II. Princi ple of sensation and motion. afterwards CLASS IV. particular actions, either of the particular parts of the brain, or of particular nerves themselves, into fluids possessing different powers, and capable of producing very different effects. And as modern experiments have induced us to believe with Galen, that the nerves are a continuation of the matter of the brain *, it is not improbable that many or all of them are endowed with something of its secernent power, and are capable of assisting in the secretion of the same fluid in its simplest state, or in some of its simpler modifications. And we may hence see the reason of that complicated mechanism which distinguishes tinuation of the higher classes of animals, and how it is possible for a nervous system to exist, though with inferior powers, under a less composite fabrication. changed by particular actions and rendered capable of producing different effects. Nervous fibres a con the matter of the brain, and hence probably themselves secernent: whence a nervous system may exist under a comparatively rude fabrication. Proofs of this. Hence sometimes a sensific power and sometimes a motory. This view accordant with Hunter's remarks. This, however, is not mere conjecture: for in acephalous and anencephalous monsters we are compelled to admit it as a fact; and in different ramifications of the nerves, we can trace such different effects actually produced; and as it has sufficiently appeared that the operative power is a quick and subtle fluid, we are directly led to conclude that such difference of effects must depend on a diversity of fluids or on various modifications of a common fluid in different trunks or ramifications: the last of which explanations is by far the simplest and easiest. And hence, in certain parts of the system, the nervous influence becomes capable of producing the effect of sensation; in others of motion. And hence, again, the sensific influence is rendered capable of exciting in one set of organs a sense of sight, in others of hearing, smell, or taste, while that of touch is diffused over the surface generally. This last by its extensive diffusion is, by Mr. Hunter, called common sensation; and his view of the subject is in perfect consonance with the present. "It is more than probable," says he, "that what may be called organs of sense (local organs) have particular nerves whose mode of action * De Hippocr. et Plat. Decret. Lib. 11. Tom. 1. p. 921. CLASS IV. II. Princi sation and motion. is different from that of nerves producing common sensation; and also different from one another; and that the ple of sennerves on which the peculiar functions of each of the organs of sense depend are not supplied from different parts of the brain. The organ of sight has its peculiar nerve: so has that of hearing; and probably that of smelling likewise : and on the same principle we may suppose the organ of taste to have a peculiar nerve, although these organs of sense may likewise have nerves from different parts of the brain; yet it is most probable such nerves are only for the common sensations of the part, and other purposes answered by nerves” *. We see farther that for the purpose of elaborating the exquisitely fine and active fluid that, differently modified, excites the local organs of sense, and excites them in perfection, it is necessary that the nervous system should exist in its highest scale of fabrication, and be crowned with the apparatus of a brain, though this is not the only use to which the brain is subservient: and hence it was long ago pointed out by Galen, that it is from the brain alone the nerves appropriated to the local senses take their riset. For though we have instances of the existence of a few of these senses where the nervous system is found in a less finished form, they are never complete in number, nor apparently in acuteness. A brain ne- where all senses are complete and perfect. sary where only a ge neral sense of touch. The sense of touch, on the contrary, which, as we Not neceshave already observed, is regarded by Cuvier as produced by the sensific fluid in its simplest and least compounded state, or as Galen has it‡" is the dullest and rudest of all the sentient powers", flows for the most part, as the latter has also remarked, from the spinal marrow alone, since it is from this column that the nerves of touch almost exclusively arise. And hence we have little difficulty in conceiving how a sense of this kind may exist in moluscæ, shell fishes, and the larves of insects, which have no On the Animal Economy, p. 261. † De Instrumentis Odoratûs. Edit. Basil. Tom. 1. p. 381. VOL. IV. D CLASS IV. II. Princi ple of sen sation and motion. Motific, or irritative power of a lower description than sen sific. organs nei ther ex other nervous system than a medullary column, with a slight increment at the upper extremity, or no increment whatever; and have no other sense, or none but in a very imperfect degree. The nervous power producing motion, and which has properly been denominated irritative, appears to be of a still lower description than that of touch. It is hence common to the great mass of muscular fibres, and is probably capable of being secreted by these fibres generally; so that every fibre supplies itself, where it receives no supply from any other source. Yet the proper source or reservoir of this modification of nervous fluid seems to be a ganglionic system; that which, in the higher classes of animals we have already noticed as formed by the curious structure and ramifications of the intercostal nerve, and that which appears to be a copy of it in worms and zoophytes, who have no other nervous Hence the organization whatever. From the copiousness with which involuntary this central system furnishes a recruit to the involuntary organs with which it is peculiarly connected in hausted nor mammals, we may see why these organs are able to persevere in one uninterrupted train of action, without exhaustion or weariness from the beginning to the end of life; and why several of them, as the heart, the lungs, and the stomach, should be able to exhibit proofs of irritative power after power for a considerable period of time after the death of the system, and especially when roused by particular stimulants. Fishes in general have few pretensions to this structure, and hence they die sooner than most other animals, and exhibit little muscular irritability afterwards. Yet it is remarkable that in those genera which make the nearest approach to a ganglionic system, as the cod and carp, we have examples of a like power. The Strikingly fish-mongers of the metropolis have taken advantage of exemplified. this endowment in the cod-kind, and introduced the Crimping of fashion of crimping or corrugating the flesh, by the sti wearied. Hence an exhibition of irritative death. cod-fish. mulus of transverse incisions; and in some curious ex Singular in- periments on the carp, lately instituted by Mr. Clift, he found its heart leaping, when out of water, four hours stance in carp. II. Princi ple of sensation and after a separation from the body *. If the apparently CLASS IV. isolated molecules found in the make of the polype and various worms are ganglions of nervous irritation, extending their vital influence through certain ranges or peripheries, we are also hence enabled to account for the peculiar tenacity with which the principle of life adheres to them, and the wonderful power of reproduction which belongs to detached segments. motion. Hence spontaneous production of polypes and worms: and propagation by sections. This view supported by various sets The curious and striking experiments which have lately been made upon animals by Dr. Philip and M. Le Gallois, confirm the general view now offered so far as they of experibear upon it. These have consisted in an examination ments. into the different effects produced on the heart and lungs by suddenly destroying or cutting off the communication of the whole brain; by slowly destroying it; by destroying it in the posterior part alone; and in the anterior part alone; and by destroying, in like manner, the spinal marrow at the neck, or where it unites with the brain; in its middle or dorsal, and in its lumbar region. The animals operated upon were chiefly rabbits. According to the experiments of M. Le Gallois †, after the destruction of the brain, the action of the heart still continues for a considerable period of time unimpaired; while on the destruction of the spinal marrow at its upper or cervical extremity, this action becomes instantly so debilitated as to be no longer capable of supporting the circulation. Whence he infers that it is from the chord of the spinal marrow, and not from the gland of the brain, that the heart derives the principle of its life and motions. + Experi ments of Le Gallois. ments of The experiments of Dr. Philip are at variance with Experithe above of M. Le Gallois, and his conclusions are, Philip. therefore, somewhat different. They seem to show that both the brain and spinal marrow may be destroyed, and yet the heart continue to act forcibly and steadily, pro Phil. Trans. 1815. p. 90. + Expériences sur la Principe de la Vie, &c. CLASS IV. vided the lungs be excited by the artificial breath of a pair of bellows. II. Princi ple of sensation and motion. Conclusion of Philip from his own experi ments, in accordance with the hints now proposed. III. Intellectual principle. The brain and spinal marrow were destroyed by a hot wire, the animal being first stupefied by a blow on the occiput. Frogs and a few other animals were here employed as well as rabbits. It is not exactly stated how long, under this process, the heart continued to beat. Yet, contrary to what Dr. Philip seems to have expected, but in perfect concurrence with the hints I have just thrown out, he found that certain stimuli applied to the brain, whether in the anterior or posterior part of the head, increased very sensibly the action of the heart, the animal being still prepared as just stated. The same effect ensued when the same stimuli were applied to the cervical and even the dorsal part of the spinal marrow : but not when applied to the lumbar. Dr. Philip hence concludes that there are three kinds of vital power: muscular, possessed by the lowest kinds of animals that are destitute of both the others; nervous, or that which is here denominated the medium of touch or simple feeling, chiefly derived from, or dependent upon the spinal marrow, and possessed by animals somewhat more advanced in the scale of life; and sensorial, constituting what we have just regarded as the medium of the local senses, and appertaining to the higher classes. He adds, that each of these may exist alone, and consequently independently of the rest; but admits that where the nervous principle co-exists with the muscular, it exerts an influence over it, so that the latter may even be overborne or destroyed by such influence; and that when the sensorial co-exists with both, it exercises over both an equal degree of control. III. But the nervous organ in its most elaborate and perfect state, as in man, is not only the seat of sensation and motion, but of intelligence: it is the instrument of communication between the mind and the body, as well as between the body and the objects by which the body is surrounded. And as a failure or irregular performance of its functions in various ways lays a foundation for an |