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CHAPTER V.

THE NERVOUS SYSTEM OF ANIMALS IN GENERAL.

SECTION I.

The Nervous System of the inferior Animals.

As the peculiarities in the structure of the human brain cannot be understood without a reference not only to the brain but to the nervous system at large of other animals; it will be necessary to take such a survey of that system as may be sufficient for the present purpose.

In the lowest species of animals, which appear to be devoid of any specific organs of digestion, motion, or sensation; whose economy indeed only enables them to contribute, in a mode as yet unknown, to the nutrition and preservation of the individual, or to the continuation of the species, no distinct nervous system has yet been discovered, or at least satisfactorily demonstrated: it is presumed rather than known that in such animals there exists a variable number of small insulated masses of nervous matter called ganglions, which are connected with each other, and with different parts of the body, by means of slender filaments that radiate from these masses in various directions.

In ascending the scale of animal existence we meet with species, in which, though devoid of organs of sense and motion, there exist distinct organs of digestion; and in such species the upper part of the passage leading from the mouth to the stomach is usually surrounded by a kind of collar, from whence distinct nerves are distributed to the other parts of the body.

In ascending still higher the scale of animal exist

ence we find, together with a greater symmetry of structure in the whole individual, additional component parts of the nervous system, and a greater degree of regularity in the distribution of these superadded parts. Thus in those classes of animals which include the leech, the centipede, and the bee, whose bodies are naturally divisible into distinct segments, we find a series of ganglions placed opposite the respective segments, and sending out nerves which are appropriated to the muscles of voluntary motion attached to these segments: and the several ganglions are reciprocally united by intervening portions of a nervous cord, which is continued from one extremity of the body to the other; so as to present the appearance of a thread in which knots have been tied at stated intervals. And in those species of these classes which have eyes, as is the case with insects, there are additional ganglions near the head; from which arise the nerves of vision, and, probably, of touch.

If, in ascending still higher the scale of animal existence, we examine the nervous system of fish, reptiles, birds, and quadrupeds, we find that those parts which are subservient to the nutrition of the individual, and to the continuation of the species, are supplied with ganglions and nerves corresponding in their general character and mode of distribution with the nervous system of the lower classes: and that the arrangement of the nerves of voluntary motion merely differs from that of the intermediate classes, in being more elaborate; the individual nerves all communicating with a continuous cord which extends from one extremity of the body to the other but which, instead of floating loosely in the general cavity of the body, as in insects, &c. is contained in a canal essentially consisting of a series of parts called vertebræ, which taken together form what is called the spine or backbone. From the structure of this spine these classes are called vertebrated: and it is deserving of notice that these classes alone have a cranium, or skull.

;

The nervous cord above described is known more familiarly under the name of the spinal marrow, a term which is derived from its resemblance, in some of its physical characters, to the oil contained in the interior of the bones of man and various other animals.

That portion of the spinal cord which is contiguous to the head is continued into the cavity of the skull; and is there apparently lost in a more or less regular mass of nervous matter called the brain: which is small, and simple in its structure, in fish; larger, and more complicated, progressively, in reptiles, birds, and quadrupeds; largest, and most complicated, in man. From the lower surface of the brain arise several pairs of nerves which are principally distributed upon the organs of the distinct senses, and the muscles of the face: and it is worthy of observation, that while the muscles of mere animal motion, as of the trunk and extremities, are derived from the spinal marrow; the muscles of the face, which may be called pre-eminently the muscles of moral and intellectual expression, are derived from the brain itself.

In ascending then from fish, the lowest of the four classes of vertebral animals, to quadrupeds which constitute the highest class, the general mass of the brain upon the whole increases in proportional size; and at the same time it also more and more resembles that of man both in its general form, and in the character and proportions of its several parts. But the human brain, when fully developed, contains parts which do not exist in the brain of those animal species which approach nearest to man in the structure of this part.*

*It may be convenient here to state that the human brain is naturally divisible into two parts, called the cerebrum and cerebellum; of which the former is eight or nine times larger than the latter. The cerebrum, which occupies nearly the whole of the cavity of the skull, consists of two parts, called hemispheres: and it should be particularly borne in mind that it is with reference to the great size of its hemispheres that the human brain exceeds that of all other animals.

It cannot be uninteresting in an inquiry like the present to add, with respect to those occasional deviations from the common form, called monsters and lusus naturæ, that nature never elevates the brain of an individual of a lower to that of a higher class; though the brain of an individual of a higher is frequently not developed beyond the degree of a lower: and this law of the developement of the brain is, with reference at least to the distinction of classes, correspondent with that of the general form. Thus a lusus naturæ or monster in the class of quadrupeds, for instance, or of birds, may have two heads, or eight legs; but the supernumerary parts will be always those of its own class, indeed of its own species: and therefore it is absurd to suppose that if there be no mixture of species in the same class, there should ever be a confusion of two distinct kingdoms of na

ture.

Horace, than whom no one better understood the principles of imaginative or artificial poetry, knew that abrupt combinations of heterogeneous subjects would certainly offend a correct taste, because unnatural: for taste, it may be affirmed, is, in one of its essential attributes, a feeling in harmony with natural combinations; whether the individual combination be that of sounds, or colours, or forms, or of intellectual images, or moral sentiments: and nature, which may be pre-eminently called the Txve TIXN, though she may occasionally surprise the mind by unusual combinations of organs natural to the species, never so couples together heterogeneous organs, as that the limbs of animals of one species should be united with those of another species; in short, as Horace himself expresses the conception,

*The subject of lusus naturæ, or monsters, will be resumed towards the conclusion of this treatise.

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SECTION II.

The Nervous System of Man.

THE nervous system of man, without any reference to that of other animals, naturally resolves itself into three sufficiently distinct divisions: of which one is appropriated to those parts, which characterize him as simply an organized being; another, to his powers of voluntary motion; the third, or the brain, to the organs of the several senses, and, probably, to the manifestation of the intellectual powers and moral affections.

Of the two first of the foregoing divisions it is not necessary to speak more at large; because no doubt exists in the minds of physiologists as to the nature of their offices. But this is not the case with respect to the brain; which therefore demands a greater share of our attention.

Of all the parts of the nervous system taken collectively, the brain has been most generally considered as the organ of the mind: and it has long been a favourite speculation to endeavour to ascertain what part of this organ is subservient to the existence and exercise of those intellectual powers and moral feelings, which to a greater or less extent are possessed by many other animals as well as man. It is presumed at least that of the existence of intellectual powers or moral feelings in brutes no one can doubt, who has been at all accustomed to observe the characters and habits of animals ;* so that when in common language it is asserted that man differs from other animals in possessing reason, while they are irrational, the term reason must be taken in its most

* Aristotle, in his History of Animals, distinctly affirms such an existence-ἔστι γὰς ἐν τοῖς πλείστοις καὶ τῶν ἄλλων ζῴων ἴχνη τῶν περὶ τὴν ψυχὴν τρόπων, ἅπερ ἐπὶ τῶν ἀνθρώπων ἔχει pavegarigas ràs diæpogás.—p. 212, lin. 7-10. ed. Bekker.