Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/49

Rh SYSTEM. ] PHYSIOLOGY 39 I ires - s is f n eye. impression, such as holding the bird, supporting its beak, or hold ing a brilliant light before its eyes, will steady it, and it has been noticed that it can, even without assistance, clean its feathers and scratch its head, its beak and foot being in these operations guided by contact with its own body.&quot; It has been supposed that the semicircular canals are concerned in the mechanism of equibration, a view urged chiefly by Mach and Crum- Brown. If a blindfolded man is seated on a horizontal rotating table, such as that used in a lighthouse for rotating or eclipsing the light, and the table is turned round, at first there is a sensation of movement in the same direction as that of the table ; then this sensation fades away, until he has no sensation of movement, although the table may be rapidly rotating ; finally, if the table be stopped without a jerk there is first a very short period in which there is no definite sensation, which is succeeded by a sense of rapid movement in the opposite direction, often accompanied by a feeling of nausea. Now it is evident that neither sight nor touch nor muscular sensations can give a sense of rotation in these circumstances, and yet it is possible to form a fairly accurate judgment of the angle through which the body has moved. It has been suggested that this is effected by the action of the semicircular canals. The membranous por tions are surrounded by a fluid called the endolymph, &quot; and are free to move through a short distance. Hence it has been supposed that rotations more or less rapid must cause variations of tension of the membranous portion. Thus, if the membranous part, especially the ampulla or dilated ends of the canals, lag behind when rotation in one direction takes place, the nerves ending in them will be subjected to a strain ; by and by both membranous portions and surrounding parts will be moving with the same velocity, when there will be no strain and no sense of movement ; and at last, when the rotatory movement is arrested, there will be a tendency on the part of the membranous portions, as they are floating in a fluid, to move on a little farther than the rest, and thus again produce a strain, causing a sensation of movement in the opposite direction. By similar reasoning it can be shown that if we take the peculiar position of the three canals into consideration any movement in space might be thus appreciated, and these appreciations enter into the judgment we form of the movements. According to this theory, the sense of equilibrium may be largely due to impressions derived from the position of the head, and, as muscular movements required for placing the body in definite positions are determined, as we have seen, by peripheral impressions, the irregular movements of the pigeon, after injury to these canals, may be accounted for. It is not improbable that in the bird, which from the structure of its extremities feet and Mings can have no peripheral impressions so delicate as those derived from the papillaj of the skin on the extremities of other animals, the sense of equilibrium is maintained chiefly by impressions from these canals, and this may account for the comparatively large size of these organs in birds and fishes. This is in correspondence also with the requirements of birds in the balancings of flight and of fishes in swimming. It is well known that disease or injury of these canals in the human being produces symptoms of vertigo and a diminution of the power of co-ordinated action, as in Meniere s disease, showing that the canals, even in man, have similar functions to those in the bird. 2. Peripheral Impressions from the Eye. Many movements are guided and controlled by the sense of vision. Simply blindfolding a bird usually makes it passive, and it will not attempt either to walk or to fly ; the same effects to a less degree may be seen in a mammal ; and a blindfolded man will stagger in his gait. The wonderfully accurate movements of the blind in walking are ac quired by long and laborious effort, and are guided by the sensa tions of hearing, of touch, and of resistance. If the optic lobes of a frog be destroyed, its power of balancing itself is lost. There are thus at least three channels by which peripheral impressions pass to the centres and seem to guide or co-ordinate movement: (1) from the periphery, by nerves of ordinary sensibility arising in the skin, muscles, and viscera ; (2) from the semicircular canals of the ear, by special nerve-fibres in the auditory nerve ; (3) from the eye, by fibres of the optic nerve. How and where these skeins of sensi tive impressions are gathered up and so arranged as to call forth the requisite movements can only be conjectured ; but the cerebellum is the organ most likely to be concerned in such a mechanism. It is in organic connexion with many of the nerve-fibres conveying sensory impressions. By the restiform bodies it receives many of the sensory fibres of the spinal cord ; the auditory nerve has roots intimately related to the cerebellum ; and it is fair to assume that there are communications between the corpora quadrigemina and the cerebellum. Stimulation of the cerebellum causes movements of the eyeballs, and disease of the cerebellum is sometimes attended by blindness. How the cerebellum co-ordinates movement is quite unknown, and the difficulty in explaining its functions is not lessened by the clinical fact that extensive disease of this organ may exist without any appreciable sensory or motor disturbance. There is no evidence to support the view of the founders of phrenology that the cerebellum has to do with the sexual functions. Cerebral Hemispheres. As these have been fully described in vol. Cls i. p. 873, it is only necessary here to point out the anatomical facts that assist in explaining the functions of the organ. It is import ant to observe, first, the general arrangements of the fibres, and, secondly, the arrangement and structure of the grey matter. The white matter of the cerebrum consists of ascending or peduncular fibres, longitudinal or collateral fibres, and of transverse or corn- missural fibres. (1.) Peduncular Fibres. The crusta of the cerebral peduncles Pedun- consists of bundles of longitudinal fibres de rived mainly from the anterior pyramid of the medulla. The crust is quadrilateral in form, but in ascending to the hemispheres it becomes flattened from above downwards, so that the fibres spread out like a fan. The fan formed by these fibres is bent into the form of an incom plete hollow cone, the IK convex surface of which is directed upwards and inwards. Thus the fibres } pass between the optic- thalamus and the lenti cular nucleus, forming the internal capsule. Higher up the fibres pursue their course be neath and to the outside of the thalamus and the caudate nucleus, and over the lenticular nu cleus. Still higher up the internal capsule has spread out from before backwards, while the anterior half forms an obtuse angle with the posterior. The angle where the halves meet is called the knee (fig. 27, K), while the divi sions themselves are called the anterior (fi pJi FIG. 27. (After Flechsig.) Horizontal section of brain of child nine months old, only a portion 2/, IK) and posterior of the right side being shown. F, frontal, TS, temporo-sphenoidal, and 0, occipital lobes ; op, operculum ; In, island of Reil ; Cls, claustrum ; /&quot;, third frontal convolution ; Th, optic thal amus ; AT, caudate nucleus; A T C&quot;, tail of caudate nucleus ; LN, lenticular nucleus ; II, III, second and third divisions of lenticular nucleus ; EK, external capsule ; IK, posterior division, IK , anterior division, and K, knee of internal cap sule ; ah, ph, anterior and posterior horns re spectively of lateral ventricles ; gcc, knee of corpus callosum ; sp, splenium ; me, middle com missure ; /, fornix ; si, septum lucidum. segments of the internal capsule &quot; (Ross). On emerging from the basal ganglia the fibres of the internal capsule radiate in all directions to reach the cortex of the brain, giving rise to the appear ance called the &quot; corona radiata.&quot; The following sets of fibres have been traced into connexion with the cerebrum. Sensory peduncular fibres, derived from the posterior root-zones and the columns of Goll. These are in connexion with the cerebellum ; but, as shown by Meynert, Flechsig, and others, many pass up through the pons to reach the crus cerebri, occupying the posterior and external portion of the pyramidal tract. They do not appear to be connected with the optic thalamus and the lenticular nucleus, but pass between them to the cortex. (6) Fibres from the roots of the optic nerves, reaching the brain by what have been called the &quot;optic radiations of Gratiolet.&quot; This bundle of fibres issues from the posterior and external border of the optic thalamus and is closely applied to the pedun cular sensory tract in its passage through the internal capsule, and the fibres seem to be connected with the convolutions of the occipital lobe. It is im portant to note that at least one of the roots of the optic nerve (the internal) passes into the external geniculate body and thence into the anterior bodies of the corpora quadrigemina. Thus there is a path for the passage of impressions from the retina to the cortex of the brain, (c) Fibres from the olfactory lobes have been traced to a junction with the optic radiations of Gratiolet, and pass with them to the convolutions of the cortex of the occipital or temporo-sphen oidal lobe, (d) The pyramidal tract has already been traced through the spinal cord, medulla, and pons. It then goes on, as already seen, to form the internal capsule and the corona radiata, the fibres ending in the parietal lobule, the paracentral lobule, the superior extremities of the ascending frontal and parietal convolutions, probably the posterior extremity of the first frontal convolution, the posterior extremity of the third frontal, and the inferior extremities of the ascending frontal and parietal convolutions, in short, in the convolutions forming the middle and parietal region of the surface of the cortex, (c) Fibres issuing from the external surface of the optic thalamus to join the internal capsule. These are distributed to the convolutions of the frontal and parietal lobes, (f) Fibres issuing from the external surface of the caudate nucleus, passing also into the corona radiata. (g) Fibres issuing from the superior and internal surface of the lenticular nucleus to join the ascending fibres of the internal capsule, (h) Fibres ascending from the superior peduncle of the cerebellum. Flechsig says that some of the fibres of the superior peduncle of the cerebellum of the opposite side pass uninterruptedly through the red nucleus and along the internal surface of the fibres of the pyramidal tract to