thought and reason:* the truth of which assertion is evident from various metaphorical terms expressive both of intellectual defect and of intellectual excellence.

It may be presumed that, without the aid afforded by the study of anatomy or natural history, the most cursory observer might discover that the indications of intelligence manifested by the various classes of animals generally correspond in degree with their approximation in physical structure to man; and that, if we confine our view to the four highest classes, namely, fish, reptiles, birds and quadrupeds, and consider them with reference to their respective degree of docility; fish and reptiles, which are the lowest in the scale, will readily be allowed to be inferior to birds, which are a degree higher in the scale, and these again will, with equal readiness, be allowed to be inferior to quadrupeds, which are the highest.

And it would be acknowledged upon a more accurate investigation, that, although there are at first sight some seeming exceptions to the regularity of gradation, the apparent anomalies vanish when put to the test of a philosophical examination. Should it be said, for instance, that the bee or the ant shows greater indications of intelligence than many species much higher in the scale of animal creation, it may be answered that those indications are manifested in actions which are referable to instinct, rather than intelligence; actions namely, which being essential to the existence of the individuals, and the preservation of the species, are apparently determined by some internal impulse which animals unconsciously obey. Nor does it militate against such a notion of instinct, that when accidental impediments prevent -and his pure brain,

Which some suppose the soul's frail dwelling house,
Doth, by the idle comments which it makes,`

Foretell the ending of mortality."

King John, Act 5, Scene 7.

the regular evolution of the comb, taking that as an instance, the bee accommodates the arrangement of its fabric to the impediment which is placed in its way for such a modification of instinct is as clearly necessary in the case of an occasional impediment, as instinct itself is necessary for the general purpose. In speaking of instinct I purposely avoid a formal definition of the term; for any attempt to define with accuracy a principle, of the real nature of which we are ignorant, usually leaves us in a state of greater darkness than we were before; of which the following extraordinary attempt, with reference to the very principle now under consideration, is a sufficient illustration. It is quoted from an author of the name of Wagner, in a work on the Brain of Man and other Animals, written by Wenzel and his brother; and is as follows: "The instincts of animals are nothing more than inert or passive attractions derived from the power of sensation: and the instinctive operations of animals nothing more than crystallizations produced through the agency of that power.

Of the general position, then, that the brain is the instrument of intelligence, and that the degree of intelligence characteristic of different classes of animals is proportional to the approximation of their structure to that of man, it may for the present be presumed that no one doubts.

* "Instinctus animalium nihil aliud sunt, quam attractiones mortuæ a sensibilitate profecta; et eorum artificia nihil aliud quam crystallizationes per sensibilitatem productæ.”. Wenzel, De penitiori Structura Cerebri. Tubingæ, fol. 1812. p. 248, lib x.

46

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 satisfactory demonstrated: it is presumed rather than known that in such animals there exists a varible 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 chord, 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 in 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 from 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 increase 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: aud 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.