CHAPTER XIV 



THE DIENCEPHALON AND THE OPTIC NERVE 



Development. In an earlier chapter we traced briefly the development of 

 the prosencephalon and showed that the cerebral hemispheres were developed 

 through the evagination of the lateral walls of the telencephalon (Fig. 16). It 

 is, however, only the alar lamina which is involved in this evagination. The 

 basal lamina of the telencephalon retains its primitive position and forms the 

 pars optica hypothalami. This part of the hypothalamus, along with the 

 lamina terminalis and the most rostral part of the third ventricle, constitutes 

 the telencephalon medium (Johnston, 1912). Through the excessive growth of 

 the hemisphere the diencephalon becomes covered from view (Fig. 17), and 

 appears to occupy a central position in the adult human brain. It is separated 

 from the hemisphere by the transverse cerebral fissure, which is formed by the 

 folding back of the hemisphere over the diencephalon. The differentiation of 

 the alar lamina of the diencephalon into the thalamus, epithalamus, and meta- 

 thalamus, and of its basal lamina into the hypothalamus was briefly traced on 

 page 34. The sulcus limitans, which separates these two plates in the embryo, 

 corresponds to the more caudal portion of the hypothalamic sulcus of the adult; 

 but, since the latter can be followed to the interventricular foramen, while the 

 former ends near the optic chiasma, the rostral ends of these two sulci are not 

 related. The roof plate of the prosencephalon remains thin and constitutes 

 the epithelial roof of the third ventricle, which along the median plane becomes 

 invaginated into the ventricle as the covering of a vascular network to form 

 the chorioid plexus. 



THE THALAMUS 



The thalamus is a large ovoid mass, consisting chiefly of gray matter, placed 

 obliquely across the rostral end of the cerebral peduncle (Figs. 154, 155). Be- 

 tween the two thalami a deep median cleft is formed by the third ventricle. 

 The rostral end is small and lies close to the median plane. It projects slightly 

 above the rest of the dorsal surface, forming the anterior tubercle of the thalamus, 

 and helps to bound the interventricular foramen (Fig. 154). The caudal ex- 

 tremity is larger and is separated from its fellow by a wide interval, in which the 



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