SYSTEM.] PHYSIOLOGY 37 rare result than loss of motion, and is always on the opposite side. According to Brown-Sequard, tactile, thermal, and painful impres sions pass through the central part of the pons. The numerous centres in the pons are associated in complex reflex movements. Nothnagel has described it as a convulsive centre, because irritation caused severe cramps, but this was no doubt due to irritation of the motor strands passing through it. Cerebral Peduncles. These contain both sensory and motor fibres, and they establish a connexion between the cerebellum and the cerebrum, and also between the ganglia at the base, corpora striata, optic thalami, and corpora qnadrigemina OH the one hand, and the pons and medulla on the other. Little is known of their functions except that they are conductors. Destruction of one peduncle causes the animal to move to the side opposite the lesion, describing a circle somewhat in the manner of a horse in a circus. Irritation may cause pain or movements of various groups of muscles. Basal Ganglia. As already shown in tracing the development of the cerebro- spinal system, the brain consists of a series of gan glia, in pairs, more or less overlapped by the cerebral hemispheres. These ganglia, termed the "basal ganglia," are usually held to include, from behind forwards, the corpora quadrigemina, the optic thalami, and the corpora striata ; but in addition there are bodies meriting an equal amount of attention, inasmuch as they cannot be regarded as belonging to the cerebral hemispheres. These are the locus niger, the red nucleus of the tegmentum, and the corpora genieulata ; but we have no knowledge of their functions. No doubt all these ganglia act along with the cerebral hemispheres, so that practically the whole mass forms one organ. Corpora Quadrigemina. These are two pairs of rounded bodies found above the Sylvian aqueduct, which passes between the third and fourth ventricles. They are situated behind the optic thalami, and are intimately related to the crura, and through these to the pons, medulla, and cord. Homologous with the optic lobes seen in the brain of the fish, frog, and bird (see figs. 17 and 18), and in marsupials and monotremes, their relative size to the mass of the encephalon is much less in the brain of man and of the higher animals. These bodies contain grey matter, covered by a thin stratum of white matter. The two posterior bodies are probably connected with the cerebellum by the superior peduncles of that organ ; at all events these peduncles disappear under the base of the corpora quadrigemina. The two posterior bodies are also related to the crura cerebri by the prominences on the sides of the crura known as the inner geniculate bodies. Both anterior and posterior bodies, more especially the anterior, are connected with the optic tracts, and finally, the two anterior bodies unite with the optic thalami. (See fig. 24 below ; also, plate XVIII., vol. i., fig. 1, g, g.) As shown by their anatomical connexions, the corpora quadri gemina are part of the mechanism of vision. Destruction caxises immediate blindness. If, in a pigeon, the encephalon be removed with the exception of these bodies, the iris will still continue to contract on the influence of light. On then destroying one of these bodies, the iris is immobile, and the power of accommodation is lost. As the third cranial nerve (which is known to contain fibres controlling the circular fibres of the iris by which the pupil con tracts, and the fibres governing the ciliary muscle by which the eye is accommodated or focused to varying distances) originates in the grey matter of the floor of the Sylvian aqueduct, close to the corpora quadrigemina, it is held that these bodies are the centres of the reflex movements of the iris and of the ciliary muscle. The corpora quadrigemina are also the first recipients of visual impres sions. When light falls on the retina changes are there induced which stinralate the optic nerve-fibres, and these fibres carry impres sions through the optic tracts to the corpora quadrigemina. What then occurs is matter of conjecture. Whether sensation is there excited, or whether to produce sensation it is necessary that the impulses be sent onwards to the cerebrum, or whether the impres sions directly received from the retina may excite, through the corpora quadrigemina and adjacent ganglia, reflex movements (like those of the somnambulist, who may see so that his steps are taken rightly, but who may at the same time not see consciously), are all speculative questions. We know that these bodies are concerned in the movements of the iris and of the ciliary muscle, but their great proportionate size in lowly -formed brains indicates that this is probably a secondary function, and that they are largely con cerned in the phenomena of consciousness of light and colour. Optic Thalami. These are two ganglionic masses placed behind the corpora striata and in front of the corpora quadrigemina. The internal surfaces are seen chiefly in the third ventricle, the upper surfaces in the same ventricle and the lateral ventricles (see vol. i. pp. 875, 876, figs. 74 and 75), whilst the external and under surface of each thalamus is united with other parts of the brain. The under surface receives fibres from the crus cerebri, whilst the upper surface is covered by fibres which diverge and pass between the thalamus and a mass of grey matter in the extra-ventricular portion of the corpus striatum (called lenticular nucleus), to form a white layer called the "internal capsule." From the internal capsule, which thus contains fibres from the optic thalamus, fibres radiate outwards to the surface of the cerebral hemispheres. The under surface of the thalamus is connected with the tegmentum, that is, with the layer of fibres forming the upper surface of the crus cerebri. They also receive fibres from the corpora quadrigemina, and according to some authorities from the superior peduncles of the cerebellum. The substance of the thalamus contains nerve- cells, scattered and also aggregated into two nuclear masses, but the relations of these to nerve -tracts have not been ascertained. It is important physiologically to notice that the thalami receive fibres from the back of the crura, and therefore are probably related to the posterior or sensory portion of the spinal cord. There is still much uncertainty as to the functions of the optic thalami. The most commonly received opinion is that they are centres for the reception of peripheral impulses, which they may elaborate and transmit forwards to the corpora striata, or directly to the cerebral hemispheres. If the sensory impulses received by the optic thalami are sent to the corpora striata, and by these trans mitted downwards and outwards through the crura cerebri, then reflex actions may occur in which the basal ganglia are the centres ; but, if the impulses are sent up, in the first place, to the cerebral hemispheres, and by these transmitted down to the corpora striata, then the action must include the higher mechanism of the grey matter of the hemispheres. In the first case it is supposed by those who hold that consciousness is specially connected with the grey matter of the hemispheres that the action would be purely reflex and unconscious. Experiment has not thrown much light on this problem, owing to the deep-seated situation of these bodies rendering the results of operative interference untrustworthy. The little that has been done shows that injury to them does not cause paralysis of motion. Nor can it be said that such injuries cause loss of sensation, the only phenomenon observed being that the animal places its limbs in anomalous positions, and does not seem to be aware of having done so. Meynert is of opinion that the optic thalami fulfil the same functions as to tactile impressions that is, impressions on the periphery of the body that the corpora quadri gemina do for visual impressions that is, impressions on the retina. In cases of apoplexy in which these bodies are involved there are always sensory disturbances on the side opposite the lesion. This would lead to the inference that the optic thalami are the sensory ganglia of the opposite sides of the body. They are not, however, the first ganglionic apparatus through which sensory impressions pass, but they probably co-ordinate in some way centripetal impulses before these are sent to the cerebral hemispheres, where they are correlated with feeling. Further, as the old name " optic thalami " indicates, these ganglia are concerned in some way in vision, because, if seriously injured, blindness, or at all events disturbance of vision, is one of the constant results. This favours the view that they are the " middlemen " between special sensory centres and the higher centres of the cerebrum. Corpora Striata. These ganglia, sometimes termed the " ganglia Corpora of the cerebral hemispheres," situated in front and on the outer striata. side of the optic thalami, are seen in the lateral ventricles. (See vol. i. pp. 875, 876, figs. 74 and 75.) The greater part of each is imbedded in the white substance of the hemisphere (extra-ventri cular portion), whilst the part seen in the floor of the lateral ventricle is called the intra-ventricular portion. Each of these contains a nucleus of grey matter, the nucleus caudatus in the intra-ventri cular and the nucleus lenticularis in the extra-ventricular. The latter is separated internally from the intra-ventricular portion by a layer of white matter called the " internal capsule," whilst on the outer side there is another layer of white matter called the " external capsule," beyond which, again, is a lamina or web of grey matter, called the "claustrum," which separates the external capsule from the island of Reil. The internal capsule is of great importance inasmuch as it is continuous with the crusta, a portion of the crus cerebri, which, in turn, is a continuation of the pyramidal fibres of the medulla oblongata and the pons. Multipolar nerve- cells are found in the nucleus caudatus ; in the claustrum the cells are small and spindle-shaped. Posteriorly, therefore, the corpus striatum is related by fibres with the optic thalamus ; iuferiorly, through the internal capsule, with the pyramidal portion of the medulla and cord ; and externally and superiorly with the grey matter of the cerebrum. The corpus striatum is a centre for the co-ordination of centrifugal or motor impulses. It may be roused into activity by impressions reaching it directly from the optic thalamus, but probably it usually acts in obedience to impulses coming from the cerebral hemispheres. When a clot of blood is formed in, say, the right corpus striatum there is motor paralysis of the opposite side of the body, and, according to the size of the clot, the paralysis may affect more or less completely the different groups of muscles. Destruction of the two bodies destroys voluntary movement, but the animal may move forwards as in running. De struction of the nucleus caudatus renders movements of progression impossible, and the animal performs movements of rotation. Noth nagel by injecting a minute drop of a solution of chromic acid destroyed the nucleus lenticularis of a rabbit, with the result of
throwing the animal into complete unconsciousness. He also states