FUNCTIONS OF THE PUPIL 153 



The whole subject requires a very careful review and deserves the 

 attention of at least one cautious investigator prepared to devote his 

 active career to it. Practically everything which has been done in the 

 way of experiments upon the mechanism of the phenomena needs re- 

 peating with better technique than has been used. Not a single experi- 

 ment has yet been made with all factors controlled, nor a single graph 

 plotted with nearly enough animals averaged at each point. The photo- 

 mechanical changes are so fascinating that their students have been a 

 little too impatient to know what makes them go. 



(C) Pupil Mobility 



Functions of the Pupil — The pupil ordinarily has two chief respon- 

 sibilities. It must fix the immediate illumination of the retina, if it can, 

 at a value above the threshold of stimulation and below the point of 

 dazzlement or injury; and it must restrict the perceived light-pencil to 

 the center of the lens as far as possible. The pupil may be relieved of one 

 or the other of these duties. Thus in the chameleons the lids are fused 

 to the surface of the eyeball and their opening is a small one which 

 'stops down' the broad lens without benefit of changes in pupil diameter. 

 Again, a strongly nocturnal animal may so conduct himself that he is 

 seldom or never exposed to bright light, and his wide pupil may not 

 need, or have, much ability to close down, to convert a dazzling external 

 illumination into a tolerable intra-ocular one. 



Though not at all exceptional, such conditions are nevertheless assoc- 

 iated with the extremes of diurnal and nocturnal adaptation and be- 

 havior. The pupil may have little to do in a night-prowling species which 

 conceals itself well in the daytime and has no wish to bask in exposed 

 positions. It may have little to do in a sun-worshipper whose pure-cone 

 retina is so insensitive that various natural brightnesses above his thresh- 

 old are about equally comfortable to him. The slit pupils, so characteristic 

 of those nocturnal forms which do court the sunshine, deserve special con- 

 sideration beyond the scope of this discussion (see Chapter 9, section C) . 



The need for considerable pupil 'excursion' — range of size change — 

 is thus the greater, the more the animal attempts to attain twenty-four- 

 hour visual capacity. In such animals the responsibility of the pupil and 

 its controlling mechanism is greatest also, for the two aims of the pupil 

 are increasingly at cross-purposes when the period of daily activity be- 

 comes longer and longer. In the brighter hours, the closure of the pupil 



