580 THE NEEVOUS SYSTEM. 



that the originally non-nucleated marginal layer becomes a densely packed granular 

 layer (stratum marginale embryonale). When this stage is reached the cerebellum 

 consists of an inner ependymal layer, a mantle layer crowded with locally developed 

 neuroblasts, a clear layer (the inner part of the original marginal layer), and the 

 superficial layer of neuroblasts which have invaded the outer part of the marginal 

 layer. As development proceeds in the mantle layer the axons of its neuro- 

 blasts are directed mainly towards the ventricular surface the reverse of what 

 happens in the spinal medulla; and as these fibre-masses increase in quantity 

 the main part of the mantle layer becomes pushed farther and farther away from 

 the ependyma by the accumulation of their own (and other) axons. Some of the 

 neuroblasts, however, do not become pushed out into the line of the embryonic 

 cerebellar cortex, but remain behind amidst the fibre-mass and receive the axons 

 that come from the cortical cells. These neuroblasts left amidst the fibres gradually 

 assume the form of the dentate, fastigial, globose, and emboliform nuclei already 

 described ; and their axons pass out (as the brachia conjunctiva) to the thalamus, 

 mesencephalon, and pons. In the meantime many of the neuroblasts of the mantle 

 layer have been converted into the large pear-shaped Purkinje-cells. 



While these events have been occurring in the true mantle layer a peculiar 

 process has been taking place in the superficial layer of immigrant cells. One by 

 one they begin to leave their places upon the surface and dip into the mantle 

 layer; many of them pass between the Purkinje-cells to a deeper plane, where 

 they cease their wanderings and form a densely packed layer of granule cells 

 (Fig. 514), the axons of which indicate the course of these migrations. 



MINUTE STRUCTURE OF A CEREBELLAR FOLIUM. 



A cerebellar folium is composed of a central core of white matter, covered with a layer 

 of gray matter. The gray cortex is arranged in two very evident layers, viz., a superficial 

 molecular layer and a subjacent rust-coloured granular layer. Between these strata a 

 single layer of large cells, termed the cells of Purkinje, are disposed in the form of a 

 very nearly continuous sheet. The cells of Purkinje constitute the most characteristic, 

 and probably the most essential, constituents of the cerebellar cortex. 



The cells of Purkinje are most numerous on the summit of the folium. At the 

 bottom of the sulci which intervene between the folia they become fewer in number, and, 

 therefore, looser in their arrangement. Each consists of a large flask-shaped or piriforin 

 cell body, the narrow end' of which projects into the molecular layer, whilst the thicker, 

 deeper end rests on the granular layer. From the latter arises a single axon, which passes 

 into the granular layer and presents the peculiarity of almost immediately assuming its 

 medullary sheath. From this axon a few collateral branches soon arise, which, taking a 

 recurrent course, enter the molecular layer, to end in connexion with certain of the 

 adjoining cells of Purkinje. They would seem to have the function of binding together 

 adjacent cells, and thus enabling them to carry on their operations in harmony with 

 each other. 



The dendritic processes spring from the narrow end of the cell in the form of 

 either one or perhaps two stout stalks. These ascend into the molecular layer, branching 

 and rebranching until an aborescent arrangement of extraordinary richness and extent 

 results. The dendritic branches extend throughout the entire thickness of the molecular 

 layer, and the branching takes place in one plane only, viz., in a plane which is transverse 

 to the long axis of the folium. Consequently, it is only when transverse sections through a 

 folium are made that the full dendritic effect is obtained (Fig. 515) ; in sections made 

 parallel to the long axis of the folium the cells are seen in profile, and are observed to 

 occupy quite a narrow area (Fig. 514). The branching of the dendrites of a cell of Purkinje 

 may, therefore, be compared to that which takes place in the case of a fruit-tree which 

 is trained against a wall. 



In the molecular layer the cells are not very numerous, and of these the most 

 characteristic are the basket-cells which lie in the deeper part of the layer. In addition 

 to numerous dendrites the basket-cell gives off an axon which runs transversely, as 

 regards the long axis of the folium, between the planes of adjacent dendritic arborisations 

 of the cells of Purkinje. At first very fine, these axons gradually become coarse and thick, 

 and at intervals they give off collaterals which run towards the bodies of the cells of 

 Purkinje. Reaching these, they break up into an enormous number of fine terminal 



