718 



THE EYE IN EVOLUTION 



intercalary cells, the latter being laigmented except in Sphenodon in which the pigment 

 is extracellular. In the lizards this pigment shows adaptive changes, moving towards 

 the sensory terminations of the cells on exposure to light (Nowikoff, 1910). Peripheral 

 to the visual cells lies a layer of bipolar ganglion cells, the nerve fibres issuing from 

 which form the j^arietal nerve which runs down the parietal stalk either to the right 

 (Anguis, Lacerta) or left {Sphenodon) habenular ganglion. The surfaces of the visual 

 cells of the retina as well as those of the lenticular cells facing the cavity of the vesicle 

 are richly provided with cilia ; the cavity itself is filled with a delicate syncytium 

 with a few oval nuclei enclosing spaces filled with fluid constitviting a " vitreous." 



It is to be noted that in all cases the svirface ectoderm takes no part in the 

 formation of the ocular vesicle, there is no secondary invagination, and the retinal 

 cells are verted, resembling the eyes of Invertebrates rather than the paired lateral 

 eyes of Vertebrates. 



Fig. 865. — The Parietal Eye of the Slow-worm, Axains fragilik. 

 CC, connective tissue ; GC, ganglion cells ; L, lens ; PC, pigment cells 

 PN, parietal nerve ; V, vitreous ; VC, visual cells (after Nowikoff). 



The function of the pineal organ in those species in which it assumes an 

 ocular formation is undoubtedly optic although it would appear that it is 

 confined to the directional appreciation of light and is incapable of optical 

 imagery (Mygind, 1949). In those species wherein a glandular structure is 

 evident, even among Mammals, the function of the pineal body is still 

 obscure despite the considerable amount of research which has been devoted 

 to the subject by morphologists, histologists, pathologists and clinicians. 

 In man it reaches maturity between the ages of G and 7 years whereafter 

 involutive phenomena begin to appear in the form of hyalinization, calcifica- 

 tion and cystic formation (Rio-Hortega, 1922-29 ; (dobus and Silber, 1931 ; 

 and others). This involution after puberty together with the variations in 

 the size of the organ observed during pregnancy, with sexual activity or after 

 castration both in human subjects ^ and in animals, ^ have confirmed the 

 clinical impression that its main association concerned skeletal growth and 

 the sexual functions. It is to be remembered, however, that Pelizzi's (1910) 



1 Brandenburg (1929), Frada and Micale (1941). 

 ^ Santamarina and Venzko (1953). 



