ingenio valemus. Generalis tamen lex, quâ commune sensorium impressiones sensorias in motorias reflectit, est nostri conservatio: ita ut impressiones externas corpore nostro noscituras sequantur certæ impressiones motoriæ, motus producturæ eo collimantes, ut monumentum a corpore nostro arceatur, amoveaturque; et vice versâ impressiones externas seu sensorias, nobis profuturas, sequantur impressiones internæ seu motoriæ, motus producturæ eo tendentes, ut gratus ille status ultro conservetur."-PROCHASKA, op. cit. p. 88. CHAPTER IV. SECONDARY NERVOUS CENTRES, OR SENSORY GANGLIA; THE HE different collections of grey matter which exist in the medulla oblongata, and at the base of the brain, the continuations of the grey matter of the spinal cord, consist chiefly of the nervous centres of the higher senses, with corresponding centres of motional reaction. Continuing the grey substance as high as the floor of the lateral ventricles, they include the optic thalami, the corpora striata, the corpora quadrigemina, and the different sensory centres that are placed in the medulla oblongata, the tuber annulare, and the cerebral peduncles. The olfactory bulls, which lie at the base of the anterior cerebral lobes, must also be included in the sensorium commune. Any one of the senses may be destroyed by injury to its sensory ganglion as surely as by actual destruction of its organ; blindness is produced by injury to the corpora quadrigemina, smell is abolished by destruction of the olfactory bulbs. These ganglionic centres are thus intermediate between the higher hemispherical ganglia above and the spinal centres below them; to those they are subordinate, to these they are superordinate. In many of the lower animals, as already pointed out, the brain consists of nothing more than the sensory ganglia, with centres of motional reaction. It is not the place here to enter into a discussion of the different opinions which have been entertained regarding the exact centres of the different senses; much of what is said on these difficult questions is still conjectural. It was maintain by Dr. Todd, who has been supported in his views by Dr. Carpenter, that the seat of common sensation is located in the thalami optici, because it is in these bodies that the anterior columns of the spinal cord seem to terminate; and that the corpora striata, to which the posterior columns of the cord pass, are the corresponding motor centres. Vulpian, however, has brought forward strong arguments in favour of assigning the seat of common sensation to the tuber annulare. After the removal of the corpora striata, the tubercula quadrigemina, and the cerebellum-the tuber annulare and the medulla oblongata being the only parts of the encephalon left-he found that dogs and rabbits evinced, by violent agitation and decided cries of suffering, the pain felt when severely pinched or otherwise irritated. Moreover, injuries of the thalami optici, pathological or experimental, do not weaken sensibility, but do often produce motor paralysis. He concludes that we are yet in entire ignorance of the special functions both of the thalami optici and the corpora striata. Notwithstanding this opinion, those who have examined the arguments on this subject will probably conclude that Vulpian's theory concerning the tuber annulare has blinded him to the import of the evidence in favour of the thalami optici and the corpora striata as sensory and motor centres respectively. It may well be that they are not the entire centres, and that there are other centres of sensibility and motion in the tuber annulare and cerebral peduncles; but that they do minister to those functions it is hardly possible to doubt. Meanwhile all that concerns us here, in dealing with the cerebral functions from a psychological point of view, is to have some general term to embrace and designate all the centres of sensation; and for this purpose we shall employ the term sensorium commune, using it to denote the common centres of sensation, and not, as Vulpian and some others have misused it, to designate the centres of common sensation. In a similar sense we shall subsequently use the terms motorium commune and intellectorium communc. The ganglionic centres of the sensorium commune are formed of numerous nerve-cells, which, like those of the spinal cord, are in connexion with afferent and efferent nerves; the afferent nerves in this case coming mostly from the organs of the special senses. The impressions which the afferent nerves bring are, therefore, special in kind, as also are the grey nuclei to which they are brought; a progressive differentiation of structure and function is manifest; and we might describe the sensorium commune physiologically as a spinal cord, the afferent nerves of which are the nerves of the special senses, or rather of the various kinds of sensibility. For although we usually distinguish only between the special senses and general sensibility, yet there are really different kinds of the latter, each probably having its special nucleus: the tactile sense, the sense of temperature, the muscular sense, and the peculiar sensibility of the glans penis, differ not in degree only, but in kind. An exact knowledge of the anatomical relations of the different grey nuclei is still wanting, notwithstanding the patient investigations of Schroeder van der Kolk. All that we are certain of is, that the fibres of the nerves are connected with the cells, as may be most easily seen in the case of the auditory nerve and ganglion; that manifold connexions exist between different nuclei; and that fibres may sometimes be traced from the nucleus of a sensory nerve to a motor nerve upon which it is known to exert a reflex action. The trigeminus, or fifth nerve, for example, passes from above downwards through the medulla, and in its downward course forms reflex connexions with all the motor nerves of the medulla as it approaches the level of their nuclei ; in this way the facial, the glossopharyngeal, the vagus, the spinal accessory, and the hypoglossal receive communications from it. The ganglionic cells of different nuclei also differ in form and size; and Schroeder van der Kolk holds that, as a general rule, at every spot where fibres are given off for the performance of any special function, there fresh groups of ganglionic cells giving origin to them appear. We justly conclude, then, that, as we should à priori expect, specially constituted ganglionic cells minister to special functions; that the central cells are, as it were, the workshops in which, on the occasion of a aitable stimulus, the peculiar current necessary for the perfomance of the specific action is excited. A message is sent up to them by the appointed channels, and they reply by sending through the regular motor channels the particular energies which it iseir function to supply. Charged with their proper force duri: the assimilating process of nutrition, it exists in them as statical power, or latent energy; and the condition of unstable vital equilibrium is upset, the force being then discharged, as the Leyden jar is, when a certain stimulus meets with a sufficient tension. The natural course of a stimulus, all the force of which is not reflected upon an efferent nerve in the spinal centres, is upwards to the sensorium commune, where it becomes the occasion of a new order of phenomena; the law of extension of reflex action excited by a spinal nerve observably being, as Pflüger has shown, from below upwards to the medulla. Having arrived at the ganglionic cells of the sensorium commune, the stimulus may be at once reflected through the motor nuclei on a motor nerve, for which there is provision in a direct physical path, and involuntary movements may thus take place in answer to a sensation, just as involuntary movements take place from the spinal centres without any sensation. The ganglionic cells of the sensory centres are unquestionably centres of independent reaction, and in association with their proper motor nuclei give rise to a class of reflex movements of their own. When a man lies with the lower half of his body paralysed in consequence of injury or disease of his spinal cord, the tickling of the soles of his feet will sometimes produce reflex movements of which he is unconscious. When a man lies with no paralysis of his limbs, but with a perfectly sound spinal cord, the sudden application of a hot iron to his foot or leg will give rise to a movement quite as involuntary as that which takes place in the paralysed limb, but, in this case, in answer to a painful sensation; the reaction takes place in the sensory ganglia, and the movement is sensori-motor. Had the hot iron been applied to the paralysed limbs, no movement would have followed, because the path of the stimulus was cut off as completely as the current of the electric stimulus is interrupted when the telegraphic wires are cut across. Take away that part of the brain of an animal which lies above the sensory ganglia, and it is still capable of sensori-motor movement, like as the animal which possesses no cerebral hemispheres is because the ganglioni cell is a centre of independent reaction—a station on the line which may either send on the message or send off an answer. (1) Make a complete transverse section of the nervous "Innumerable Mr. James Mill clearly recognised this class of movements. facts are capable of being adduced to prove that sensation is a cause of muscular |