An analogical argument in favour of this opinion may be derived from a very curious observation which has been lately made by Mr. C. Bell on the spinal nerves. These nerves have a double origin ; one set of their fila ments proceeding from the anterior, and the other from the posterior part of the spinal column ; and it has been discovered by this eminent anatomist, that if the anterior filaments are divided, the part to which the nerve is sent is deprived of voluntary motion, while the division of the posterior filaments destroys its sensation. Now there ap pears some ground for supposing that the anterior part of the column is more directly connected with the cerebrum, and the posterior with the cerebellum ; and hence we may conjecture, that these two parts are respectively the more immediate seats of the two faculties.
Mr. Bell has also brought to light another very inter esting circumstance in the anatomy of the nerves, which throws considerable light upon their operations and upon their connexion with the other parts of the animal econo my. He arranges all the nerves into two classes; to the first of these he gives the title of symmetrical or original, and to the second of irregular-or superadded. The first set consists of the fifth pair of the cranial and of all the spi nal nerves : they resemble each other in the mode of their origin, and in the circumstance of their passing late rally to the two halves of the body, the two sides having no connexion with each other, and are distributed to all the voluntary muscles. The second set, which arise prin cipally -from the medulla oblongata, or the parts contigu ous to it, proceed, in a very irregular manner, to all the organs which are concerned, either directly or indirectly, in the function of respiration; hence they may be called the respiratory nerves. They pass from one organ to an other in the most irregular manner, connecting them toge ther, crossing the symmetrical nerves, and uniting the two halves of the body. The parts to which they are dis 4ributed, are not under the control of the will, and they appear to be principally concerned in what, according to Dr Philip, may be styled simple nervous action, i. e. in transmitting the nervous influence from one part to ano ther without exciting perceptions.
The conclusion then that we form respecting the ner vous system, and its functions, arc, that it has two distinct powers, that of transmitting impressions, which is exer cised by the nerves and spinal cord, and that of perception and volition, more immediately depending upon the brain, or upon the brain in conjunction with the nerves. To these two powers or classes of functions, we must add a third class of intellectual ; but the nature of these, as well as the connexion which they have with the physical part of our frame, is generally regarded as being essen tially different from the two former classes. With re spect to these two classes, the mere nervous and the sen sorial functions, as contrasted with the intellectual, we observe that they exist in very different degrees in differ ent classes of animals ; and it has been a subject of inves tigation by anatomists, whether we can trace any corres pondence between the respective states of these functions and the different parts of the nervous system. Many of
the inferior animals possess the nervous and sensorial functions in a more acute state than man, but man deci. dedly excels then) all in his intellectual powers.
The great size of the human brain, compared to that of other animals, was observed by Aristotle ; and he laid it down as a general principle, that the faculties of the brain are in proportion to its size, when ssmsidered in re lation to that of the whole body. But although this rule appears to hold good with respect to many of the animals which fall under our daily observation, there are se veral important exceptions. The moderns have disco vered that in some of the mammalia, the proportion of the size of the brain to the body is equal to that of the human subject, and that there are certain species of birds, in which the proportionate size is even greater. In man, the average ratio of the weight of the brain to that of the whole body, is about 1-28th ; in the dog, about 1-160th ; in the horse about 1-400th ; in the elephant about 1-500th, while in the Canary bird it is 1-14th ; but these numbers evidently bear no ratio to the faculties of the respective animals.
An observation has been made by Seemmerring, which appears to be much more correct, and to which we have hitherto met with no exceptions, that the functions do not correspond to the size of the brain compared with that of the body, but to the size of the brain compared to the ag gregate bulk of the nerves that proceed from it. In those cases where the size of the brain is as great or even greater than that of man, comparing it with the bulk of the whole body, we shall find it to be very small when compared with the size of the nerves that proceed from it, thus seeming to prove that acuteness of the organs of sense, or of the sensorial functions, is more particularly connected with the nerves, and that the intellectual pow ers have their seat in the brain, or that these are the or gans to which the respective nervous operations are more especially attached. It may, however, be supposed, that the size of the nervous system is a less important circum stance than the perfection of its organization ; but this is a subject which is entirely involved in obscurity ; for al though considerable pains have been taken to develop the minute texture of the brain, so far as we are aware, no attempts have been made to establish any comparative ob servations on this point.
The use of ganglia is a question which has given rise to much discussion, but which still remains undecided. From their situation, and the manner in which they are composed, it has been thought that they serve to produce a more perfect connexion or sympathy between the dif ferent parts of the body ; but this, so far as we are able to judge, might have been accomplished by the mere union. of the nerves, as in the plexuses, without the additional ap paratus which is found in the ganglia. Many writers of considerable authority have supposed that these organs are to be regarded as small subsidiary brains, affording independent sources of nervous power, and constituting a number of minor seats of sensation ; but we have no suf ficient evidence of this being the case ; and, upon the whole, it appears the most prudent to confess our igno rance of the subject.