196 THE NERVOUS SYSTEM 



opment, which is stimulated by the growth into it of afferent fibers from the 

 vestibular nerve and of sensory fibers of the second order, bringing afferent 

 impulses from other sources, chiefly from the somatic musculature. This 

 part of the alar lamina, which may be regarded as an overgrown portion of the 

 vestibular nucleus, develops into the cerebellum. As the paired cerebellar plates 

 increase in thickness during the second month of embryonic development, they 

 bulge inward toward the ventricle and take up a transverse position (Fig. 137, 

 5). As they increase in size they invade the roof plate and unite in the median 

 plane forming a transverse bar above the fourth ventricle. The lateral ex- 

 tremities of this bar expand, and the entire structure assumes a dumb-bell 

 shape, the lateral masses representing the future cerebellar hemispheres and the 

 intermediate part the future vermis. 



At the close of the third month transverse sulci begin to appear in the vermis. 

 The first of these, the fissura prima or sulcus primarius, extends into the lateral 

 masses on either side and separates an anterior lobe from the remainder of the 

 cerebellum. Other transverse fissures soon appear, due to the rapid expansion 

 and resultant folding of the cortical layers. 



The cerebellum differs from the other parts of the nervous system, which we 

 have thus far studied in detail, in that the relative position of the gray and white 

 matter is reversed. The gray substance forms a thin superficial layer, the 

 cerebellar cortex, which covers a central white medullary body (corpus medullare). 

 Originally the cerebellar plate is formed, like other parts of the neural tube, of 

 an ependymal, a nuclear or mantle, and a cell-free marginal zone. The neuro- 

 blasts of the mantle zone take no part in the formation of the cortex, but become 

 grouped in the internal nuclear masses of the cerebellum. The superficial or 

 marginal zone is at first devoid of nuclei; the neuroblasts, from which the cere- 

 bellar cortex is differentiated, migrate into this zone from the ependymal and 

 perhaps also from the mantle layers of the rhombic lip. These developing neu- 

 rons send their axons inward instead of outward as in the case of the spinal cord. 

 These axons accumulate, along with others which enter the cerebellum from 

 without, in the deep part of the marginal layer and form the central medullary 

 body of the cerebellum, separating the developing cortex from the deep nuclear 

 masses that are differentiating from the mantle layer. 



THE ANATOMY OF THE CEREBELLUM 



It is customary to consider the cerebellum as composed of three parts: a 

 small unpaired median portion, called the vermis, because superficially it re- 



