THE NERVOUS SYSTEM. 



525 



neurone group apparently owes its great development principally to its close 

 association with the cerebellum. These two long descending intersegmental 

 tracts as they grow downward envelop the differentiating reticular formation 

 of more caudal regions of brain (and cord) and thereby come to occupy an 

 external position in the fully differentiated reticular formation. 



The reticular formation is thus composed of a gray portion containing the 

 neurone bodies and shorter tracts and a white portion composed of the longer 

 tracts. Axones from certain nuclei (especially N. ruber, N. of Darkschewitsch 

 and N. of Deiters) form long, principally descending, tracts which envelop the 

 gray reticular formation mesially (medial longitudinal fasciculus including 

 fibers from nuclei of Darkschewitsch and Deiters as well as other reticulo- 

 spinal fibers) and laterally (rubro-spinal, lateral uncrossed tract from Deiters' 



Alar plate 



Fig. 451. — Transverse section through the mid-brain of a 10.2 mm. human embryo. His. 



nucleus and other reticulo-spinal fibers) and constitute the white reticular 

 formation. These long tracts descend to the cord and there similarly envelop 

 its ventro-lateral ground bundles. 



While the above differentiation of the reticular formation has been taking 

 place, changes in the alar plate have begun which lead to the formation of 

 terminal nuclei of peripheral afferent nerves, as well as terminal nuclei of other 

 tracts, all of which send fiber bundles to suprasegmental structures. 



The formation of the receptive nuclei of the afferent nerves of peripheral 

 (segmental) structures is complicated by the fact that the central continuations 

 of the peripheral afferent nerves are not confined to their own respective seg- 

 ments but form longitudinal tracts which continue to grow upward (columns of 

 Goll and Burdach) or downward (descending solitary, vestibular and trigeminal 

 tracts) passing* into other segments and overlapping externally structures 

 already in process of formation there. In each segment, then, the terminal 

 nuclei of the afferent nerves of that segment must be distinguished from the 



