794 NEUROLOGY 



veil, descends upon its upper part from between the inferior colliculi, and on either 

 side of this the trochlear nerve emerges. 



The posterior medullary velum (velum medullare posterius; inferior medullary velum) 

 is a thin layer of white Substance, prolonged from the white center of the cerebellum, 

 above and on either side of the nodule; it forms a part of the roof of the foun;h 

 ventricle. Somewhat semilunar in shape, its convex edge is continuous with the 

 white substance of the cerebellum, while its thin concave margin is apparently 

 free; in reality, however, it is continuous with the epithelium of the ventricle, 

 which is prolonged downward from the posterior medullary velum to the liguke. 



The two medullary vela are in contact with each other along their line of emer- 

 gence from the white substance of the cerebellum; and this line of contact forms 

 the summit of the roof of the fourth ventricle, which, in a vertical section through 

 the cavity, appears as a pointed angle. 



The Fibrae Propriae of the cerebellum are of two kinds: (1) commissural fibers, 

 which cross the middle line at the anterior and posterior parts of the vermis and 

 connect the opposite halves of the cerebellum; (2) arcuate or association fibers, 

 which connect adjacent laminae with each other. 



Gray Substance. The gray substance of the cerebellum is found in two situations: 

 (1) on the surface, forming the cortex; (2) as independent masses in the anterior. 



(1) The gray substance of the cortex presents a characteristic foliated appearance, 

 due to the series of laminae which are given off from the central white substance; 

 these in their turn give off secondary laminae, which are covered by gray substance. 

 Externally, the cortex is covered by pia mater; internally is the medullary center, 

 consisting mainly of nerve fibers. 



Microscopic Appearance of the Cortex (Fig. 706). The cortex consists of two 

 layers, viz., an external gray molecular layer, and an internal rust-colored nuclear 

 layer; between these is an incomplete stratum of cells which are characteristic of 

 the cerebellum, viz., the cells of Purkinje. 



The external gray or molecular layer consists of fibers and cells. The nerve fibers 

 are delicate fibrillae, and are derived from the following sources: (a) the dendrites 

 and axon collaterals of Purkinje's cells; (6) fibers from cells in the nuclear layer; 

 (c) fibers from the central white substance of the cerebellum; (d) fibers derived 

 from cells in the molecular layer itself. In addition to these are other fibers, which 

 have a vertical direction, and are the processes of large neuroglia cells, situated 

 in the nuclear layer. They pass outward to the periphery of the gray matter, 

 where they expand into little conical enlargements which form a sort of limiting 

 membrane beneath the pia mater, analogous to the membrana limitans interna 

 in the retina, formed by the sustentacular fibers of Miiller. 



The cells of the molecular layer are small, and are arranged in two strata, an 

 outer and an inner. They all possess branched axons; those of the inner layer 

 are termed basket cells ; they run for some distance parallel with the surface of the 

 folium giving off collaterals which pass in a vertical direction toward the bodies 

 of Purkinje's cells, around which they become enlarged, and form basket-like 

 net-works. 



The cells of Purkinje form a single stratum of large, flask-shaped cells at the 

 junction of the molecular and nuclear layers, their bases resting against the latter; 

 in fishes and reptiles they are arranged in several layers. The cells are flattened 

 in a direction transverse to the long axis of the folium, and thus appear broad 

 in sections carried across the folium, and fusiform in sections parallel to the long 

 axis of the folium. From the neck of the flask one or more dendrites arise and pass 

 into the molecular layer, where they subdivide and form an extremely rich arbores- 

 cence, the various subdivisions of the dendrites being covered by lateral spine- 

 like processes. This arborescence is not circular, but, like the cell, is flattened at 

 right angles to the long axis of the folium; in other words, it does not resemble 



