On dissection, we find that the encephalon fills the cavity of the cranium; during life, therefore, it must be pressed upon, more or less, by the blood in the vessels, and by the serous fluid exhaled by the pia mater into the subarachnoid tissue. Thence it penetrates into the ventricles, -according to M. Magendie, at the lower end of the fourth ventricle, at the calamus scriptorius. The quantity varies according to the age and size of the patient, and usually bears an inverse proportion to the size of the encephalon. It is seldom, however, less than two ounces, and often amounts to five. M. Magendie is of opinion, that the fluid is secreted by the pia mater, and states, that it may be seen transuding from it in the living animal. The results of chemical analysis appear to show, that it differs from mere serum. It is obviously, however, almost impracticable-if not wholly so-to separate the consideration of this fluid from that met with in the cavity of the arachnoid.

The spinal marrow does not, as we have seen, fill the vertebral canal; the cephalo-spinal fluid exerts upon it the necessary pressure; added to which, the pia mater seems to press more upon this organ than upon the rest of the cerebro-spinal system. A certain degree of pressure appears, indeed, necessary for the due performance of its functions; and if this be either suddenly and considerably augmented, or diminished, derangement of function is the result. M. Magendie,' however, asserts, that he has known animals, from which the fluid had been removed, survive without any sensible derangement of the nervous functions. It is this fluid, which is drawn off by the surgeon when he punctures in a case of spina bifida.

When the brain is examined in the living body, it exhibits properties, which, some years ago, it would have been esteemed the height of hardihood and ignorance to ascribe to it. The opinion has universally prevailed, that all nerves are exquisitely sensible. Many opportunities will occur for showing, that this sentiment is not founded on fact; even the encephalon itself,-the organ in which perception takes place,——is insensible, in the common acceptation of the term; that is, we may prick, lacerate, cut, and even cauterize it, yet no painful impression will be produced. Experiment leaves no doubt regarding the truth of this, and we find the fact frequently confirmed by pathological cases. Portions of brain may be discharged from a wound in the skull, and yet no pain be evinced. In his "Anatomy and Physiology," Sir C. Bell' remarks, that he cannot resist stating, that on the morning on which he was writing, he had had his finger deep in the anterior lobes of the brain; when the patient, being at the time acutely sensible, and capable of expressing himself, complained only of the integument. A pistol-ball had passed through the head, and Sir Charles, having ascertained, that it had penetrated the dura mater by forcing his finger into the wound, trephined on the opposite side of the head, and extracted it. By the experiments, instituted by MM. Magendie,3 Flourens and

1 Précis Elémentaire, seconde édit., i. 192; and Recherches Physiologiques et Cliniques sur le Liquide Céphalo-rachidien ou Cérébro spinal, Paris, 1842.

Fifth Amer. edit. by J. D. Godman, ii. 6, 1827.

3 Précis Elémentaire, i. 325.

others, it has been shown, that an animal may live days, and even weeks, after the hemispheres have been removed; nay, that in certain animals, as reptiles, no change is produced in their habitudes by such abstraction. They move about as if unhurt. Injuries of the surface of the cerebellum exhibit, that it also is not sensible; but deeper wounds, and especially such as interest the peduncles, have singular results,-to be explained hereafter. The spinal cord is not exactly circumstanced in this manner. Its sensibility is exquisite on the posterior surface; much less on the anterior, and almost null at the centre. Considerable sensibility is also found within, and at the sides of, the fourth ventricle; but this diminishes as we proceed towards the anterior part of the medulla oblongata, and is very feeble in the tubercula quadrigemina of the mammalia.

It has been shown, that the spinal nerves, by means of their posterior roots, convey general sensibility to the parts to which they are distributed. But there are other nerves, which, like the brain, are themselves entirely devoid of general sensibility. This has given occasion to a distinction of nerves into those of general and of special sensibility. Of nerves, which must be considered insensible or devoid of general sensibility, we may instance the optic, olfactory, and auditory. Each of these has, however, a special sensibility; and although it may exhibit no pain when irritated, it is capable of being impressed by appropriate stimuli-by light, in the case of the optic nerve; by odours, in that of the olfactory; and by sound, in that of the auditory. Yet we shall find, that every nerve of special sensibility seems to require the influence of a nerve of general sensibility; the fifth pair.

Many nerves appear devoid of sensibility, as the third, fourth, and sixth pairs; the portio dura of the seventh; the ninth pair of encephalic nerves; and, as has been shown, all the anterior roots of the spinal

nerves.

The parts of the encephalon, concerned in muscular motion, will fall under consideration hereafter.

2. PHYSIOLOGY OF SENSIBILITY.

Sensibility we have defined to be-the function by which an animal experiences feeling, or has the perception of an impression. It includes two great sets of phenomena; the sensations, properly so called, and the intellectual and moral manifestations. These we shall investigate in succession.

a. Sensations.

A sensation is the perception of an impression made on a living tissue; or, in the language of Gall, it is the perception of an irritation. By the sensations we receive a knowledge of what is passing within or without the body; and, in this way, our notions or ideas of them are obtained. When these ideas are reflected upon, and compared with each other, we exert thought and judgment; and they can be recalled with more or less vividness and accuracy by the exercise of memory.

The sensations are numerous, but they may all be comprised in two divisions, the external and the internal. Vision and audition afford us examples of the former, in which the impression made upon the organ is external to the part impressed. Hunger and thirst are instances of the latter, the cause being internal; necessary; and depending upon influences seated in the economy itself. Let us endeavour to discover in what they resemble each other.

In the first place, every sensation, whatever may be its nature,-external, or internal,-requires the intervention of the encephalon. The distant organ-as the eye or ear-may receive the impression, but it is not until this impression has been communicated to the encephalon, that sensation is effected. The proofs of this are easy and satisfactory. If we cut the nerve proceeding to any sensible part, put a ligature around it, or compress it in any manner;-it matters not that the object, which ordinarily excites a sensible impression, is applied to the part, no sensation is experienced. Again, if the brain, the organ of perception, be prevented in any way from acting, it matters not that the part impressed, and the nerve communicating with it, are in a condition necessary for the due performance of the function, sensation is not effected. We see this in numerous instances. In pressure on the brain, occasioned by fracture of the skull; or in apoplexy, a disease generally dependent upon pressure, we find all sensation, all mental manifestation, lost; and they are not regained until the compressing cause has been removed. The same thing occurs if the brain be stupefied by opium; and, to a less degree, in sleep, or when the brain is engaged in intellectual meditations. Who has not found, that in a state of reverie or brown study, he has succeeded in threading his way through a crowded street, carefully avoiding every obstacle, yet so little impressed by the objects around as not to retain the slightest recollection of them! On the other hand, how vivid are the sensations when attention is directed to them! Again, we have numerous cases in which the brain itself engenders the sensation, as in dreams, and in insanity. In the former, we see, hear, speak, use every one of our senses apparently; yet there has been no impression from without. Although we may behold in our dreams the figure of a friend long since dead, there can obviously be no impression made on the retina from without."

The whole history of spectral illusions, morbid hallucinations, and maniacal phantasies, is to be accounted for in this manner. Whether, in such cases, the brain reacts upon the nerves of sense, and produces an impression upon them from within, similar to what they experience from without during the production of a sensation, will form a subject for future inquiry. Pathology also affords several instances where the brain engenders the sensation, most of which are precursory signs of cerebral derangement. The appearance of spots flying before the eyes, of spangles, depravations of vision, hearing, &c., and a sense of numbness in the extremities, are referable to this cause; as well as the singular fact well known to the operative surgeon, that pain is often

Adelon, Art. Encéphale (Physiologie), in Dict. de Méd., vii. 514, Paris, 1823, and Physiol. de l'Homme, tom. i. p. 239, 2de édit., Paris, 1829.

felt in part of a limb, months after the limb has been removed from the body.

These facts prove, that every sensation, although referred to some organ, must be perfected in the brain. The impression is made upon the nerve of the part, but the appreciation takes place in the common sensorium.

There are few organs which could be regarded insensible, were we aware of the precise circumstances under which their sensibility is elicited. The old doctrine-as old indeed as Hippocrates'-was, that the tendons and other membranous parts are among the most sensible of the body. This opinion was implicitly credited by Boerhaave, and his follower Van Swieten; and in many cases had a decided influence on surgical practice more especially. As the bladder consists principally of membrane, it was agreed for ages by lithotomists, that it would be improper to cut or divide it; and, therefore, to extract the stone dilating instruments were used, which caused the most painful lacerations of the parts. Haller considered tendons, ligaments, periosteum, bones, meninges of the brain, different serous membranes, arteries and veins, entirely insensible; yet we know, that they are exquisitely sensible when attacked with inflammation. One of the most painful affections to which man is liable is the variety of whitlow that implicates the periosteum; and in all affections of the bone which inflame or press forcibly upon that membrane, there is excessive sensibility. It would appear, that the possession of vessels or vascularity is a necessary condition of the sensibility of any tissue.

Many parts, too, are affected by special irritants; and, after they have appeared insensible to a multitude of agents, show great sensibility when a particular irritant is applied. Bichat endeavoured to elicit the sensibility of ligaments in a thousand ways, and without success; but when he subjected them to distension or twisting, they immediately gave evidence of it. It is obvious, that before we determine that a part is insensible, it must have been submitted to every kind of irritation. M. Adelon affirms, that there is no part but what may become painful by disease. From this assertion the cuticle might be excepted. If we are right, indeed, in our view of its origin and uses, as described hereafter, sensibility would be of no advantage to it; but the contrary. In the present state, then, of our knowledge, we are justified in asserting, that bones, cartilages, and membranes are not sensible to ordinary external irritants, when in a state of health,—or in other words, that we are not aware of the irritants, which are adapted to elicit their sensibility.

That sensibility is due to the nerves distributed to a part is so generally admitted as not to require comment. Dr. Todd has affirmed, that the anatomical condition necessary for the developement of the greater or less sensibility in an organ or tissue is the distribution in it of a greater or less number of sensitive nerves; and that the anatomist can determine the degree to which this property is enjoyed by any

Foësii Econom. Hippocr. "Neupov.”

3 Oper. Minor., tom. i.

Aphorism. 164, and 165, and Comment.

♦ Art. Sensation, Cyclopedia of Anat. and Physiology, pt. xxxiv. p. 511, Jan., 1849.

tissue or organ by the amount of nervous supply, which his research discloses. It may well be doubted, however, whether such sensibility be by any means in proportion to the number of nerves received by a part. Nay, some parts are acutely sensible in disease into which nerves cannot be traced. To explain these cases, Reil' supposed that each nerve is surrounded at its termination by a nervous atmosphere, by which its action is extended beyond the part in which it is seated. This opinion is a mere creation of the imagination. We have no evidence of any such atmosphere; and it is more philosophical to presume, that the reason we do not discover nerves may be owing to the imperfection of our vision.

The

Fig. 33.

We may conclude, that the action of impression occurs in the nerves of the part to which the sensation is referred. As to the mode in which this impression affects them we are ignorant. Microscopic examination of the nerves connected with sensory organs would seem to show, that they come into relation with a substance very analogous to the gray matter of the encephalon, although its elements are somewhat differently arranged. nervous fibres, too, appear to terminate in close approximation with a vascular plexus; and a granular structure is present, whichas in the cortical portion of the brainseems to be intermediate. This point has been regarded as the origin of the afferent fibres; and as the seat of changes made by external impressions.2

The facts mentioned show, that the action of perception takes place in the encephalon; and that the nerve is merely the conductor of the impression between the part impressed and that organ. If a ligature be put round a nerve, sensation is lost below the ligature; but it is uninjured above it. If two ligatures be applied, sensibility is lost in the portion included between the ligatures; but it is restored if the upper ligature be removed. The spinal marrow is sensible along the whole of its posterior column, but it also acts only as a conductor of the impression. M. Flourens destroyed the spinal cord from below, by slicing it away; and found, that sensibility was gradually extinguished in the parts corresponding to the destroyed medulla, but that the parts above evidently continued to feel. Perception, therefore, occurs in the encephalon; and not in the whole, but in some of its parts. Many physiologists-Haller, Lorry, Rolando, and Flourens3-sliced away the brain, and found that the sensations continued until the knife reached the level of the corpora quadrigemina; and, again, it has been found, that if the spinal cord be sliced away from below upwards, the sensations persist until we reach the medulla oblongata.

Exercitat. Anatom. Fascic., i. p. 28, and Archiv. für die Physiologie, B. iii.

ร Carpenter, Human Physiology, p. 85, Lond., 1842.

Rolando, Saggio sopra la vera Struttura del Cervello, Sassari, 1809; and Flourens, Re. cherches Expérimentales sur les Propriétés et les Fonctions du Système Nerveux, &c., 2de édit., Paris, 1842.

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