542 THE EYE IN EVOLUTION 



tecti ; it is related to the reflex centres in the mid-brain and has no connec- 

 tion with the cortex, persisting in man after lesions in the geniculo-calcarine 

 path.^ The ventral nucleus thus probably retains the primitive photostatic 

 functions, while the higher visual functions are probably all taken over by 

 the more lately differentiated dorsal nucleus with its elaborate laminated 

 structure and point-to-point retinal representation ^ (Ingvar, 1923 ; 

 Woollard, 1926 ; Le Gros Clark, 1941-42). 



THE PULViNAR. This nucleus occupying the posterior extremity of the thalamus 

 appears late in phylogenetic history, becoming of considerable size only in Primates 

 in which its development may be correlated with the adoption of the erect posture. 

 While tindoubtedly associated with visual and auditory integrations, its connections 

 with these systems are still obscure. It would appear to have no direct connections 

 with the retina or the visual cortex, nor with the ascending somatic tracts (Minkowski, 

 1913 ; Brouwer and Zeeman, 1926) but projects to the parastriate area of the cortex 

 (area 18) and to the posterior Sylvian receptive area adjacent to the auditory area 



Figs. 716 to 718. — The Development of the Telencephalon. 



R 



DL R DL 



O CD 00 



^—T^ VL F VI 



VL 



Fig. 716. Fig. 717. Fig. 718. 



Fig. 716. — Initially the telencephalon appears as a tube with thick lateral 

 walls of nervous tissue and a thin non-nervous roof, R, and floor, F. The lateral 

 walls are divided into dorso- lateral, DL, and ventro-lateral, V L, segments. 



Fig. 717. — During further development the lateral walls turn inwards leaving 

 a narrow area representing the roof and floor. 



Fig. 718. — The inturned dorsal and ventral edges of each lateral wall fuse, 

 forming out of the unpaired vesicle two cerebral hemispheres, each containing a 

 lateral ventricle. The dorso-lateral wall, DL, forms the cortex ; from the ventro- 

 lateral wall, VL, develop the nuclei of the corpus striatum. 



(area 22) (Le Gros Clark and Northfield, 1937). Its association with the cerebellum 

 (Clarke and Horsley, 1905) and the red nucleus (Sachs, 1909) and with the thalamo- 

 cortical fibres for the arm region in the precentral convolution of the brain may perhaps 

 reflect the importance of the hand and fingers in Primates in exploration and manipula- 

 tion, and the nucleus may act as an integrating area for the coordination of the eye and 

 the hand, being thus related to the higher visual functions of stereognosis (Winkler, 

 1919 ; Kappers, 1920). 



The telencephalon is present in all Vertebrates, the dorsal part of its lateral 

 walls forming the cortex, the ventral walls the nuclei of the corpus striatum 

 (Figs. 716 to 718). Initially it was built up as a receptor station for the 

 olfactor\r nerves, and the dominance of the higher Vertebrates is essentially 

 due to tlie replacement of the original palseocortex based upon the sense of 

 smell by tlie neocortex built around the sense of vision. 



1 B.,: '.vver (1917-26), Minkowski (1920), Winkler (1921), v. Monakow (1924). 

 * p. :. 



