I. Nature

CLASS IV. new combinations, modifications, and deficiencies in others, of the brain, that it is impossible for us to avail ourselves of any such its ramifica- diversities.

tions and substitutes. But no

on this

[The principle of improvement in the nervous system, throughout the ascending scale of animals is, as Mr. Mayo* reasoning has explained, the progressive accumulation of nervous ground con- matter in larger masses upon that part of the central organ cerning the which is nearest the head or mouth. In proportion as this superiority alteration from the simplest type of organization is effected, the animal becomes more and more individualized, portions separated from it are found to be less capable of independent existence, and the destruction of one organ provement observed to produce more derangement of the rest.]

of the hu

man intellect. Principle of im

in the nervous

system.

is

Aristotle endeavoured to establish a distinction by laying it down as a maxim, that man has the largest brain of all How com- animals in proportion to the size of his body; a maxim, pared with the brain by which has been almost universally received from his own time to the present period. But it has of late years, and

Aristotle.

The com

in various

cases.

upon a more extensive cultivation of comparative anatomy, been found to fail in various instances: for, while the brain parison fails of several species of the ape kind bears as large a proportion to the body as that of man, the brain of several Aristotle's kinds of birds bears a proportion still larger. 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

maxim corrected by Sömmering:

Diss. de Basi Encephali. Götting. 1778. 4to. G. R. Treviranus has given the results of a series of observations on the relative weight and breadth of the brain to that of several of its parts in different mammiferous animals: also a table of the proportion of the several parts of the brain to the greatest breadth of the spinal chord. The particulars may be found in the Edinb. Med. and Surgical Journ. for January, 1829.-EDITOR.

CLASS IV.

I. Nature

its ramifica

tions and

Sömmering has hence endeavoured to correct the rule of Aristotle by a modification under which it appears to of the brain, hold universally; and, thus corrected, it runs as follows: "Man has the largest brain of all animals in proportion to substitutes. the general mass of nerves that issue from it.' Thus the and thus brain of a horse gives only half the weight of that of a holds uniman, but the nerves it sends forth are ten times as bulky. versally. The largest brain, which Sömmering ever dissected in the horse kind, weighed only 1lb. 4oz., while the smallest he has met with in an adult man was 2lb. 5oz.

corrected,

Rule ap

plies to ani

scent of the

But the remark applies farther than to man: for this acute physiologist has been able to trace a direct propor- mals in the tion between the degree of intelligence in every class of general deanimals, and the bulk of the brain, where the latter bears scale of anian inverse proportion to the nerves that arise from it. And mal life. we may hence observe, in passing, as indeed we have already hinted, that the nerves seem rather to be a product of the brain, than the brain of the nerves: for it is much more easy to conceive how a fountain may become exhausted in proportion to the magnitude of its streams, than how a reservoir can be augmented in proportion to the minuteness of its channels *.

Distinctive

character

ture of

Upon a general survey, I may observe, that the nervous structure of all vertebral animals, comprising the first four of the nervclasses of the Linnéan classification, mammals, birds, am- ous strucphibials, and fishes, is characterized by the two following vertebral properties: Firstly, the central organ of the nervous sy- animals. stem consists of a brain, with a long chord or spinal marrow descending from it; and, secondly, that both are securely enclosed in a bony case or covering.

In man, as I have already observed, the brain is (with a few exceptions) larger than in any other animal in proportion to the size of the body; and, without any exception whatever, in proportion to the size of its dependent column.

In other animals even of the vertebral classes, or those In what immediately before us, we meet with every variety of pro- respect

The doctrine that the nerves arise from, or are a production from the brain, has of late years been considerably weakened, if not quite refuted, by various facts, amongst which it is only necessary to mention here the existence of nerves in acephalous monsters.-ED.

the scale

descends.

I. Nature

CLASS IV. portion, from the ape, which, in this respect, approaches of the brain, nearest to that of man, to tortoises, and fishes, in which. its ramifica- the brain does not much exceed the diameter of the spinal marrow itself.

tions and

substitutes.

Nervous

animals.

Possess a

nervous

chord pro

It is not, therefore, to be wondered at that animals of a structure in still lower description, and without a vertebral column, in vertebral should exhibit proofs of a nervous chord or spinal marrow, without a brain of any kind at the top; 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 either of a cranium or a 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 portionally spinal marrow, as it is called, in animals possessing a larger than in vertebral spine; often consisting of one, and sometimes of two animals, ganglions, designed, apparently, to correspond with the organ of a brain; the descending column chiefly taking ganglions. the course of the esophagus, and surrounding it. The nervous chord, however, in these animals is proportionally larger than in those of a superior rank; and, though sometimes simple, as in molluscous worms, in other cases, as in insects, is possessed at various distances of minuter Ganglions ganglions or little knots, from which fresh ramifications of probably nerves shoot forth like branches from the trunk of a tree, minute and which have sometimes been regarded as so many dicerebels. stinct cerebels or little brains: having a close resemblance to the subordinate system of the intercostal nerve in man, as we have already traced it in its various ramifications and connexions*.

and enrich

ed with

In animals, which do

no spinal

chord.

[On this part of the subject, Mr. Charles Bell has prenot breathe, sented some rather different, but very ingenious views. In animals, which do not breathe by an uniform and general motion of their bodies, he observes, there is no spinal marrow, but only a long compound and ganglionic nerve, extending through the body for the purpose of sensation and motion. In such creatures, the chord does not actuate the animal machine with alternate dilatation and

See on this subject Sir E. Home's Croonian Lecture on the Internal Structure of the Human Brain, when examined in the microscope, as compared with that of fishes, insects, and worms. Phil. Trans. 1824, p. 1.

I. Nature

substitutes.

contraction. There may be a motion of some part, which CLASS IV. admits and expels air from a cavity, or agitates the water, of the brain, and which motion is subservient to oxygenation of the its ramifica blood; and there may be a nerve, supplied to that appa- tions and ratus, with sensibility and power suited to the function thus to be performed, and resembling our par vagum in office; but there is no regular and corresponding distribution of a respiratory system of nerves to both sides of the body, and no arrangement of bones and muscles for a general and regular motion of the frame, like that which takes place in vertebral animals, and which is necessary to their mode of existence *.]

structure in

animals?

In worms of apparently the simplest make, as zoophytes Whether a and infusory animals, no distinct structure can be dis- nervous cerned, and particularly nothing like a nervous system. zoophytes The hydra or nearly transparent polypus found so fre. and infusory quently in the stagnant waters of our own country, with a body of an inch long, and arms or tentacles in proportion, seems, when examined by the largest magnifying glasses, Their pecuto consist of a congeries of granular globules or molecules, liar make. not unlike boiled sago, surrounded by a gelatinous substance; in some tribes solitary, in others catenated. And hence, whatever degree of sensation or voluntary motion exists in such animals can only be conceived as issuing from these molecules acting the part of nervous ganglions detached, or connected. And on this account M. Virey Virey's has elegantly divided all animals into three classes accord- tion of ing to the nature of their nervous configuration; as, first, animals animals with two nervous systems, a cerebral and sympa- supposed thetic, including mammals and birds, amphibials and fishes. difference Secondly, animals with a sympathetic nervous system structure. alone, surrounding the esophagus, as mollusca and shellfishes, insects and proper worms. And, thirdly, animals with nervous molecules, as echini, polypes, and infusory animalcules, corals, madrepores, and sponges; all which in M. Virey's classification are included under the term zoophytes +.

C. Bell's Exposition of the Natural System of the Nerves of the Human Body, p. 23. Lond. 8vo. 1824.

+ When the nerves, the spinal marrow, and medulla oblongata (as Mr. Mayo observes) are proved to be sufficient for the continuance of sensation, volition,

classifica

from their

of nervous

CLASS IV.

of the brain,

tions and

substitutes. supposes

Touch the

common to

The only sense which seems common to animals, and 1. Nature which pervades almost the whole surface of their bodies, its ramifica- is that of general touch, or feeling; whence M. Cuvier that the material of touch is the sensorial power in its simplest and uncompounded state; and that the only sense other senses are only modifications of this material, though all animals: peculiarly elaborated by peculiar organs, which are also hence sup- capable of receiving more delicate impressions *. Touch, posed by Cuvier to 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 though dif- nervous papillæ of the tongue, lips, and extremities of the nerally, has fingers. Its situation in other animals I shall advert to presently.

be the base

of the other

senses.

Touch,

fused ge

its local

organs, as

The differences in the external senses of the different well as the other senses, orders and kinds of animals consist in their number and when situdegree of energy.

ated in man.

Vertebral

All the classes of vertebral animals possess the same animals the number of senses as man. Sight is wanting in zoophytes,

same num

ber of ex

ternal senses

as man.

and the commonest instincts, we naturally suppose, that the cerebral masses which seem to grow in the scale of animals with their approximation to reason, are the seat of the higher affections of the mind. When, indeed, we read over an enumeration of the different affections of consciousness, and compare the phænomena of the mind with the anatomy of the brain, we see not the most remote correspondence between the structure of the organ and the mutual dependence of the mental phænomena. When, again, we compare the brain in man with the brain in the higher animals, we are surprised to find exactly the same complexity of structure, the same series and order of internal parts, the same general exterior structure in both. The brain, however, in the higher animals, (contrasted with their other organs in general, and their spinal chord and medulla oblongata in particular) is relatively smaller, than in man, and its external structure much less complicated. The brain of the monkey, for example, has the internal parts of the same general form with those of the human brain; but the surface of the hemispheres, instead of the numerous intricate furrows and sinuosities, by means of which, the quantity of cineritious matter upon the cerebrum and cerebellum in man is so prodigiously increased, exhibits but a few straight depressions, and has proportionately a vastly less extent of the cortical superficies, and of the immediate medullary substratum. "It is here, then, that we are tempted to place the seat of reason; and, as it appears that the human brain excels that of the monkey by the number and intricacy of its folds and convolutions, so should we analogically be led to suppose, that the difference in mental endowments between one man and another may be connected with a greater amplitude of the surface of the brain in those who are most highly gifted."-(Outlines of Physiology, p. 298, 2d edit.)

Anatom. Comparat. 1. 25.