Sivut kuvina
PDF
ePub

the top of St. Paul's, has returned home through the air in a straight line, in four or five hours.

Buffon asserts, that a hawk or eagle can travel two hundred leagues in ten hours, and relates a story of one that travelled two hundred and fifty leagues in sixteen hours.

A Newfoundland dog has in like manner been brought from Plymouth to London by water, and having got loose, has run home by land with a speed so rapid as to prove that his course must have been nearly in a straight line, though every inch of it was unknown to him.

At such instances we start back, and, as far as we can, we disbelieve them, and think we become wise in proportion as we become skeptical. Meanwhile, nature pursues her wonder-working course, equally uninfluenced by our doubts or our convictions.*

Even among mankind, however, we occasionally meet with a sort of sensation altogether as wonderful and inexplicable. For there are some persons so peculiarly affected by the presence of a particular object, that is neither seen, smelt, tasted, heard, or touched, as not only to be conscious of its presence, but to be in an agony till it is removed. The vicinity of a cat not unfrequently produces such an effect; and I have been a witness to the most decisive proofs of this in several instances. It is possible that the anomalous sense may in this instance result from a peculiar irritability in some of the nervous branches of the organ of smell, which may render them capable of being irritated in a new and peculiar manner: but the persons thus affected are no more conscious of an excitement in this organ of sense than in any other; and from the originality of the sensation itself find no terms in any language by which the sensation can be expressed.

Sharks and rays are generally supposed by naturalists to be endowed with a peculiar sense in the organ of a tubular structure found immediately under the integuments of the head though they have not agreed as to the exact character of this additional sense. Trevannius calls it generally a sixth organ of sensation. M. Jacobson, and Dr. de Blainville, who quotes his authority, regard it as a local organ of touch. M. Roux, who seems to have examined it with great attention, believes it to be the source of a feeling of a middle nature between the two senses of touch and hearing.† The bat appears to have, in like manner, an additional sensific power, for it is observed to avoid external objects when in their vicinity, while the eye, ear, and nose are closed, and there is no direct touch: and this peculiar feeling has been called a sixth sense generally by naturalists, without discriminating it farther.

What is the cause of those peculiar sensations which we denominate hunger and thirst? A thousand theories have been advanced to account for them, but all have proved equally unsatisfactory, and have died one after another almost as soon as they have received a birth. We trace indeed the organs in which they immediately reside, and know by the sensations themselves that the one exists in the region of the stomach, and the other in that of the throat: but though we call them sensations, they have neither of them any of the common characters of touch, taste, hearing, seeing, or smelling.

*The fact of the migratory power of one kind of animals confirms the fact of the migratory power of others. While the question was confined to birds it was too often denied by many naturalists, merely from the difficulty of accounting for it; and it was said, in opposition to Catesby and White, and all our hest ornithologists, that our summer birds only disappear by creeping into holes and crevices to hibernate. And hence, even so late as 1823, the late Dr. Jenner felt himself called upon to examine such assertions with a view of disproving them; which he has done in one of the most agreeable essays on the natural history of migratory birds to be found in our own or any other language. "A little reflection," says he, "must compel us to confess that they are endowed with discriminating powers totally unknown to, and for ever unattainable by, man. I have no objection to admit the possibility that birds may be overtaken by the cold of winter, and thus be thrown into the situation of other animals which remain torpid at that season; though I must own I never witnessed the fact, nor could I ever obtain evidence on the subject that was to me satisfactory; but, as it has been often asserted, may I be allowed to suppose that some deception might have been practised with the design of misleading those to whom it might seem to have appeared obvious?" Phil. Trans. 1824, p. 11. The strongest argument against all such disbelief, arising from the difficulty of accounting for the migration of birds, is to turn to the migration of fishes, and to the parallel cases of remote travel in other animals, which are given above. The respective marvels give support to sach other, till disbelief itself becomes at length,the greatest marvel of the whole.

See farther on this subject, Edino. Journ. of Science, No. iii. Art. iii. p. 87, 1825.

Foods and drinks are the natural and common means of quieting their pain, but there are other means that may be also employed for this purpose, and which are often found to answer as a temporary substitute; as, for instance, pressure against the coats of the stomach in the case of hunger, and stimulating the salivary glands in the case of thirst. It is hence that chewing a mouthful of hay alone, or merely moistened with water, proves so refreshing to a tired horse, and is found so serviceable when we dare not allow him to slake his thirst by drinking. Savages and savage beasts are equally sensi-ble of the advantage of pressure in the case of hunger, and resort to it upon all occasions in which they cannot take off the pain in the usual way.

The manis or pangolin tribes, that swallow their food whole, will swallow stones or coals or any other substance, if they cannot obtain nutriment: not that their instinct deceives them, but for the purpose of acquiring such a pressure as may blunt the sense of hunger, which is found so corroding. Almost all carnivorous beasts pursue the same plan; and a mixture of pieces of coal, stone, slate, and earth is often met with in the stomach of ostriches, cassowaries, and even toads. The Kamtschatkadale obtains the same purpose by swallowing saw-dust; and some of the northern Asiatic tribes by a board placed over the region of the stomach, and tightened behind with cords, in proportion to the severity of the suffering. Even in our own country we often pursue the same end by the same means; and employ a tight handkerchief, instead of a tightened stomach-board.

In consequence of this difference in the mode in which the matter of touch or general feeling is secreted under different circumstances, we may also perceive why some parts of the body, although perhaps as largely furnished with the nerves of touch or general feeling as other parts, are far less sensible and irritable; as the bones, the teeth, and the tendons; and why the very same parts should, under other circumstances, as when morbidly affected, become the most sensible or irritable of all the organs of the system; a fact well known to all, but I believe not hitherto satisfactorily accounted for by any one. We may see also why inflammation, attacking different organs of the body, should be accompanied with very different sensations. In the bones and cartilages, except in extreme cases, it is accompanied with a dull and heavy pain; in the brain, with an oppressive and stupifying pain; and in the stomach, with a nauseating uneasiness. So, again, in the skin, muscles, and cellular membrane, it is a pain that rouses and excites the system generally; but in those parts which are supplied with the two branches of nerves which are called par vagum and sympathetic, as the loins and kidneys, the patient is affected with lowness of spirits from the first attack of the inflammation.* Dr. Gall, whose physiological theory has excited so much attention of late years on the Continent, has endeavoured to account for all these varieties of feeling, and indeed for all the animal senses of every kind, both external and internal, by supposing some particular part of the brain to be allotted to each, and that the general character and temperament of the individual is the result of the different proportions which these different parts or chambers of the brain bear to one another. He supposes, also, that this organ is possessed of two distinct sets of nervous fibres-a secernent and an absorbent; both directly connected with what is called the cineritious or ash-coloured part of the brain; the former issuing from it and secreting the fluid of the will, or that by which the mind operates on the muscles; and the latter terminating in it, and conveying to it the fluid of the external senses, secreted by those senses themselves, and communicating a knowledge of the presence and degree of power of external objects. This elaborate theory, and the facts to which it appeals, were very minutely investigated, a few years ago, by a very excellent committee of the physical class of the French National Institute, assisted by Mr. (now Dr.) Spurzheim, the intimate friend and coadjutor of its inventor, and who is well known to have contributed quite as much to the establishment of this speculation as himself. This committee, after a very minute

* Hunter on Blood, p. 289, 290

and cautious research, gave it as a part of their report, that the doctrine of the origin and action of the nerves is probably correct; but that this doctrine does not appear to have any immediate or necessary connexion with that part of Dr. Gall's theory which relates to distinct functions possessed by distinct parts of the brain.* The origin, and distribution, and action, however, of the nervous trunks have since been far more accurately traced out by Mr. Charles Bell, M. Magendie, and various other physiologists; while, in refutation of the doctrine that ascribes distinct functions to distinct parts of the brain, it may be sufficient to observe, for the present, that many of the nerves productive of different functions originate in the same part, while others, productive of the same function, originate in different parts.

There is no animal whose brain is a precise counterpart to that of man; and it has hence been conceived, that by attending to the distinctions between the human brain and that of other animals, we might be able to account for their different degrees of intelligence. But the varieties are so numerous, and the parts which are deficient in one animal are found connected with such new combinations, modifications, and deficiencies in others, that it is impossible for us to avail ourselves of any such diversities. Aristotle endeavoured to establish a distinction by laying it down as a maxim that man has the largest brain of all animals in proportion to the size of his body; a maxim which has been almost universally received from his own time to the present period. But it has of late years, and upon a more extensive cultivation of comparative anatomy, been found to fail in various instances: for while the brain of several species of the ape kind bears as large a proportion to the body as that of man, the brain of several kinds of birds bears a proportion still larger. M. Sömmering has carried the comparison through a great diversity of genera and species: but the following brief table will be sufficient for the present purpose. The weight of the brain to that of the body forms

[blocks in formation]

M. Sömmering has hence endeavoured to correct the rule of Aristotle by a modification, under which it appears to hold universally; and, thus corrected, it runs as follows: "Man has the largest brain of all animals in proportion to the general mass of nerves that issue from it."

Thus, the brain of the horse gives only half the weight of that of a man, but the nerves it sends forth are ten times as bulky. The largest brain which M. Sömmering ever dissected in the horse-tribe weighed only 1lb. 4oz. while the smallest he ever met with in an adult man was 2lb. 5oz.t

It is a singular circumstance, that in the small heart-shaped pulpy substance of the human brain, denominated the pineal gland, and which Des Cartes regarded as the seat of the soul, a collection of sandy matter should invariably be found after the first few years of existence; and it is still more singular, that such matter has rarely, if ever, been detected but in the brain of a few bisulcated animals, as that of the fallow-deer, in which it has been found by Sömmering;‡ and that of the goat, in which it has been traced by Malacarne.§

The nervous system of all the vertebral or first four classes of animals,mammals, birds, amphibials, and fishes,--are characterized by the two following properties :-first, the organ of sense consists of a gland or ganglion with

*For an examination of the general subject of craniology and physiognomy, see Series 111. Lecture xiii. † Study of Med. iv. 11, 2d edit.

Dissertatio de basi Encephali, 1778, and Tabula baseos Encephali, 1799. See Blumenb. p. 292.
Dissert. p. 10. See also Blumenbach, Anat. Comp. 206.

[ocr errors]

a long and bifid chord or spinal marrow descending from it, of a smaller diameter than the gland itself; and, secondly, both are severally enclosed in a bony case or covering.

In man, as we have already observed, this gland, or ganglion, is (with a few exceptions) larger than in any other animal, in proportion to the size of the body; without any exception whatever in proportion to the size of the chord or spinal marrow that issues from it.

In other animals, even of the vertebral classes, or those immediately before us, we meet with every variety of proportion; from the ape, which, in this respect approaches nearest to that of man, to tortoises and fishes, in which the brain or ganglion does not much exceed the diameter of the spinal marrow itself.

It is not therefore to be wondered at that animals of a still lower description should exhibit proofs of a nervous chord or spinal marrow, without a superior gland or brain of any kind; and that this chord should even be destitute of its common bony defence. And such is actually the conformation of the nervous system in insects, and, for the most part, in worms; neither of which are possessed of a cranium or spine, and in none of which we are able to trace more than a slight enlargement of the superior part of the nervous chord, or spinal marrow, as it is called in other animals-a part situ ated near the mouth, and apparently intended to correspond with the organ of a brain. The nervous chord, however, in these animals, is, for the most part, proportionally larger than in those of a superior rank; and at various distances is possessed of little knots or ganglions, from which fresh ramifications of nerves shoot forth, like branches from the trunk of a tree, and which may perhaps be regarded as so many distinct cerebels or little brains.

In zoophytic worms we can scarcely trace any distinction of structure, and are totally unable to recognise a nervous system of any kind. The common and almost transparent hydra or polype, which is often to be found in the stagnant waters of our own country, with a body about an inch long, and arms or tentacles in proportion, appears to consist, when examined by the best glasses, of nothing but a granular structure, something like boiled sago, connected by a gelatinous substance into a definite form.* Hydatids and infusory animals exhibit a similarity of make. The common formative principle of all these may be reasonably conjectured to consist in the living power of the blood alone, or rather of the fluid which answers the purpose of blood; and their principles of action to be little more than instinctive.

Can we, then, conceive that all these different kinds, and orders, and classes of animals, thus differently organized and differently endowed with intelligence, are possessed of an equality of corporeal feeling? or, to adopt the language of the poet, that-

the poor worm thou tread'st on, In corporal suffering, feels a pang as great As when a giant dies?

This is an interesting question, and deserves to be examined at some length. It may, perhaps, save the heart of genuine sensibility from a few of those pangs which, even under the happiest circumstances of life, will be still called forth too frequently; and if there be a human being so hardened and barbarized as to take advantage of the conclusion to which the inquiry may lead us, he will furnish an additional proof of its correctness in his own person, and show himself utterly unqualified for the discussion.

Life and sensation, then, are by no means necessarily connected: the blood is alive, but we all know it has no sensation; and vegetables are alive, but we have no reason to suppose they possess any. Sensation, so far as we are able to trace it, is the sole result of a nervous structure. Yet, though thus limited, it has already appeared that it does not exist equally in every kind of the same structure, nor in every part of the same kind. The skin is

* Blumenbach, Anat. Comp. § 203.

more sensible to pain than the lungs, the brain, or the stomach; but even the skin itself is more sensible in some parts than in others, which are apparently supplied with an equal number of nerves, and of nerves from the very same quarter. It is perhaps least sensible in the gums; a little more so on the hairy scalp of the head; much more so on the front of the body; and most of all so in the interior of the eyelids: while the bones, teeth, cartilages, cuticle, and cellular membrane, though largely supplied with nerves, have no sensation whatever in a healthy state.

As the degree of intelligence decreases, we have reason to believe that the intensity of touch or corporeal feeling decreases also, excepting in particular organs, in which the sense of touch is employed as a local power. And hence we may reasonably conjecture that in some of the lowest ranks of animals, the sensibility may not exceed, even in their most lively organs, the acuteness of the human cellular membrane, cuticle, or gums.

This, however, does not rest upon conjecture or even upon loose indefinite reasoning. We find in our own system that those parts which are most independent of all the other parts, and can reproduce themselves most readily, are possessed of the smallest portion of sensation; such are all the appendages of the true skin, the cuticle, horn, hair, beard, and nails: some of which are so totally independent of the rest, that they will not only continue to live, but even to grow, for a long time after the death of every other part of the body.

Now it is this very property by which every kind of animal below the rank of man is in a greater or less degree distinguished from man himself. All of them are compounded of organs which in a greater or less degree approach towards that independence of the general system which, in man, the insensible or less sensible parts alone possess; and hence all of them are capable of reproducing parts that have been destroyed by accident or disease, with vastly more facility and perfection than mankind can do.

I have once or twice had occasion to apply this remark to the lobster, which has a power not only of reproducing its claws spontaneously, when deprived of them by accident or disease, but of throwing them off spontaneously whenever laid hold of by them, in order to extricate itself from the imprisoning grasp. The tipula pectiniformis, or insect vulgarly called fatherlong-legs, and several of the spider-family, are possessed of a similar power, and exercise it in a similar manner. These limbs are renewed by the formative effect of the living principle in a short period of time: but it would be absurd to imagine that in thus voluntarily parting with them the animal puts himself to any very intolerable degree of pain; for in such case he would not exert himself to throw them off. The gad-fly, when it has once fastened on the hand, may be cut to pieces apparently without much disturbance of its gratification; and the polype appears to be in as perfect health and content'ment when turned inside out as when in its natural state. This animal may be divided into halves, and each half by its own formative and instinctive effort will produce the half that is deficient, and in this manner an individual of the tribe may be multiplied into countless numbers.

In many animals of the three classes of amphibials, insects, and worms, the most dreadful wounds that can be inflicted, unless actually mortal, seem hardly to accelerate death; and hence we have a decisive proof that the pain endured by such animals must be very considerably and almost infinitely less than would be suffered by animals of a more perfect kind, and especially by man; since in these the pain itself, and the sympathetic fever which follows as its necessary result, would be sufficient to kill them independently of any other cause.

The life of man is in jeopardy upon the fracture or amputation of a limb; and even at times when his body has been spattered over with a charge of small shot, or only of gunpowder. But M. Ribaud, with a spirit of expe rimenting that I will not justify, has struck different beetles through with pins, and cut and lacerated others in the severest manner, all of which lived through their usual term of life as though no injury had been committed on

« EdellinenJatka »