17 



NERVOUS SYSTEM. 



NERVOUS SYSTEM. 



13 



is lets regularity of origin, the nervea of peculiar sensation having no 

 muscular nerves corresponding to them, the seventh nerve, or portio 

 dura, being exclusively motor, without any corresponding sensitive 

 root, &c. Thus, of the nerves proceeding from the brain, some are in 

 all their course sensitive only ; others are (except for the occasional 

 connection of some of their branches with branches of the fifth) entirely 

 motor ; others (part of those of the fifth) mixed, that is, containing in 

 a common sheath both sensitive and motor filaments, like all the 

 nerves derived from the spinal chord. 



The following table will give the student an idea of the functions of 

 the cerebral nerves : 



Nerves of Special 1 Olfactory. Optic. Auditory. Part of Glosso-Pharyngeal. 

 Sense . . ] Lingual branch of 5th. 



Greater portion of 5lh, and part of Glowo-Pharrngeal. 



Nerve of Motion 3rd, 4th, leaser division of ith, 6th, Facial, and Hrpoglossal. 

 Mixed Nervea . Pncumogastric and Accessory. 



The conveyance of those impressions which produce common sensa- 

 tion U the property exclusively of those nervous filaments which arise 

 from the posterior columns of the spinal chord and their continuations 

 in the brain. To possess this power, their connection with the brain, 

 either directly or through the medium of the spinal chord (which, in 

 this view, may be regarded merely as a collection of a vast number of 

 the nervous filaments which have their centre in the brain), must 

 remain uninjured. Hence, when any sensitive nerve is cut through, 

 all the parts to which its filaments are distributed beyond the place of 

 division are rendered perfectly insensible. The nearer to the brain 

 that the injury is inflicted, the more extensive is the destruction of 

 sensibility. When the spinal chord is injured by fracture of the ver- 

 tebrae, or by disease, or any other means, all the parts whose nerves 

 come out from the portion below that which is destroyed, become 

 insensible ; they lose also all power of voluntary motion, but for the 

 present we shall consider the sensitive filaments and their functions 

 only. If the posterior roots of a nerve be divided, all the parts sup- 

 plied by that nerve lose their sensibility ; or if a nerve be divided in 

 any part of its course, then all the parts supplied by branches given 

 off between the point of division and the brain retain their sensibility, 

 while those which are supplied by branches given off in the other part 

 of the nerve more distant from the brain are rendered insensible. 

 These facts prove that the influence of an impression upon the distal 

 or peripheral extremity of a nervous filament can only be conveyed to 

 the brain so as to produce sensation through a continuity of nervous 

 tissue, and only through the very filaments that are impressed ; and 

 that there is no such communication of adjacent filaments, that if one 

 is injured, another can convey the impression made on it, as in the 

 circulation, by the anastomosis or communication of the bloodvessels, 

 when one La obliterated, the blood which should traverse it passes 

 through another. The same facts evidently explain also many of the 

 phenomena of partial paralysis. 



If that part of a divided nerve which is still connected with the 

 brain be stimulated, the same sensation is perceived as if the stimulus 

 were applied to all the parts in which the branches that the nerve 

 gives off below the division are distributed. This is shown in the 

 sensation which every one must have felt on striking the inside of the 

 elbow (the 'funny bone," as it is commonly called); the tingling pain 

 that the blow produces, and which appeal's to have its seat in the inner 

 side of the fore arm and hand, and in the little and ring fingers, is 

 owing to the filaments of the ulnar nerve (which passes behind the 

 elbow) being distributed to those parts, and to the sensation being 

 always referred to the peripheral termination of the filament, in what- 

 ever part of its course it is irritated. It is of course impossible to 

 experiment upon a single nervous filament, but the accuracy with 

 which the imagined seat of sensation produced by irritating a bundle 

 of filaments accords with their distribution leaves no doubt of the 

 fact just mentioned. The nearer to the brain that the stimulus is 

 applied, the more extended is the fensation ; hence in disease of the 

 spinal chonl, pain is often felt in all the parts supplied with nerves 

 coming off from the chord below the diseased portion, and the pain of 

 dividing a nerve is felt over all the parts to which its branches are 

 distributed. The same circumstance gives rise to the apparently 

 strange ideas which those who have lost a limb entertain that they 

 still possess it, till by their sight or some other means they correct 

 the erroneous impressions of their sense of touch. For example, the 

 constant cry of a patient who has just lost his leg, while the stump 

 is being dressed, is that bis attendants are squeezing his knee, or 

 cutting his foot, or injuring some other part of the limb, which he 

 cannot believe has been cut off. The reason is, that when those fila- 

 ments of the nerves in the stump which are destined for the knee are 

 touched, the knee seems to be touched ; when those which are going 

 to the. foot are injured, the foot appears to suffer. So deceptive are 

 these sensations, that even years after the loss of a limb, or for the 

 rest of their lives, persons occasionally endeavour to perform sbme act 

 with the stump which they were accustomed to do with the part they 

 have lost. Krom the same circumstance of the sameness of sensation, 

 whatever be the part of a nervous filament to which the stimulus is 

 applied, arises the well-known feeling of the foot having falleu asleep, 

 when the ischiatic nerve has been pressed upon, and the peculiar 



HAT. HIST. niv. vox,, iv. 



character of the pain in many cases of tic doloureux, in which the 

 trunk of a nerve being diseased, the pain is referred to all the parts 

 to which its filaments are distributed. 



The laws deduced from these facts, and which are of great import- 

 ance in the explanation of many diseases of the nervous system, are 

 briefly these : The impressions producing sensation always pass from 

 the circumference of the distribution of the nervea to the centre ; they 

 are always centripetal, and their centre, the seat of sensation, is the 

 brain. They can pass only through the tissue of the nervous filaments 

 to which they have been applied, and whose connection with the brain 

 must therefore be unimpaired. The sensation is the same on whatever 

 part of the nervous filament the impression is first made, and it is 

 always referred to the peripheral end of the filament. 



If the connection of the nerve with the brain be broken, the same 

 impression may pass along ita filaments to the part where its con- 

 tinuity is destroyed, but it will produce no sensation ; though when 

 the spinal chord remains, its passage may be indicated by the pheno- 

 mena of reflex motion, which will be presently considered. It is 

 chiefly through the medium of the nerves of common sensation that 

 webecome acquainted with all that the contact of other palpable 

 bodies teaches, as their form, size, hardness, heat, &c. For the per- 

 ception of these properties we must suppose a peculiar condition of 

 nervous matter ; for although the laws under which impressions are 

 conducted through all the sensitive nervea to the brain are the same, 

 yet the nerves of peculiar sensation, as the optic [EYK] and the auditory 

 [EAR] are no more capable of perceiving the impressions of contact of 

 non-vibrating bodies, than the nerves of the limbs, &c., are of perceiving 

 light and sound. If the retina be touched in an operation on the cyo 

 (as in the experiments which M. Magendie has often made in operating 

 for cataract), the sensation produced is that of a brilliant flash of light; 

 if the auditory nerve be electrified, the sensation is thut of a sudden 

 loud noise ; but in neither case is paiu produced, unless the nerves 

 of common sensation, with which parts of the eye and ear are supplied, 

 be irritated at the same time. 



Our ideas of the form and size as well as of the hardness of bodies 

 are dependent in some degree on another sense, which is peculiarly 

 connected with the muscles [MUSCLE], and is called Muscular Sensi- 

 bility. By it we know what degree and extent of muscular force we 

 exert at any given time. Thus, if the hand grasps a ball, we estimate 

 its size by the degree in which the fingers are extended to inclose 

 its circumference, for the mere impression of its contact on the fingers 

 would be the same whether the ball were large or small; we estimate 

 ita weight by the muscular exertion which we feel it necessary to make 

 to prevent it from falling; and its hardness by the degree of exertion 

 which is sufficient or insufficient to make an impression on its exterior. 

 In like manner, if the body be larger than the hand can grasp, then 

 (unless we can see it) we estimate its size by the distance through 

 which the hand moves in passing over the whole surface, and its form 

 by the differences of position in which, in thus passing over it, the 

 hand it from time to time placed. It is only the smaller differences 

 in the form of the surfaces of bodies, their smoothness and roughness, 

 and other similar characters, that are estimated by the touch alone. 

 Thus, when a sensitive part (as the hand) is placed or moved with a 

 certain force on the surface of a body, we know that it is smooth if 

 all the hand receives the same sensation, or rough if the sensatiou of 

 contact is perceived at points distant from each other. In like manner, 

 from the extent of surface touched, we form our ideas of the sharpness 

 or obtuseness of.bodies. 



Perceptions of temperature are also the peculiar attributes of the 

 nerves of common sensation, but they do not admit of our forming 

 very accurate ideas of the heat of bodies, because our sensations depend 

 rathe.- on the rapidity with which heat is abstracted from or added to 

 our bodies, than on the quantity which we lose or receive. Hence we 

 feel marble, which conducts heat rapidly, as if it were much colder 

 than a slowly-conducting carpet of the same temperature ; and when 

 the difference of temperature between the hand or any other part of 

 the body and that which it touches is very great, we lose all power of 

 judging, and cannot directly tell whether it is in contact with (fur 

 example) ice or boiling water. Peculiar states of the circulation in and 

 around the nerves give rise to very different sensations of heat, as in 

 fever or in shivering, in which, though the skin feels as if it were 

 burning or freezing, the actual temperature of the surface does not in 

 either case differ more thau five or six degrees from that which is 

 natural to it. 



The acuteness of the common sensibility of parts probably depends, 

 csoteris paribus, in the healthy state, on the closeness with which they 

 are beset with the termination of sensitive nervous filaments. E. H. 

 Weber (' A nnot. Anatom. et Phys.') has suggested a mode of estimating 

 the degrees of sensibility of different parts of the surface of the body by 

 touching two adjacent points of it at the same instant, and observing 

 at what distance from each other the two contacts can be discerned 

 as two distinct impressions. This may be effected by putting small 

 pieces of cork on the points of a pair of compasses, and pressing both 

 nt the fame instant on the part to be examined ; the angles which the 

 legs of the compass form will thus measure the distances at which the 

 two impressions are distinguished or are confounded into one. By 

 this method of examination it is found that the tip of the tongue is the 

 most sensible of all the surface of tho body, being able to d stinguish 







