PUPILLARY VS. RETINAL ADAPTATION 157 



ical adaptation of the pupil is to be sharply distinguished from the imme- 

 diate reaction it gives to increased illumination. Upon a reduction of 

 illumination the pupil only dilates to a new constant size, there being 

 rarely a brief preliminary contraction. The rate of the complex light- 

 adaptation and of the simpler dark-adaptation of the pupil depends upon 

 the method by which the retina changes its sensitivity. When only cones 

 are present, as for example in diurnal snakes, the change in sensitivity is 

 slight and rapid and the pupil also makes quickly the slight adjustment of 

 which it is capable. Where many rods (but no photomechanical changes) 

 are present, as in the guinea-pig for instance, the light-adaptation of the 

 pupil is governed by the relatively rapid destruction of rhodopsin. Where 

 both rods and photomechanical changes are conspicuous the rhodopsin is 

 more abundant and, especially in fishes and owls, slow to bleach. The 

 pupil then adapts much more slowly (frogs) or not at all (fishes) , since 

 retinal sensitivity is altered primarily by the relatively slow pigment 

 migration. It is in fact quite probable that in the teleosts the rhodopsin is 

 seldom all bleached, since the rods are completely shielded by expanded 

 pigment. Higher vertebrates, it would seem, must be able to form more 

 rhodopsin since so much must be destroyed at every light-adaptation. 

 Very likely, the greater instability to light of the rhodopsins of higher 

 vertebrates (through which the rhodopsin is quickly destroyed, and the 

 threshold of the retina as quickly raised) is partly a consequence of the 

 lack of such perfectly protective photomechanical changes as the lower 

 vertebrates possess. 



Pupil movements are thus not only less marked, but less rapid, in prim- 

 itive forms which still depend primarily upon retinal migrations. Phyloge- 

 netically there has been a steady perfection not only of the pupil as an 

 adjusting mechanism, but also of its method of actuation and control. In 

 the few fishes which have iris muscles, these are pigmented and respond 

 directly and autonomously to light — the sphincter by contracting, the 

 dilatator by losing tonus. These actions are extraordinarily slow — elas- 

 mobranch pupils take two or three minutes to close in bright light and 

 an hour or so to re-open in the dark! Such muscles are unresponsive to 

 electrical stimulation and to neurotropic drugs like atropin, since such 

 agencies operate through nervous connections. In the amphibians and 

 some reptiles some degree of autonomy persists, although in the intact 

 animal it is masked by the superposition of a control through the nervous 

 system by means of reflexes originating in the retina. In the frog it has 

 recently been reported that an intra-ocular reflex occurs — the pupil of the 



