38 PHYSIOLOGY [NERVOUS that in the corpus striatum of the same animal there is a point, the nodus cursorius, the excitation of which caused the rabbit to rush forwards. This observation agrees with the statement of Magendie that, when he injured the corpora striata, the animal seemed to have an irresistible propulsion forwards. Ferrier states that when the corpora striata were stimulated by an interrupted current convulsive movements of the opposite side of the body took FIG. 24. Three pairs of cerebellar peduncles (from Sappey, after Hirschfeld and Leveille. Quain). On the left the three cerebellar peduncles have been cut short ; on the right side the hemisphere has been cut obliquely to show its connexion with the superior and inferior peduncles. 1, median groove of fourth ventricle ; 2, the same groove at the place where the auditory strise emerge from it to cross the floor of the ventricle ; 3, inferior peduncle or restiform body ; 4, funiculus gracilis ; 5, 5, superior peduncles, on the right the dissection shows the superior and inferior peduncles crossing each other as they pass into the white substance of cerebellum ; 6, 6, fillets at the side of crura cerebri ; 7, lateral grooves of crura cerebri ; 8, corpora quadrigemina. place ; and when the current was powerful the side of the body opposite to the side of the brain stimulated i was forcibly drawn into an arch. Cerebellum. In connexion with the phy siology of this organ it is important to note its connexions with the rest of the cerebro- spinal axis. It has three peduncles : (1) the superior peduncles (see fig. 24) crura ad cere brum, or processes ad testes together with! the valve of Vieussens, connect the cerebellum |K| to the cerebrum ; (2) the inferior peduncles, or crura ad medullam, are the superior ex- i] tremities of the restiform bodies ; (3) the middle peduncles, or crura ad pontem, much the largest, are the lateral extremities of the transverse fibres of the pons Varolii. They) act as commissural fibres for the hemispheres I of the cerebellum. All these peduncles pass into the interior of the cerebellum at its fore- > part. In the interior of the organ, where the peduncles enter, we find a nucleus of grey) matter, the corpus dcntatum. The cortical substance consists of two layers, an outer. -molecular layer, consisting of a delicate ma- trix containing a few round cells and fibres, and an inner or granule layer, containing 1 granules or nucleated corpuscles closely packed together. The corpuscles are from, isVtfth to -j-sVsth of an inch in diameter, and are mixed with a network of delicate nerve-fibres. At the junction of the granu- 5? j lar layer with the molecular layer there are r peculiar large cells called " Purkinje s cells. " i They are flask -shaped and about ^ a th to! TTnmth O f a n inch in diameter, and the long process is directed towards the surface of the .-^^^E^ cerebellum (see fig. 25). The white centre of each lamina consists of delicate nerve-fibres, the terminations of which have not been -: satisfactorily made out. Probably they end _ in the plexus of nerve-fibres in the granule FIG. 25. Vertical section layer, or in the processes of Purkinje s cells, through cortex of cere- On comparing the section of cerebrum (fig. 28) with that of cerebellum (fig. 25) the con trast is striking. The structure of cerebellum is more like that of the retina (vol. i. p. 888, Experi- fig. 78) than of any other nerve-centre, ments on Results of Experiments. The cerebellum is insensible to mechan- cere- ical excitations. Puncture causes no indications of pain, but there bellum. may be twisting of the head to the side. Ferrier states that Faradaic bellurn (Sankey). a, pia mater; b, external layer; c, layer of cells of Pur- kinje ; d, inner or gran ule layer ; e, medullary centre. irritation causes movements of the eyeballs and other movements indicative of vertigo. Section of the middle peduncle on one side causes the animal to roll rapidly round its longitudinal axis, tho rotation being towards the side operated on. If the cerebellum be removal gradually by successive slices an operation easily done in a pigeon there is a progressive effect on locomotive actions. On taking away only the upper layer there is some weakness and a hesitation in gait. When the sections have reached the middle of the organ the animal staggers much, and assists itself by its wings in walking. The sections being continued further, it is no longer able to preserve its equilibrium without the assistance of its wings and tail ; its attempts to fly or walk resemble the fruit less efforts of a nestling, and the slightest touch knocks it over. At last, when the whole cerebel lum is removed, it can not support itself even with the aid of its wings and tail ; it makes vio lent efforts to rise, but only rolls up and down ; then, fatigued with struggling, it remains for a few seconds at rest on its back or ab domen, and then again commences its vain struggles to rise and walk. Yet all the while sight and hearing are Fio. 2(5. Pigeon from which the cerebellum has been removed. perfect. See fig. 20. It attempts to escape, and appears to have all its sensa tions perfect. The results contrast very strongly with those of removing the cerebral lobes. "Take two pigeons," says Longet ; "from one remove com pletely the cerebral lobes, and from the other only half the cerebellum ; the next day the first will be firm on its feet, the second will exhibit the unsteady and uncertain gait of drunkenness." There is thus a loss of the power of co-ordination, or of regula tion of movement, without the loss of sensibility, and hence it has been assumed that in some way or other the cerebellum acts as the co-ordinator of movements. Co-ordination of Movement. The nervous mechanisms by which Co-or- movements are co-ordinated that is, adapted to specific ends are dinatioi not thoroughly understood, but a short description of what is of move known may be here given. Muscular movements may be either ment. simple or complex. In winking, the movement of the eyelid is effected by two muscles, one bringing the lid down, the other rais ing it. But picking up a pen from the table, taking a dip of ink, and writing a few words involve a complicated set of movements of the muscles of the trunk, shoulder, arm, forearm, fingers, and thumb. To perform the movements with precision each muscle or group of muscles must act at the right time and to the proper amount. It is also clear that all this is accomplished automatic ally. "We are not conscious of the requisite combinations ; but it must be noted that many of these complicated movements are first acquired by conscious efforts, and that they become automatic only by repetition. Again, in walking, equilibrium is maintained by a delicate series of muscular adjustments. When we swing forward one leg and balance the body on the other many muscular move ments occur, and with every change in the position of the centre of gravity in the body there are corresponding adjustments. It would appear that in all mechanisms of co-ordination the first part of the process is the transmission of sensory impressions from the periphery. These sensory impressions may be derived from the skin or muscles, and may be caused by variations of pressure arising in them. Thus, if we lift a heavy weight, as a large stone, by the right hand and raise it to the bend of the elbow we throw the body to the other side by the action of the muscles of that side, thus maintaining the equilibrium. We judge of the amount of force necessary to overcome an obstruction by the feeling of resistance we encounter. All the movements of the body, therefore, give rise to feelings of varying pressures, and these feelings regulate the amount or degree of muscular action necessary to maintain equilibrium, or to perform a requisite movement. This is at first a conscious ex perience, and a child has to pass through an education, often involv ing pain, before the nervous mechanisms become automatic and the movement is done without effort. But the ordinary sensory nerves, coming from skin and muscle, are not the only channels by which such guiding mechanisms are set in action. As one would expect, sensory impressions, such as those associated with sight and hearing, may be brought into play. 1. Peripheral Impressions from Semicircular Canals. If the Impres- membranous portion of the horizontal semicircular canal in the sions internal ear of a pigeon be cut, the bird moves its head from side to from side, and if one of the vertical canals be divided it moves the head semi- up and down. The effects may pass off in a few days if only one circular canal has been cut. If the canals on both sides be divided the canals, movements are exaggerated and the condition becomes permanent. It will then be observed that the animal has lost the power of co ordinating its movements. It can rest with only a twitching, perhaps, of the head, but if it attempt to fly or walk its movements are indefinite and irregular, like those of a dizzy person, or like those described as following injury to the cerebellum. The irregular movements do not arise from deafness, or noises in the ears, or partial
paralysis, or from an uncontrollable impulse. Any strong sensory