12, eighth pair; 13, hypoglossal; 14, spinal nerves; 15, spinal accessory of right side, separated from par vagum and glosso-pharyngeal. Fig. 160 (on the following page), the sensory tract (from Sir C. Bell). A, pons varolii; B, B, sensory tract separated; C, union of posterior columns; D, D, posterior roots of spinal nerves; E, sensory roots of the fifth pair. The sensorium. The ganglia at the base of the brain are regarded by Dr. Carpenter as constituting the true sensorium, a doctrine which he has established by many weighty arguments, and which is doubtless one of the most important thus far introduced by any physiologist. The idea here intended to be conveyed is, that the thalami, striata, sensory ganglia, and nervous arrangements below, constitute an isolated. apparatus; distinct from which, and superadded, are the cerebral hemispheres. From observations on the animal series, the conclusion seems to be un avoidable that the chain of ganglia now under consideration must constitute a sensorium, the centripetal fibres communicating their impression and motion ensuing, the impressions being attended with consciousness. This view is moreover substantiated by observations made after excision of the cerebrum, a certain degree of consciousness remaining, not unlike that exhibited by a man who is half asleep. This condition of things is naturally presented in the amphioxus. dition of the cerebrum. But after the cerebral hemispheres are added, an impression received Effect of the ad- upon the thalamus, whether it has come in through the sensory ganglia, or any other sensory part of the cranio-spinal axis, is transmitted to the convolutions along the radiating fibres. From the convolutions, the influence which is to produce motion descends along the converging fibres to the striatum, thence along the inferior layers of the crus, through the mesocephalon to the anterior pyramids, and by their decussation to the opposite side of the cord. Such is the view which Dr. Carpenter presents of the functions of the sensory ganglia and spinal axis; or, employing the terms we have previously defined, the cord alone is a longitudinal series of automatic arcs; on the addition of the thalamus and striatum, it becomes a compound registering arc, the cerebral hemispheres finally annexed to it constituting an influential arc. In a simple arc, an impression is at once converted into motion, and leaves behind it no traces; its expenditure is instantaneous and complete. In a registering arc, a part of the impression is stored up or remains It is not to nay, even the whole of it may be so received and retained. be overlooked that, as soon as this effect occurs, the evidences of sensation arise; and, since sensation necessarily implies the existence of ideas, ideas themselves are doubtless dependent on this partial retention or registry of impressions. We may therefore adopt the doctrine of Dr. Carpenter, as regards the sensorial functions of the cranio-spinal apparatus, not only from the arguments he has presented, but also from other considerations. in the size and There can be no doubt that the cerebral hemispheres constitute the instrument through which the mind exerts its influences on the General result body. Any injury of sufficient severity inflicted upon them of variations is at once attended with a total loss of intellectual power; weight of the any malformation or lesion by disease is attended by a dete- hemispheres. rioration below the customary mental standard; any unusual development with correspondingly increased powers of intellection; and this not only as regards animals of different tribes, or individuals at special periods of their lives, but also of different men when compared with one another. The general impression is founded in fact that those who have distinguished themselves for mental attainments or intellectual power have been marked by the unusual development of their cerebral hemispheres. It is to be understood that, in thus asserting a correspondence between the development of the cerebrum and intellectual capability, Instrumental we are not to overlook the instrumental nature of that organ. nature of cereThough imperfections in it may produce a manifest inferior brum. ity, that inferiority is by no means to be referred to the intellectual principle itself. The mode of action being by an instrument, if that instrument becomes imperfect the action becomes imperfect too. Under such circumstances, in any human contrivance, we should never think of imputing inferiority to the prime mover. From this point of view we may therefore consider the intellectual principle as possessing powers, properties, and faculties of its own; as being acted on by impressions existing in the thalamus, and delivered through the intervening fibrous structures to the vesicular material of the convolutions of the cerebral hemispheres. In this region they act upon the intellectual principle and are acted upon by it, the returning influence, if any, coming down through the converging tubular structures to the corpus striatum, and by its commissural connections sent off to particular ganglia, passing along the inferior strand of the crus through the mesocephalon to the anterior pyramids, and by their decussation to the opposite side of the cord. Having thus spoken of the sensory ganglia and the cerebral hemispheres, it remains to add some remarks respecting the cerebellum. It X 322 lum. STRUCTURE OF THE CEREBELLUM. The cerebel- arises, as has been stated, from the triple strand of the crus cerebelli, of which one layer of fibres is connected with the corpora quadrigemina, and through them with the optic thalami; a second with the restiform bodies; and the third is commissural, and passes forward as the pons varolii. Like the cerebrum, this organ is vesicular on its surface, which presents a number of parallel lines, which are fissures descending to the interior. Their object is apparently the same as that of the convolutions of the brain, the augmentation of surface. Of these fissures, the deep are termed the primary: they divide the organ into lobes. Those which descend to a less depth are termed secondary: the divisions they give rise to are lobules. The gray vesicular material does not, however, descend to the bottom of the primary fissures, and in this respect they differ from the cerebral convolutions. Moreover, from this circumstance, that material is not continuous all over the cerebellum, but is in divided portions. Structure of Such are the appearances presented on an exterior examination of the cerebellum. Viewed as a development upon the crura the cerebellum. cerebelli, it may be described as consisting of a median lobe and two hemispheres; the former is, however, found existing alone in fishes and reptiles, the latter being subsequently added in the higher tribes. From the central column of each hemisphere white fibrous planes are given off, and from these, again, secondary, and again, tertiary planes proceed. The planes are covered with vesicular matter, and thus give rise to the appearance spoken of in the preceding paragraph, in the exterior examination of the cerebellum, as primary and secondary fissures. They are lined with pia mater. The median lobe is formed on the same plan. Its fibrous stem comes from the processus cerebelli ad testes, or, more properly, from the optic thalamus. The weight of the cerebellum, compared with that of the cerebrum, is usually stated as being about 1 to 8. Much diversity of opinion prevails respecting the true function of the cerebellum, some supposing that it is the centre of common sensation, others that it is for the purpose of co-ordinating muscular movement, and others that it is the seat of sexual instinct. That the cerebellum is one of the sensory ganglia may be inferred from Function of the the history of its development and its anatomical conneccerebellum. tions. Its median lobe is the first to appear, as in fishes, and the hemispheres arise subsequently as appendages thereto, as in birds. The size which these eventually attain gives them a deceptive prominence, and hides their subordinate character. Regarding the lobe, therefore, as the essential and fundamental portion of the structure, the significance of its cerebral connection with the thalamus through the processus ad testes is too obvious to be overlooked. As by this its senso FUNCTIONS OF THE CEREBELLUM. 323 ry character is displayed, so the same holds good for the hemispheres, their relations with the spinal cord through the restiform bodies being also of a sensory nature. It seems probable that the superficial vesicular material is in anatomical connection with the thalamus, and the corpus dentatum or inner ganglia with the posterior or sensory columns of the cord. nates muscular The arguments which have been brought forward by those who suppose the cerebellum to have for its office the co-ordination of The doctrine general muscular movement, may be briefly quoted as fol- that it co-ordilows: There appears to be a general correspondence between motion. its size and the degree of energy and complication of the motor powers in various animals. Thus, in fishes, and likewise in birds, those tribes which excel in their powers of motion, or are distinguished by the complication of their movements, are characterized by the manner in which this organ is developed; and the same may be said even of the mammalia, quadrupeds whose locomotive mechanism is simple possessing it in a lower state of development than those which either temporarily or constantly move on the posterior extremities. Among apes, those which more frequently assume the erect posture, which is normal to man, have their cerebellum of a size more closely approaching to his. On examining such facts, it appears that it is not so much muscular power as the quality of co-ordinating and governing minute muscular motions. To maintain the standing position motionless, there are, in reality, a great many muscular movements required, which serve to antagonize all the little incidents producing a tendency to fall; and if this be so in standing, how much more difficult must such antagonizing and compensating actions become in walking, running, and such movements. Theoretically, it might be expected that some special organ is necessary to combine such various actions, and that organ seems to be the cerebellum. the cerebel In confirmation of this are the experimental results which have been obtained. The cerebellum, on irritation, gives rise to no Results of exconvulsive motions, nor to sensations. If removed by de- periments of grees in successive slices, the motions of the animal become lum. irregular, and, finally, it loses all power of walking or of maintaining its equilibrium. Though the powers of the animal in bringing its muscles into contraction seem not to have suffered, it can not co-ordinate or combine the necessary muscular exertions, and, as is graphically stated, staggers and falls over like a drunken man, still making efforts to maintain its balance. Such experiments have been repeatedly made in the case of different animals, and with the same results. Connected with these results of experimental lesions of the cerebellum are the rotations, as they are termed, which occur, for example, when one |