PUPILLARY VS. RETINAL ADAPTATION 155 



A pure-rod animal may need neither photomechanical changes nor a 

 mobile pupil, if he is content to be strictly nocturnal; and a diurnal ani- 

 mal will need neither very badly if he has a pure-cone retina and can 

 afford to be blind in dim light and utterly dependent then upon other 

 senses — as a pure-cone retina necessitates. But if a form whose retina is 

 duplex is to be able to appear indifferent to depth of water or to night 

 and day, or if a pure-rod animal is to be able to bask in comfort and to 

 defend itself from an enemy which routs it out of its daytime slumber, it 

 must have a widely excursive pupil or effective photomechanical changes. 

 Only one other mechanism, of limited value and with primarily other 

 functions, can sometimes be called into play for the regulation of stim- 

 ulation : the lid apparatus — sometimes, as regards its awning-effect, sub- 

 stituted for by projections on the upper part of the iris. The importance 

 of the lids in this connection can best be judged from instances in which 

 they are absent. Thus for example in the rays, the vipers, and the geckoes, 

 movable lids are lacking and the pupil is capable of an exceptional degree 

 of closure as compared with relatives which do have functional lids. 



We can expect to find that a pupil will tend to open unless something 

 makes it close. The inherent elasticity of the connective-tissue stroma of 

 the iris tends to insure this, and in some animals, notably certain small 

 mammals in which a dilatator is lacking, it is the only antagonist of the 

 sphincter. Where there is a dilatator, it is a thin sheet, but a broad one ; 

 and its total bulk compares favorably with that of the sphincter. One or 

 the other may be the stronger in a given case; but the orientation of the 

 dilatator, other things being equal, gives it a big advantage over the 

 sphincter. It is as though the two muscles are pulling on opposite ends of 

 a lever of the first class, the ratio of whose arms is 3.1416:1; for the 

 sphincter, contracting around the periphery of a circle, must shorten 

 TT units while the radially-oriented dilatator relaxes one unit, and the 

 sphincter cells must be capable of contracting TT times as fast. Of course 

 if a pupil can move at all it can both open and close; but it is sometimes 

 more important for it to open in dim light than for it to close in bright. 

 Where the pupil is static, it is even more necessary for a nocturnal an- 

 imal to have a large one than for a diurnal animal to have a very small 

 one, for the latter can always partly close his lids. Where it is mobile, 

 it is more desirable for a nocturnal pupil to close promptly in bright 

 light than for it to open so suddenly in dim light, where the accumu- 

 lation of rhodopsin is very slow anyway. Hence it is that many a small 

 nocturnal animal has a powerful sphincter muscle and no dilatator at all. 



