14 THE EYE IN EVOLUTION 



midnight and minimal at noon, a rhythmic variation first noted in plants ^ and 

 discovered in mammalian tissues (including the corneal epithelium) by van 

 Leijden (1917), confirmed in the human epidermis by Cooper (1939) and noted 

 in the cornea of tadjDoles by Meyer (1954).^ It is interesting that it is a physio- 

 logical process not seen in cancer cells. As will be fully discussed in a subsequent 

 volume, a diurnal variation of soine 3-5 inin. Hg occurs in the ocular tension.^ 



Established by constant repetition, these rhythms persist for some 

 time in the absence of environmental reinforcement when the external 

 rhythm has been artificially altered or has ceased. The mechanism of 

 these changes is unknown ; most of them are probably maintained by 

 rhythmic changes of activity in the neuro -vegetative centres of the 

 diencephalon, while the endocrine system, particularly the pituitary 



8 '° ^M» 



Fig. 8. — The Normal Diurnal Variation in the Intra-ocular 

 Pressure. 



The abscissae are times of the day ; the vertical line denotes when the 

 patient was asleep. 



complex, probably has some effect. Apart from the long-known 

 centres controlling thermo-regulation and urinary output, functions 

 such as the variation in circulating eosinophils are governed by the 

 diencephalo-hypophyseal system through the secretion of cortico- 

 steroids by the adrenal cortex (Hume, 1949 ; Porter, 1953), and the 

 evidence is rapidly accumulating that a region in the same neighbour- 

 hood exerts control over the intra-ocular pressure (v. Sallman and 

 Lowenstein, 1955 ; Gloster and Greaves, 1957). However they are 

 controlled, these rhythms are real and autochthonous. Thus in man 

 the normal variation in temperature persists for a considerable period 

 after the commencement of habitual night work, and the diurnal 

 rhythm of urinary flow survives a uniform intake of fluid throughout 



Kellicott (1904). 



See Blumenfeld (1939), Halberg (1953). 



Mailenikow (1904) ; see Duke-Elder (1952). 



See also p. 560. 



