Page:The New International Encyclopædia 1st ed. v. 14.djvu/428

NERVOUS SYSTEM AND BRAIN. 378 NERVOUS SYSTEM AND BRAIN. rating the lolies, the entiio suifuce of the cere- bellum is crossed Ijy a iiuiltitude of transverse grooves whioli divide it into plates or lamella and give it a foliated or laminated appearance. On cutting into the cerebellum it is seen to con- sist of a central portion or core of white matter which extends outward into the cortex as a series of transversely disposed hranching plates. These, covered over by a layer of gray matter, form the lamina already referred to, which on cross- section present the characteristic leaf-like ap- pearance known as the "arbor vitae." The white matter of the cerebellum is composed of mcdul- lated nerve fibres which pass out in a radial manner into the gray matter of the cortex. Thesc fibres are axones coming from cells situated in the cerebellum and passing out to terminate either in some other part of the eerebellum or in some other part of the nervous system, or coming from cells located in other parts of the nervous system to their terminations in the cerebellum. The gray matter of the cerebellum is composed of three layers, an external or molecular layer, an internal or granular layer, and. between the two, a layer composed of a single layer of large cells, the layer of Purkinje cells. The molecular layer contains multipolar cells of various sizes known as "basket cells' from the fact that while their dendrites pass mostly toward the surface, their axones pass downward and end in a basket like network around the I'urkinje cells. The granular or nuc-lear layer derives its name from the fact that after the ordinary methods of staining it appears to be comi)osed of a mass of small close- ly packed cells, each cell consisting of a nucleus surroimded by an extremely small amount of protoplasm. Intermingled with these cells are inedullated and nun-medullated fibres. By the application of the niethoil of Golgi these cells have been divided into small granule cells (by far the more numerous) and large granule cells. In the case of the small granule cell the den- drites ramify in the granular layer, while the axone terminates freely in the molecular layer. The large granule cells, on the other hand, send their dendrites into the molecular layer, while X>endrUef yerve /process NERVE CKI.I. (CEbb OK l-CRKIXJE). From a section through tho human cerebellar cortex. their axones form a rich plexus within the granular layer. The dense network of fibres found in the granular layer is derived from the cells just di'scribcd. from axones and col- laterals of Purkinje cells, and from fibres coining from the white matter. The cells of Purkinje form a single layer along the boundary between the molecular layer and the granular layer. Krom the outwardly directed part of the cell a thick neck is given oil', from which arise two main dendritic processes, which subdivide and give rise to an enormous number of processes which ramify and terminate in the molecular layer. From the opposite pole of the cell is given olT the axone which passes through the granular layer and becomes a meduUated fibre of the white matter. JSesides the gray matter of the cortex, certain independent masses of gray matter occur in the eerebellum. These are the corpus dentatum, a convoluted body of gray matter situated to the inner side of the centre of the core of white mat- ter, and the roof-nuclei of Stilling, situated at the anterior end of the su])erior vermiform proc- ess and projecting forward into the roof of the ventricle. The functions of the cerebellum have been made the subject of much discussion and inves- tigation. It is itself insensible to irritation, and has been cut away in various animals, without eliciting signs of pain; moreover, its removal or disorganization by disease is generally unaccom- panied with loss or disorder of sensibility, ani- mals from whom it has been removed being ap- parently able to smell, see, hear, and feel as ]ierfcctly as before. I'Tourens extirpated the cere- bellum in birds by successive layers. Feebleness and want of harmony of the movements resulted from the removal of the superficial layers. When he reached the middle layers, the animals became restless; their movements were violent and irregular. By the time that the organ was en- tirely removed, the animals had completely lost the power of Hying, walking, standing, and pre- serving their equilibrium. lieu a pigeon iu this slate was laid upon its back, it could not recover its former position, but lluttered its wings, and saw and tried to avoid a threatened blow. Hence volition, sensation, and memory were not lost, but merely the faculty of combining the actions of the muscles. From a large series of experiments of this kind, subseciuently made on all classes of animals, Floureiis inferred that the cerebellum belonged neither to the sensitive nor to the intellectual apparatus; that it was not the source of voluntary movenients, although belong- ing to the motor apparatus: that it is the organ for the coordination of the voluntary move- ments, or for the excitement of the combined and harmonious action of the muscles. In spite of the above facts, the extent to which muscular coordination is dependent upon the cerebellum cannot be considered as satisfactorily determined. The cerebrum — sometimes called the brain proper — constitutes in man the largest part of the cerebrospinal axis. Lying within the cav- ity of the skull, its shape eonforiiis to that of the cranial cavity. Anteriorly, supcriiuly. and posteriorly, a deep fissure runs lengthwise of the brain, called the great longitudinal fissure, separating the brain into two equal halves or hemispheres. At the bottom of the fissure a broad band of fibres — the corpus callosiim — tmitcs the two hemispheres. Kaih lieniisphere ])resents three surfaces — .'in outer, which is con- vex and corresponds to the inner surface of the skull; an inner, which is Hat. lying against the similar surface of the opposite hemisphere, from