HISTOGENESIS AND MINUTE STEUCTUKE OF CEEEBELLUM. 579 



Fiss.secunda. 



Fiss. suprapyramidalis. 

 ,' Fiss. prima. 



Lobus 

 lareralis. 



In its inferior part the roof of the ventricle is exceedingly thin and is not all 

 formed of nervous matter. The posterior medullary velum is a mere ridge which 

 can hardly be said to enter into its formation : the epithelial lining of the cavity, 

 supported by pia mater, is carried downwards towards the inferior boundaries of 

 the floor of the ventricle. At the lowest 

 part of the calamus scriptorius, and also 

 along each lateral boundary of the floor, 

 the epithelial roof becomes thickened 

 at its attachment to the parts of the 

 medulla oblongata. The small semilunar 

 lamina which stretches across between 

 the inferior parts of the two clavse at 

 the calamus scriptorius and overhangs 

 the opening of the central canal is 

 termed the obex (Fig. 482, p. 550). A 

 downwardly directed protrusion of the 

 epithelial roof is often found behind 

 the obex. 



Velum 



medullare. 



posl-erius./ 



/.Parafloc. 



'Floe. 



Recessus 

 lareralis 

 venh.quarrt. 



Tuberculum acushcum. 

 Medulla oblongara. 



Modulus. Obex. Taenia ventriculi quarH. 



FIG. 513. THE POSTERIOR ASPECT OF A FCETAL 

 CEREBELLUM AND MEDULLA OBLONGATA. 



THE HlSTOGENESIS AND MlNUTE STRUCTURE OF THE CEREBELLUM. 



The developmental history of the cerebellum presents certain peculiar features 

 which seem quite enigmatic unless considered from the point of view of the evolu- 

 tion of the connexions and functions of the organ. The cerebellum is derived from 

 part of the alar lamina of the rhombencephalon, and at an early stage of its develop- 

 ment the rudiment shows the regular lamination into ependyma, mantle layer, and 



marginal layer, which has already been de- 

 scribed as distinctive of the corresponding 

 place of development in the whole nervous 

 system. The cells of this mantle layer are to 

 be looked upon as an outlying (superior) part 

 of the receptive nucleus , of the vestibular 

 nerve, the cells to which information con- 

 cerning the position and movements of the 

 body as a whole or of the head will be trans- 

 mitted from the semicircular ducts of the 

 internal ear. But, if such information is to 

 be put to any use in influencing behaviour, it 

 is obvious that the activity of these cerebellar 



'ff**J I 1 ffl ^oT \\l '/ I / ce ^ s mus ^' fi rs kty > be correlated with visual 

 !//*) fe *jO I \ -^L impassions, which also supply information 

 *"x/VrW >J?vlA/ ^ concerning the position and movements of the 



*X/y Jn^<tVJ [xL / jir* kody, an( l w ^h all those nerves which are 



*** bringing into the encephalon or spinal medulla 



impressions of touch, pressure, or any other 

 information concerning the state of the 



OF A ^CEREBELLAR FOLIUM (after Kolliker). muscles, tendons, joints, Or other structures 



which are concerned in movements; and, 

 secondly, they must be brought to bear upon 

 the various motor nuclei and other motor 

 regulating parts of the brain (such as the red 

 nucleus, tectum mesencephali, basal ganglia, 



and cerebral cortex) to which the co-ordinating influence of the cerebellum is 

 essential for the properly adjusted performance of complex actions. 



The neuroblasts which receive all these extrinsic sensory impulses, visual, tactile, 

 musculo-sensory, et cetera, collect at the threshold of what was originally the vesti- 

 bular cerebellar rudiment ; and they can be seen during the latter part of the second 

 month migrating from the rhombic lip into the marginal layer of the cerebellum, 

 til eventually its whole surface has been invaded by these alien ueuroblasts, so 



38 a 





FIG. 514. SECTION THROUGH THE MOLECULAR 

 AND GRANULAR LAYERS IN THE LONG Axis 



Treated by the Golgi method. 



P. Cell of Purkinje. 



GR. Granule cells. 



N. Axon of a granule cell. 



N 1 . Axons of granule cells iu molecular layer. 



un 



