560 THE EYE IN EVOLUTION 



Birds, on the other hand, and perhaps in some Mammals, hght appears to 

 activate the pituitary or the central nervous system directly, perhaps through 

 the spectral sensitivity of a coproporphyrin (Parker et al., 1952). ^ In ducks, 

 for example, Benoit and his collaborators (1952-54) have shown that light, 

 concentrated as it traverses the eye, travels through the orbit and reaches 

 the hypothalamus, thus regulating the gonadotropic action of the hypo- 

 physis ; excision of both eyes does not inhibit but, by increasing trans- 

 missibility, rather enhances the gonadotropic activity, and the pituitary 

 body of immature ducks stimulated by increased illumination can excite 

 cestrus when implanted into immature mice. 



The seasonal migrations associated with the sexual cycle of Birds and 

 Mammals is similarly controlled by photoperiod,^ as well as the seasonal 

 moults and changes of colour in the feathers or hair of many Birds or 

 Mammals.^ In these cyclic changes the pituitary is the most potent factor 

 (Witschi, 1935 ; Brown and Rollo, 1940 ; Lesher and Kendeigh, 1941 ; 

 Kobayashi and Okubo, 1955) ; similarly, hypophysectomy abolishes the 

 cyclic moulting of ferrets (Bissonnette, 1935-38). It would seem, indeed, 

 that the pituitary is the only endocrine organ involved in these activities 

 in Mammals since castration or thyroidectomy has no such effect on the 

 varying hare (Lyman, 1943). 



The influence of the hypothalamo -hypophyseal system on the growth, meta- 

 morphosis and metabolism, of Vertebrates and its pressor effects on the circulation are 

 potent but are without marked interest in our survey of the development of the visual 

 system. An associated optic -diencephalic relationship, however, may be seen in the 

 observation that in the rabbit exposure to light increases the urinary excretion of 

 17-ketosteroids (Siliato, 1955). Another exception niay be constituted by the photo- 

 glyccemic reflex recently explored by Italian workers but not otherwise investigated. 

 It was originally claimed by Cavallacci (1937) that stimulation of the retina by light 

 altered the metabolism of sugar, the blood -sugar curve being normally different if the 

 sugar were ingested by day or by night. This finding has been confirmed by Bassi 

 (1945) and Rubino and his collaborators (1948) who concluded that abnormalities 

 occurred in persons affected by glaucoma and primary pigmentary degeneration of 

 the retina, both of which diseases may have hypothalamic implications. In this 

 connection the suggestion that dark-adaptation is impaired in adiposo -genital 

 dystrophy, a disease associated with hypothalamic disturbances, may possibly be of 

 interest (Landau and Bromberg, 1955). 



A relationship, still vague but yet undoubted, exists between the intra-ocular 

 pressure and the endocrine system, particularly the hypothalamo -hypophyseal complex, 

 and claims have been put forward from time to time that primary glaucoma is often 

 an expression of a diencephalic disturbance (Hess, 1945 ; Zondek and Wolfsohn, 

 1947 ; Magitot, 1947 ; Alajmo and Rubino, 1952 ; and many others). The pupillary 

 changes described by Lowenstein and Schoenberg (1944) point to some neurogenic 

 sympathetic disturbance in this region of the brain in this disease. That a hypo- 

 thalamic centre exercises some control over the intra-ocular pressure is clear (v. 



* Ivanova (1935) produced evidence that the skin may also be a possible receptor in the 

 house span iw, Passer domesticus. 

 ' p. 16. 

 ^ p. 21. 



