Imágenes de página

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 formsIn man, from

to z's part.
- several tribes of simia 1


310 sparrow

IT goose

do - turtle smallest) 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 ilb. 4oz., while the smallest he ever met with in an adult man was 21b. 5 oz.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 ;f 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 follow. ing 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 1. Lecture xiii.
1 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 Blurnenbach, Anat. Comp. 0 206.

canary bird


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.

În 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

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 surnish an additional prout 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. 5 203.

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 ani. mals, 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 contentment 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 lise 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 experimenting 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


them. Vaillant, wishing to preserve a locust of the Cape of Good Hope, took out the intestines, and filled the abdomen with cotton, and then fixed it down by a pin through the chest; yet after five months the animal still moved its feet and antennas.

In the beginning of November, Redi opened the skull of a land-tortoise, and excavated it of the whole brain. He expressly tells us that the tortoise did not seem to suffer: it moved about as before, but groped for its path, for the eyes closed soon after losing the brain, and never opened again. A fleshy integument was produced, which covered the opening of the skull, but the instinctive power of the living principle was incompetent to renew the brain, and in the ensuing May, six months afterward, the animal died.*

Spallanzani has incontestibly proved that the snail has a power of reproducing a new head when decapitated: but it should be remarked that the brain of the snail does not exist in its head.

I will not pursue this argument any farther; it is in many respects painful and abhorrent; and consists of experiments in which I never have been, and trust I never shall be, a participant. But I avail myself of the facts themselves in order to establish an important conclusion in physiology, which I could not so well have established without them.

Let us turn to a more cheerful subject, and examine a few of those peculiarities in the external senses which characterize the different classes and orders of animals, so far as we are acquainted with such distinctions; and admire the wisdom which they display.

The only sense which seems common to animals, and which pervades almost the whole surface of their bodies, is that of general touch or feeling; whence M. Cuvier supposes that the material of touch is the sensorial power in its simplest and uncompounded state ; and that the other senses are only modifications of this material, though peculiarly elaborated by peculiar organs, which are also capable of receiving more delicate impressions.t Touch, however, has its peculiar local organ, as well as the other senses, for particular purposes, and purposes in which unusual delicacy and precision are required ; in man this peculiar power of touch is well known to be seated in the nervous papillæ of the tongue, lips, and extremities of the fingers. Its situation in other animals I shall advert to presently.

The differences in the external senses of the different orders and kinds of animals, consists in their number and degree of energy.

All the classes of vertebral animals possess the same number of senses as man. Sight is wanted in zoophytes, in various kinds of moluscous and articulated worms, and in the larves of several species of insects. Hearing does not exist, or at least has not been traced to exist, in many molluscous worms, and several insects in a perfect state. Taste and smell, like the general and simple sense of touch, seem seldom to be wanting in any animal.

The local sense of Touch, however, or that which is of a more elaborate character, and capable of being exercised in a higher degree, appears to be confined to the three classes of mammals, birds, and insects: and even in the last two it is by no means common to all of them, and less so among insects than among birds.

In apes and macaucoes, constituting the quadrumana of Blumenbach, it resides partly in the tongue, and tips of the fingers, as in man, but equally, and in some species even in a superior degree, 'their toes. In the racoon (ursus lotor) it exists chiefly in the under surface of the front toes. In the horse and cattle orders, it is supposed by most naturalists to exist conjointly in the tongue and snout, and in the pig and mole to be confined to the snout alone; this, however, is uncertain; as it is also, though there seems to be more reason for such a belief, that in the elephant it is seated in the proboscis. Some physiologists have supposed the bristly hairs of the tiger, lion, and eat, to be an organ of the same kind; but there seems little ground for such an opinion. In the opossum (and especially the Cayenne opossum) it exists

• Dalzell's Introd. to his Transl. of Spallanzani, p. xlv.

† Anatom. Comparat. 1. 25.

very visibly in the tail; and M. Cuvier suspects that it has a similar existence in all the prehensile-tailed mammals.

Blumenbach supposes the same sense to have a place in the same organ in the platypus, or ornithorhynchus, as he calls it, that most extraordinary duckbilled quadruped which has lately been discovered in Australia, and, by its intermixture of organs, consounds the different classes of animals, and sets all natural arrangement at defiance.

The local organ of touch or feeling in ducks and geese, and some other genera of birds, appears to be situated in the integument which covers the extremity of the mandibles, and especially the upper mandible, with which apparatus they are well known to feel for their food in the midst of mud in which they can neither see nor perhaps smell it.

We do not know that amphibials, fishes, or worms possess any thing like a local sense of touch: it has been suspected in some of these, and especially in the arms of the cuttle-fish, and in the tentacles of worms that possess this organ; but at present it is suspicion, and nothing more.

În the insect tribes, we have much reason for believing such a sense to reside in the antennas, or in the tentacles; whence the former of these are denominated by the German naturalists fühlhorner or feeling-horns. This belief has not been fully established, but it is highly plausible, from the general possession of the one or the other of these organs by the insect tribes, the general purpose to which they apply them, and the necessity which there seems for some such organ from the crustaceous or horny texture of their external coat.

The senses of TASTE and SMELL in animals bear a very near affinity to the local sense of touch; and it is difficult to determine whether the upper mandible of the duck-tribe, with which they distinguish food in the mud, may not be an organ of taste or smell as well as of touch; and there are some naturalists that in like manner regard the cirrous filaments or antennules attached to the mouths of insects as organs of taste and touch equally. Taste in the more perfect animals resides jointly in the papillæ of the tongue and the palate ; but I have already had occasion to observe that it may exist, and in full persection, in the palate alone, since it has been found so in persons who have completely lost the tongue from external force or disease."

In animals that possess the organ of nostrils this is always the seat of smell; and in many quadrupeds, most birds, and perhaps most fishes, it is a sense far more acute than in man, and that which is chiefly confided in. For the most part it resides in the nerves distributed over a mucous membrane that lines the interior of the bones of the nostrils, and which is called the Schneiderian membrane, in honour of M. Schneider, a celebrated anatomist, who first accurately described it. Generally speaking, it will be found that the acuteness of smell bears a proportion' in all animals to the extent of surface which this membrane displays; and hence, in the dog and cattle tribes, as well as in several others, it possesses a variety of folds or convolus tions, and in birds is continued to the utmost points of the nostrils, which in different kinds open in very different parts of the mandible.

The frontal sinuses, which are lined with this delicate membrane, are larger in the elephant than in any other quadruped, and in this animal the sense is also continued through the flexible organ of its proboscis. In the pig the smelling organ is likewise very extensive; and in most of the mammals possessing proper horns it ascends as high as the processes of the fron tal bone from which the horns issue.

It is not known that the cetaceous tribes possess any organ of smell; their blowing holes are generally regarded as such; but the point has been by no means fully established. We are in the same uncertainty with respect to amphibials and worms; the sense is suspected to exist in all the former, and in several of the latter, especially in the cuttle-fish, but no distinct organ has hitherto been traced out satisfactorily.

In fishes there is no doubt; the olfactory nerves are very obviously distributed on an olfactory membrane, and in several instances the snouts are

« AnteriorContinuar »