We proceed, in the second place, to consider the pro perties of the nervous system, and the connexion which it has with the other parts of the body. It is obvious that the office of the nervous system is, to produce sensa tion; but it is extremely difficult to comprehend the man lier in which this is accomplished, and it is equally diffi cult to ascertain the relation which the different parts of the nervous system bear to each other. The general doc trine on the subject is, that the brain is the centre of the system, the part to which all the rest are subservient ; that the spinal cord is merely a prolongation or continua tion of the brain ; that the nerves receive impressions from external objects, and transmit these impressions to the brain, where they become sensible to the mind, and the intellectual faculties. This view of the subject is pro bably in the main correct, although the experiments and discoveries of the modern anatomists have led to some modifications which deserve to be noticed.
All questions respecting the action of the nervous sys tem are involved in much obscurity, which appears, in some measure, necessarily attached to the nature of the subject. Although there are many difficulties connected with the operations of the muscular system, yet we can form a plausible 'conjecture respecting the mode in which it acts, and we can distinctly trace it from its commence ment in the fibre to the part that is moved ; so that we are able to see the connexion between the shortening of the fibre and the ultimate effects, although we cannot assign any reason why the application of stimulants should pro duce this immediate effect upon the fibre. But we have no analogy of this kind to guide us in explaining the phe nomena of the nervous system ; so that, although we have sufficient evidence that the nerves arc the media by which external impressions are conveyed to the brain, we are unable to account for the mode of the conveyance.
Sensibility, the term which is generally employed by physiologists to express the power which the nervous sys tem possesses of producing sensation, is here used in a metaphorical sense, its original import implying a certain state of the mind or character, and having no express re lation to the physical state of the nerves. When, how ever, we adopt it as a physiological term, we restrict its operatibns to the nervous system, and define it to be, the power which this system possesses of receiving and trans mining impressions. The affection of the nerves appears, in some cases, to be confined to themselves, and at other times to be transmitted to the mind, when it constitutes a perception ; but this is not a necessary consequence, as there arc many instances in which the nerves transmit im pressions to distant parts, of which our feelings give us no indication. Hence we perceive the distinction between a simple sensation and a perception, a distinction which nearly coincides with Bichat's division of sensibility into organic and animal ; for this division would appear to have a real foundation in nature, although we may conceive that he is not always correct in the classification of the, different cases which he notices, In relation to this subject, we may notice the learned and animated discussion that was carried on about sixty years ago, between Haller and Whytt, respecting the mode in which the muscular and nervous systems are connected with each other. The question at issue was, i whether the nerves are necessarily concerned in muscular contraction ; whether, when a stimulant acts upon a mus cle so as to produce contraction, it acts directly upon the fibre, or always produces its effect through the interven tion of the nerve. Before the time of Haller, no clear
conception had been formed of the living body, as endow ed with distinct faculties or powers, but as simply possess ing life. He very accurately examined the nature of the vital power, and showed that it may be referred to the two heads of contractility and sensibility, which are ex ercised by two sets of organs. And he did not rest here, but he endeavoered to show, that these functions not only differ in their nature, but that they arc independent of each other in their operation ; and he concluded that the muscular fibre, without the co-operation of the nerve, pos sesses the power of being directly acted upon by stimu lants; to this power he gave the name of irritability. Whytt, on the contrary. denies the existence of this in nate irritability, but supposes, that in all cases, stimulants act primarily upon the nerves connected with the muscle, and that the impression is conveyed front the nerve to the muscular fibre.
The great argument of Haller was an appeal to the ana tomical structure of the body. He pointed out many parts that are extremely contractile, but which are pos sessed of very little sensibility; and he endeavoured to show generally, that these two properties bear no relation to each other, which they should do were they necessari ly connected together. The heart was particularly re ferred to by Haller in this controversy, as being a muscu lar organ in perpetual motion, and capable of strong con traction, yet scantily furnished with nerves, and almost destitute of feeling. It was also advanced, as a strong proof of this doctrine, that muscular parts remain con tractile for a considerable time after they are removed from the body; and, of course, when their communication with the brain is destroyed ; and this is particularly the case with the heart itself; which, in many of the ion phibia and cold-blooded animals, remains susceptible of the impression of stimulants for some hours after its se paration from the body; yet how, it was asked, can the nervous system be affected since the seat of sensation is removed? Upon the same principle, and in furtherance of the same train of reasoning, cases were adduced of acepha Ions fectuses, which had vet grown to their full size, and seemed to possess the various vital functions in a perfect state, so that they must have enjoyed muscular contracti lity, although totally destitute of a brain. It was also urged, that, among the lower classes of animals, there are many that are entirely without a nervous system; yet where the muscles are easily excited by stimulants, and capable of very powerful contractions. It was also re marked, that, in the more perfect animals, if we watch the progress in which the different parts are formed, and of aourse the order in which the different functions are developed, we shall find that the heart exists before the brain; and, of course, that there must be contractility without sensibility. And this circumstance accords with our ordinary opinion upon the subject ; for we can easily conceive the possibility of mere life being continued with out sensation, while we could not imagine any species of existence to which some kind of internal motion was not essential.