THE CEREBELLUM. 1093 



addition to a short and uncertain centrally directed process, the irregular cell gives 

 off a brush-like group of tibrillce which penetrate the molecular layer, seldom branch- 

 ing, as far as the free surface of the folium, when they end beneath the pia in expan- 

 sions that become condensed and unite into a delicate limiting membrane. The 

 radial disposition of the neuroglia fibres, as well as of the Purkinjean dendrites, 

 climbing fibres and the larger blood-vessels, confer upon the molecular layer a 

 vertical striation that is often marked. 



The Medullary Substance. — The white matter composing the core of the cerebellar 

 hemispheres exhibits several fairly definite subdivisions, among which may be distinguished : 



1. The subcortical layer, from .2-. 5 mm. in thickness, that extends beneath the granule 

 layer, parallel to the surface, and sweeps around the bottom of the deeper fissures. Within the 

 series of festoons thus formed lie the association tracts that connect the folia and lobules of the 

 same hemisphere. 



2. The commissural tracts, of which the larger lies in front of the dentate nucelus and the 

 smaller behind this nucleus, are continued across the mid-line and into the opposite hemisphere 

 as the anterior (superior) and the posterior (inferior) cerebellar decussations. 



3. The peridentate stratum that comprises a fibre-complex that surrounds the nucleus 

 dentatum. 



Within the medullary substance of the worm, lie : 



1. The superior cerebellar commissure, a robust tract of transversely coursing fibres that 

 passes in front of the roof-nucleus and, beyond the worm, expands on each side into the main 

 limbs of the medullary tree. It is chiefly by the decussating fibres within this commissure that 

 the cortex of the two hemispheres is connected. 



2. The inferior cerebellar commissure passes behind the roof-nucleus and consists of a 

 number of small transversely coursing bundles. 



3. The decussation of the roof nuclei constitutes a commissural and decussating tract distinct 

 from that of the cerebellar commissures just described. The rounded bundles traverse the 

 roof -nucleus, particularly its superior (anterior) part, more distally skirting its dorsal margin 

 and, still farther backward, invading the beginning of the horizontal medullary limb. 



4. The median sagittal bundle extends from the superior medullary velum beneath the roof- 

 nucleus into the medulla of the worm ; above, these fibres are continued upward through the 

 velar frenum and into the inferior quadrigeminal coUiculus. 



In addition to the foregoing tracts, the central parts of the branches of the medullary tree, 

 not only of the hemispheres but also of the worm, are occupied by longitudinally coursing fibres 

 that pass directly into the white core, and thence are continued into the cerebellar peduncles as 

 the- afferent and efferent paths by which the cerebellar cortex is brought into relation with other 

 parts of the brain and spinal cord. 



Fibre-Tracts of the Cerebellar Peduncles. 



Repeated mention has been made of the three robust arms of white matter, 

 the peduncles, that enter the medullary substance of the cerebellum and serve 

 to transmit the fibre-tracts that connect the cerebellum with the cerebrum, the 

 brain-stem and the spinal cord. The general features of the inferior, middle and 

 superior cerebellar peduncles are described in connection with the medulla, the pons 

 and the mid-brain respectively. It will be convenient in this place, in connection 

 with the cerebellum, to consider more in detail the constituents of these important 

 pathways. 



The Inferior Cerebellar Peduncle.— This robust stalk (corpus restiforme), 

 also known as the restiform body, includes not only the tracts connecting the cere- 

 bellum with the spinal cord, but also those that link the cerebellum and the medulla. 

 Two divisions, the spinal and the bulbar, are therefore often recognized. 



The chief constituents of the inferior peduncle are : 



1. The direct cerebellar tract, the fibres of which arise from the cells of Clarke's column, 

 course through the lateral part of the inferior peduncle and end in the cortex of the anterior part 

 of the superior worm on the same side, some fibres reaching the opposite side of the worm by 

 way of the superior commissure. 



2. The arcuate fibres (anterior and posterior superficial), from the gracile andcuneate nuclei 

 of the opposite and the same side. Additionally, perhaps, some fibres are continued, without inter- 

 ruption in the medullary nuclei, from the posterior fasciculi of the cord. All of these, direct and 

 indirect, end chiefly within the cortex of the superior worm of the same and the opposite side. 



