860 



THE NERVE SYSTEM 



portions, atrophies in others, while in certain localities it is crowded away from 

 the central cavitv by the intrusion of white fibre masses which are chiefly com- 



/ */ t/ 



missural. A total atrophy occurs in a part of the dorsal wall of both fore-brain 

 and hind-brain; partial atrophy is observable in the floor of the third ventricle, 

 near the optic chiasm, once perhaps the optic centre in the earliest of the ancestral 

 vertebrates, but atrophied in higher forms as the visual tract became secondarily 

 projected in the mid-brain. Great hypertrophy characterizes the growth of the 

 ganglionic gray in the floor of the lateral ventricle (cerebral vesicle), resulting 

 in the formation of nugget-like masses, the caudate, lenticular, and amygdaline 

 nuclei. The central gray of the primary fore-brain also undergoes great hyper- 

 trophy, but in the lateral walls only, to form the large, compact thalami. 



Thalamns. 



Foramen of 



Posterior commissure. 

 Pineal body. 



Crus cerebri. 

 Aqueduct 



Quadrigeminal plnte* 

 \ Cerebellum. 



Cerebral hemisphere. 



Olfactory lobe 01 '. 



rhinencephalon > 



Lamina terminalis. 



Optic nerve., < 

 Optic chiasm.' 



~~IV. Ventricle, 



Corpus 

 albicans. 



Spinal cord. 



Pituitary body. ' 

 Tuber cinereum. 



FIG. 628. Median section of brain of human fetus during the third month. (After His.) 



As in the cord, fibre masses develop ectad of the central tubular gray in some 

 localities, while in other regions the ganglionic gray remains at the surface and 

 the white conducting substance is developed on its inner aspect. Thus we have, 

 secondarily, the formation of superficial gray matter as the cortex (or rind) of the 

 cerebrum and cerebellum. The isolation of ganglionic gray masses from the 

 primitive central tubular gray and their differentiation into nerve cell nidi is also 

 observable in the reticular ganglionic formation of the medulla oblongata and pons 

 as well as in the roof of the mid-brain. Certain aggregations of gray ganglionic 

 tissue are intercalated in the course of fibre strands, receive an admixture of these, 

 and are regarded as terminal, interrupting, or as condensing stations not unlike 

 some very complex relay telegraph system. The olive, dentate nucleus, red nu- 

 cleus, the nuclei of the gracile and cuneate funiculi, the basketwork intercalations 

 of the reticular and lemniscus fields belong to this intrafascicular type of ganglionic 

 structures. 



