218 



ADAPTATIONS TO NOCTURNAL ACTIVITY 



Value of the Slit Form — The slit pupil, like nearly all pupils, dilates 

 in dim light to a perfect or almost perfect circle. Very many years ago, a 

 generalization had already been found possible, to the effect that the slit 

 pupil is associated with nocturnal habits. Yet under nocturnal conditions 

 the slit pupil becomes as round as any. Obviously it has nothing to do 

 with vision in dim light; what then does it accomplish? 



The broadly oval pupil of a frog can contract to a diameter which is 

 one-third of its fully dilated size; but to bring about this degree of con- 

 traction, the intensity of light must be increased 200 times. We have 



Fig. 85 — Diagrams of mammalian iris musculatures. 



a, round pupil of diurnal and strictly nocturnal forms, showing simple sphincter (solid 

 lines) and symmetrical dilatator (broken lines). 



b, vertical slit (of cat), characteristic of nocturnal forms which bask. Part of the sphincter 

 surrounds the pupil, but two bundles which cross above and below and continue to the 

 periphery have a scissor-aciion upon the pupil, compressing it laterally. The dilatator 

 (broken lines) is quite symmetrical — contrast Figure 88, page 223. Redrawn from Raselli. 



c, horizontal pupil (of horse), characteristic of ungulates, some whales, and other species. 

 Some sphincter fibers are oriented radially and are anchored in connective-tissue sectors 

 (stippled) which are devoid of dilatator fibers (broken lines). The pupil can expand to a 

 circle; but when the sphinaer fibers contract, the terminal ones force the pupil to become 

 a horizontal rectangle, indented by the corpora nigra (white). Based upon drawings and 

 descriptions of Eversbusch. 



already noted that the frog is more dependent upon the photomechanical 

 changes of its retina for avoiding dazzlement in bright light. His pupil 

 cannot cope with the situation; but for that matter, neither can any pupil 

 whose closure depends upon a ring-shaped, sphincter muscle. We our- 

 selves can easily be dazzled even when our pupils are closed as far as they 

 will go. True, a lizard or a garter-snake is comfortable in even brighter 

 light despite the practical immobility of the pupil — but these forms have 

 only the relatively insensitive cones in their retinae. 



Where the rods are very much in the ascendant, the circular pupil 

 ceases to be adequately protective. The sphincter may contract fully, but 



