The corticipetal fibers, which rise either in other parts of the brain or in the spinal cord and ganglia, terminate in varicose fibrils chiefly in the superficial layer. These fibrils entwine about the "primary and secondary stems of the Purkinje dendrites" (Cunningham).
2. The deep, granular layer (stratum granulosum, Figs. 108 and 109) is of uniform thickness. It blends centrally with the medullary projection. It contains a few superficial granules which are large in size (60-80/.4) and many small granules in which the nucleus occupies nearly the whole cell-body.
Cells of the Granular granules are small, round, or stellate cell-bodies (7--io,u), largest near Purkinje's cells, closely packed externally, but scattered among the projection fibers centrally. Each small granule has three to five short dendrites, which soon break up into claw-like tufts in contact with adjacent granules, and one long axone. The axone runs out into the superficial layer, branches, T-like, and, piercing the dendritic planes of Purkinje, gives off collaterals to them until exhausted by multiple division. It runs parallel with edge of the gyrus. The large granules (6o-8o p) are dendraxones, the type of Golgi. The axones form remarkable arborizations toward the medullary projection, touching and associating many granules. The dendrites, branching freely, ramify in the superficial layer.
Fibers of the Granular Layer (Figs. rob and I09).—The nerve fibers of the granular layer are as follows: (a) The processes of the granules, (b) the axones of Purkinje's cells running down into the medullary projection, together with their recurrent collaterals, and (c) corticipetal fibers, most of which run through the granular layer, without branching, to end in the first layer; the remainder terminate in the deep layer in the moss like appendages of Cajal.
The functions of the stellate cells, the "basket cells" and the granule cells are probably receptive and associative; they re ceive impulses through the projection fibers and transfer those impulses to the dendrites or bodies of Purkinje's cells. Purk inje's cells originate impulses for the coordination of muscu lar action, the preservation of muscle-tone, and the produc tion of powerful tonic contractions. Lesions in the cerebel
lum produce incoordination, chorea, athetosis and, rarely, convulsions.
The neuroglia of the cerebellum is similar to that in the cere brum. The short-rayed cells are scattered throughout the gray substance, while the long-rayed are located near, or within, the white substance. In the region of Purkinje's cells, near the surface of the deep layer, are the bodies of the arborescent cells, whose processes form a fine interlacement about the cell bodies of Purkinje and then extend in parallel lines out to the surface. They form a neuroglia felt-work just beneath the pia mater (lamina basalis).
Histogenesis of Cerebellar Cortex and Nuclei.—The dorsal lamine and roof-plate of the metencephalon, in which the cerebellum is developed, very early show a stratification into three layers. The stratification here is the same as occurs elsewhere in the neural tube, the layers being the Oen dymal, mantle and marginal layers, from within outward. The neurones of the cerebellum are derived from two sources: r. The mantle layer of the cerebellum and 2. the rhombic lip of the medulla oblongata. All cortical neurones, except those of Purkinje, are emigrant cells from the rhombic lip. The nuclear neurones are natives of the dorsal laminae, intrinsic neurones.
1. The intrinsic neuroblasts of the mantle layer throw out their primary pseudopods and develop their axones in the direction of the ventricular cavity. In accordance with the relations they acquire, these neurones fall into two classes, nuclear and cortical. (1) The nuclear neuroblasts push their axones (cerebello-tegmental fibers) out into other parts of the brain and form synapses with neurones in the thalamus, mid-brain, pons and medulla; their cell-bodies receive the end-tufts of the cortical neurones. (2) The cortical neuroblasts of the mantle layer develop wholly within the cerebellum. As just stated, their axones (cortico-nuclear fibers) form synapses with the dendrites and bodies of the nuclear neurones.