, in a state of perfect health, the imagination can conjure up when looking into a burning fire, or upon an irregularly shaded surface, it is easy to conceive how the masses of coloured light which float before the eye may be moulded, by the same power, into those fantastic and unnatural shapes which so often haunt the couch of the invalid, even when the mind retains its energy, and is conscious of the illusion under which it labours. In other cases, temporary blindness is produced by pressure upon the optic nerve, or upon the retina; and under the excitation of fever or delirium, when the physical cause which produces spectral forms is at its height, there is superadded a powerful influence of the mind, which imparts a new character to the phantasms of the senses."*

Many circumstances render it extremely probable, that the pictures drawn in the mind by memory, or created by imagination, do not merely exist "in the mind's eye," but are actually figured on the retina. During health, and in ordinary cases, these images are faint, and are easily distinguished from the sensations resulting from real perception. It is only when the body is affected by certain diseases, or during sleep, that the impressions on the retina appear to proceed from objects in actual existence.

Several instances might be brought forward to illustrate the illusions of the senses. By the well-known experiment of making a galvanic circuit through the tongue, a piece of zinc and one of silver, there is produced a pungent metallic taste, in the same manner as would have followed the real application of a sapid substance. Thus it may be seen that, if we communicate an impression to the nerve on its passage to the central mass, the mind will be affected in the same manner as if the impression had been made on the external organ.

By the terms central masses, we understand a certain portion of the nervous system, which is always more circumscribed as the animal is more perfectly constructed. In Man it is exclusively a limited portion of the brain. On the contrary, in reptiles the central mass may include either the brain, the entire marrow, or any portion of them taken separately; so that the absence of the entire brain does not deprive them of sensation. The extension of the term, when applied to lower classes of animals, is much greater, as their sensitive power is still more widely diffused.

We are hitherto completely ignorant of the nature of the changes which take place in the nerves and brain during perception, and of the manner in which the process is carried on. Analogies

Letters on Natural Magic, by Sir David Brewster.

derive from matter, sensible species, images, and vibrations, obscure rather than explain this mysterious subject.

A certain state of mind follows a certain impression upon an external organ. We refer the cause of the sensation to some external object. This constitutes perception; and the mind is said to form an idea of the object. By a necessary law involved in the constitution of the mind, all the ideas of material objects are in time and in space. When an impression has once been made through the medullary masses upon the mind, it possesses the power of recalling the impression after the exciting cause has been removed. This is memory, a faculty which varies much with the age and health of the individual.

During childhood, and in youth, the memory is very vivid. Accordingly, this period of life is most favourable to the acquisition of knowledge, especially of those subjects involving a great extent of detail, such as languages, geography, civil history, and natural history. The memory fails with increasing years.

Vivid perceptions and sensations are easily conceived; but the memory of a former mental impression is in general more faint.

Certain diseases, such as apoplexy, destroy the memory, either entirely or partially. A disordered state of the stomach will deprive the mind of the power of following a continued train of deep thought. This is also the case in the first stages of fevers. Blows and other injuries of the head will often affect the memory in a manner altogether incredible and surprising; and similar effects are sometimes produced by a high degree of nervous excitement.

Ideas which resemble [which contrast], or which were produced at the same time [or in the same place], have the power of recalling each other. This is termed the association of ideas. The order, the extent, and the quickness in which this power is exercised, constitutes the perfection of the memory.

Every object presents itself to the memory with all its qualities, and all the ideas associated therewith. The understanding possesses the power of separating these associated ideas from the objects, and of combining all the properties resembling each other in different objects under one general idea. This power of generalization, by which an object is imagined to be divested of certain properties, which in reality are never found separate, is termed Abstraction.

The power of abstraction appears to belong exclusively to Man; who, by the invention of general terms, is enabled to reason concerning entire classes of objects and events, and to arrive at general conclusions, comprehending a multitude of particular truths.

Every sensation being more or less agreeable, or disagreeable, experience and repeated trials readily point out the movements necessary to procure the one, or to avoid the other. The understanding thence deduces general rules for the direction of the will relatively to pleasure and pain.

An agreeable sensation may produce unpleasant consequences; and the foresight of these consequences may react upon the first sensation, and thus produce certain modifications of the abstract rules framed by the understanding. This is prudence or self-love.

The lower animals seem influenced only by their present or very recent sensations, and they invariably yield to the impulse of the moment. Man alone appears able to form the general idea of happiness, and, by taking a comprehensive view of things, to lay down a plan for the regulation of his future conduct, and the attainment of his favourite objects.

But an inseparable barrier is placed between man and inferior intelligences, by the power of perceiving those qualities of actions which are termed right and wrong, and the emotions which attend their perception. The supremacy of conscience, and its claim to be considered an original faculty of the mind, are clearly pointed out by Bishop Butler. Virtue," he elsewhere observes, "is that which all ages and all countries have made profession of in public-it is that which every man you meet puts on the show of it is that which the primary and fundamental laws of all civil constitutions over the face of the earth make it their business and endeavour to enforce the practice of upon mankind, such as justice, veracity, or a regard for the common good."

By applying terms to express our general ideas, we obtain certain formulæ or rules, which are easily adapted to particular cases. This is judgment or reasoning [which may be either intuitive or deductive].

When original sensations and associations forcibly recur to the memory [the mind