The grey matter which covers the surface of the cerebellum, its cortex, is singularly regular in microscopic pattern ([Fig. 23]). It is divided into three sheets: superficially, the molecular layer in which the dendrites of the cells of Purkinje branch; beneath this, the thin layer in which are situate the cell-bodies of these neurones; thirdly, the layer of small cells, or granules. Cells of Purkinje and granules have been already described ([p. 303]). To these must be added the stellate, bracketing cells of the molecular layer, the axons of which divide to form baskets about a number of Purkinje-cells, and the cells of Golgi of the granular layer. These last are comparatively large cells, which have thornless dendrites, and axons which branch repeatedly in the granular layer, without passing into the white matter which underlies the cortex. Two kinds of nerve-fibre bring impulses to the cortex: (1) “Mossy” fibres, which bear rosettes of filaments which distribute impulses to the granules; and (2) “climbing” fibres or “tendril” fibres, which, passing through the granular layer, cling like ivy to the trunk and principal boughs of the dendritic processes of Purkinje-cells. The axons of the cells of Purkinje undoubtedly carry impulses away from the cortex, but their destination is not certainly known.
The uniformity of structure of the cerebellum suggests that it “acts as a whole.” Anatomy gives no warrant for the expectation that work of different kinds is done by its several lobes. Its simplicity leads one to hope that its mechanism may some day be understood; but at present there are so many gaps in our knowledge that it is difficult, perhaps hardly profitable, to attempt to string together the few anatomical facts of which we are sure.
By means of tracts of afferent fibres the cerebellum has a very extensive connection with the grey matter of the cerebro-spinal axis (including the optic thalamus) into which sensory impulses of all kinds are poured. Experimental results indicate that the organ distributes impulses to the whole length of the cerebro-spinal axis, from the level of the neurones which govern the muscles which move the eyes to its far hinder end. No nerve-roots enter it. Its afferent fibres are the axons of cell-bodies which lie in the posterior horns of the grey matter of the spinal cord and in the corresponding grey matter of the axis of the brain, especially that part related to the nerve from the semicircular canals. Another set of afferent fibres lies at the periphery of the spinal cord, forming one of the best defined of the spinal tracts. It is also one of the oldest, being found in the same situation in all vertebrate animals. Its fibres, which are exceptionally large, are the axons of cells which form a very definite column—the “vesicular column of Clarke”—on the median side of the posterior horn. Further than this we cannot go. We are ignorant of the nature of the sensory impressions collected by the cells of Clarke. The cerebellum also receives through its middle peduncle the axons of cells which lie in the pons Varolii on the opposite side; which cells are discharged by impulses descending from the cortex of the great brain. It is not improbable that it gives to the great brain as many fibres as it receives from it.
If we had no experimental evidence as to the part which the cerebellum plays in the harmonious working of the whole nervous system, we should infer from its structure and connections that it is somewhat mechanical, a co-ordinator of the activities of other parts rather than in itself a functionally independent organ. Pathological and physiological observations very definitely justify this conclusion. They show that the cerebellum is not essential to life. It may be completely destroyed by disease or removed by operation without robbing the individual of any single function or capacity. Disease of the cerebellum does not diminish the patient’s sensitiveness to every kind of stimulus, nor does it deprive him of the use of any single muscle; but it reduces him to the condition of a person who in gait, but not in mind, is habitually drunk. When he walks he staggers from side to side; when he stretches out his hand it trembles. His movements are jerky; his head shakes, his eyes oscillate; he suffers from a feeling of giddiness; his speech comes haltingly. Cerebellar ataxia, which is a rare disease, resembles in many respects the much commoner “locomotor ataxia” produced by disease of the spinal ganglia and the parts of the cord connected with posterior roots; but careful analysis of the symptoms shows that they are due, not to want of the sensations which guide movements, but to inability to regulate the force of muscular contractions. A man suffering from locomotor ataxia falls when he closes his eyes, because, not being able to feel with his feet, he is dependent upon vision for information as to his attitude. When the cerebellum is diseased, the patient is no less unsteady with his eyes open than he is with them closed.
The results of cerebellar disease or injury bring home to us the fact that a nice adjustment of movements is needed to maintain equilibrium. A dog from which the cerebellum has been removed retains all its natural enterprise, all its instincts, all its emotions; but every action which requires it to maintain its centre of gravity in an unstable position gives it trouble. Placed in water so that its body is supported, it swims almost as well as a normal dog. It is, however, easy to lay too much stress upon the balancing function of the cerebellum. The disturbance of this function attracts our attention; yet it is probably but the indirect result of the suppression of activities of a more widespread character. No animal ventures such liberties with its centre of gravity as the biped Man accomplishes, without thinking, every time that he descends a flight of stairs. Yet the cerebellum of the limbless whale, that lives in a medium which decentralizes its gravity, so to speak, bears the same proportional relation to the rest of the nervous system as that of Man. Strangely enough, it is the only cerebellum in the animal kingdom which so closely resembles Man’s that it might be passed off as belonging to a human giant; another reminder of the difficulty of deducing the functions of the several parts of the organ from a study of their relative development. What have a man and a whale in common which determines the identity in form of their cerebella? How has it come about that two cerebra as widely unlike as a man’s and a whale’s should be associated with a common form of cerebellum?
If we apply to grey matter the distinction between sensory and motor nerve-tissue—having no exact terminology, it is difficult to avoid these metaphorical expressions—the cerebellum is essentially a sensory development. It grows from the very margin of the infolding groove, which, when closed, becomes the central canal of the brain and spinal cord, its elements being marshalled in intimate association with sensory root-fibres. Its millions of loops formed by the axons of granules and the collecting processes of Purkinje-cells, are by-paths which tap the conductors of sensory impulses. From some—those, for example, which originate in the muscles and tendons, and in the semicircular canals—more of the impulse is diverted to the cerebellum, from others less. The organ has no motor functions. It does not discharge neurones which control skeletal muscles, or plain muscle, or glands. Yet it influences the passage of impulses through sensori-motor chains, and apparently its influence is universal. It regulates tone, reflex action, voluntary action. There is no part of the nervous system over which its control is not felt. By its action on the apparatus which binds the infinity of receptors which the body contains to its muscle-fibres and other effectors, it unifies the body. The cerebrum, as we shall see, is the organ which unifies the personality. In the progress of evolution two functions which were originally combined have, for convenience of concentration, been divorced. The great brain has been set free from the more mechanical part of the work. That it can perform the functions of the cerebellum as well as its own is proved in cases of congenital deficiency of that organ. In several instances malformation, amounting to a very considerable reduction in the size of the cerebellum, was not detected until after death, there being no symptoms of a sufficiently pronounced character to call attention to it during life.
The Cerebrum.—All observations made on the great brain prior to 1870 showed it as absolutely inexcitable. Surgeons and physiologists agreed that cutting, burning, passing electric currents through its substance, neither yielded evidence of sensation nor movement of any part of the body. Concerning its structure little was known beyond the fact that whereas the grey matter, or cortex, which covers its surface contains nerve-cells, only fibres are to be found in the white matter which constitutes the greater part of its bulk. It seemed a hopeless task to attempt to make anything out of a mass of tissue so uniform in constitution and so irresponsive to experiment. Removing portions of it appeared to cause a general dulling of the intellect without loss of any particular mental quality. Physiologists, therefore, spoke of the cerebrum as “functioning as a whole.” Phrenologists, having classified the various phases of mental activity as “faculties,” discovered “bumps” on the surface of the skull which they correlated with the possession of the several faculties in a marked degree. They parcelled out the brain in organs concerned with different kinds of thought; but their localization of function was anatomically as baseless as their classification of the various aspects of mind, viewed as a system of philosophy, was absurd. In 1870 it was announced that electrical stimulation of certain areas of the cortex of the cerebrum of an animal under the influence of an anæsthetic, and therefore incapable of voluntary action, induces definite movements. Although the surgical applications of this discovery have proved immensely important, its physiological value, as affording a method of investigating the functions of the brain, is extremely small. Yet the discovery gave an impetus to the further study of the cortex, which has been rewarded with many exact results. By the discovery of its excitability to electric currents it was proved that the whole cortex has not exactly the same work to do, or—perhaps this is the safer form of statement—does not do its work in exactly the same way. As soon as it was known that it is divisible into areas differing in function, many methods by which the delimitation of the areas might be attempted were devised. The converging efforts made during the past forty years by comparative anatomists, histologists, physiologists, pathologists, and physicians, have resulted in the acquisition of an accurate, if very restricted, understanding of the construction and mode of working of the apparatus of thought. Of some of the new data the psychologist is able to make use; but so far as the physiologist is concerned, it is the vehicle of mind which is the subject of study, not its contents.
A new subject has been created since 1870. There is therefore nothing to be gained, so far as our present purpose is concerned, from the consideration of views which were current before that date; and since, as must always occur when a science is rapidly advancing, observations which logically should have been the first to be made were not thought of until it became necessary to devise methods of checking results obtained in other ways, we will consider the various sources of our information without regard to the chronological order in which they were opened up.
The cerebral hemisphere contains two large central masses of grey matter, the nucleus caudatus and the nucleus lenticularis, often described as a single structure under the name “corpus striatum.” Their functions are unknown. The nerve-fibres which connect the cerebral hemispheres with the rest of the central nervous system form two thick limbs or crura on the under side of the brain. Each crus turns upwards into its hemisphere, between the nucleus caudatus and optic thalamus (the latter belongs to the “between-brain”) on the inner side, and the nucleus lenticularis on the outer. In this passage the compact crus, which is somewhat flattened, is termed the “internal capsule.” Immediately above the three grey masses the internal capsule disperses as a fountain of fibres which go to all parts of the cortex. Mingled with these radiating fibres are vast numbers of others, proper to the hemispheres, which run tangentially. Some, crossing the median plane, as the corpus callosum, bind the two hemispheres together. Others form tracts which can be followed from one end or pole of the hemisphere to the other. Groups of fibres, dipping but little below the cortex, unite nearly adjacent spots or neighbouring convolutions.
The folding of the cortex beneath fissures is due to the necessity of disposing of a certain bulk of grey matter without increasing its thickness beyond the proper limit. Since the superficial area of a sphere varies as the square of its radius, whereas its capacity varies as the cube, it is possible for a fixed relation to be maintained between the amount of cortex and the amount of white matter in the brain, only by the folds increasing in depth as the size of the brain increases. Fissuring is a response to a mechanical need. This does not imply, however, that the lines along which it takes place are devoid of morphological meaning. The similarity in pattern of the convolutions and fissures in various animals, and the regular progress of their development in each individual, prove the contrary. If they are not absolutely trustworthy as boundaries of areas of separate function—and further evidence will be needed before a decision can be pronounced upon this disputed question—they are in the main satisfactory as landmarks.