HORMONAL CONTROL 559 



Some evidence, however, has been made available in the frog. Derman (1949) found 

 that while hyiDophysectomy had no effect on the retraction of the retinal pigment 

 during dark-adaptation, it slowed but did not inhibit the migration of pigment during 

 light adaptation, while injection of an extract of the intermediate lobe of the pituitary 

 body provoked the migration characteristic of light-adaptation in the dark-adapted 

 hypophysectomized frog. This action was abolished after section of the optic nerve. 

 Damage to the hypothalamus has also been said to influence the migration of the 

 retinal pigment in this animal (Kitashoji, 1953 ; Nakamura, 1954), an effect also shared 

 by epinephrine (Nakamura, 1955) and Pregnenolone, a relation of the adrenal cortico- 

 steroids (Paiynarale, 1952). It is clear, however, that nervous influences predominate 

 over any effect that may be exercised by the pituitary-hypothalamic system in 

 Amphibians. 



In the higher Vertebrates any such effect is even more insignificant. It may, 

 however, be of interest that Rubino and Pereyra (1948-50) have claimed that 

 the degree of light -sensitivity in man undergoes a diurnal rhythm, being increased 

 during the night ; the fact that this faculty is maintained unimpaired in the amblyopic 

 eye or in patients affected by primary pigmentary degeneration suggests that this 

 cyclic change is centrally determined. There is, indeed a considerable body of opinion 

 which maintains that this latter disease may sometimes be associated primarily with 

 a hypothalamic-endocrine disturbance, the most dramatic instance of which is seen 

 in the Laurence-Moon-Biedl syndrome (see Zondek and Koehler, 1932 ; Zondek and 

 VVolfsohn, 1940 ; Alajmo and Rubino, 1952). 



The gonadotropic action of the hormones elaborated in the anterior lobe 

 of the j^itiiitar}' in Vertebrates is well established, ^ both in determining the 

 development of the organs of sex and governing the cyclic activities of 

 reproduction. The rate and rli}i:hmic variation of the secretion of the 

 gonadotropic hormone in Mammals are regulated by the tuber nuclei of the 

 hj'jjothalamus, isolated injuries to which have caused sexual disturbances 

 in all Mammals so far studied - ; delicacy of adjustment and integration is 

 thus achieved and in the absence of this nervous control the secretion 

 continues without coordinated balance. In many Vertebrates the sexual 

 rhythm is adapted to the most favourable season of the annular solar cycle 

 and one of the most potent influences in determining this process is light. 

 We have already seen ^ that the sexual maturation of many Fishes, Reptiles, 

 Birds and Mammals is determined in this way by jjhotoperiodism, and that 

 the process can be accelerated or retarded by altering the relative duration 

 of light and darkness in the diurnal cycle. In most cases the stimulus is 

 retinal in origin and neural in conduction along the optic nerve and is relayed 

 not to the visual centres of the brain but to the h;y|3othalamus which 

 activates the j^ituitary (Le Gros Clark et ol., 1937-39) ; and blinding, 

 hypophysectomy or section of the nervous connections between the 

 hypothalamus and the jjituitary destroys the cycle, while the injection of 

 pituitary extract activates it (Hill and Parkes, 1933 ; Thomson, 1951-54 ; 

 Thomson and Zuckerman, 1953-54 ; Donovan and Harris, 1956). In some 



1 For reviews, see Allen (1939), Burrows (1949), Brown (1950), Galgano and Mazzi (1951). 



2 Guinea-pig, rabbit, ferret — Brooks (1938-40), Bard (1940), and others. 

 » p. 16. 



