156 ADAPTATIONS TO ARHYTHMIC ACTIVITY 



In emergency situations, in the higher vertebrates, the pupil seems to 

 try to make sure of enough light — its response to pain, to rapid deep 

 breathing, or to strong emotion of any kind is to dilate, sometimes so fast 

 and far (as in the hyaenas) as to seem to be under the animal's control. 

 Close scrutiny of an object, ordinarily a calm and non-emergency proce- 

 dure, is on the other hand accompanied by contraction. This 'accommo- 

 dation reflex' is not a true reflex, but a fortunate accident of innervation. 

 The iris sphincter and the ciliary muscle are supplied by the same nerves. 

 The 'reflex', which in man occurs more with convergence than with the 

 accompanying accommodation, and is in that aspect truly reflex, is of 

 some value in all its possessors (though of most value in those vertebrates 

 whose irides actually aid in accommodation; see Chapter 11, section C). 

 In its accommodated form the lens has more spherical aberration. It 

 therefore needs more stopping-down, and receives this upon the reflex 

 closure of the pupil, thus increasing resolving power for approaching ob- 

 jects. As objects approach, the amount of light received from them in- 

 creases enough to compensate adequately for any accommodatory reduc- 

 tion of the area of the pupil.* Another pupil reflex in man, of no obvious 

 value, has recently been described : it consists of a slight contraction at 

 the moment of fusion of the two monocular images into a single stereo- 

 scopic one, as when one is observing through a stereoscope. 



In the birds, whose photomechanical changes are more conspicuous 

 than one would expect from the phylogeny of the changes and of the 

 pupil, the reason appears to be that the bird pupil pays less attention to 

 illumination than to accommodation and emotion. The 'play' of the 

 pupil of a captive wild bird will readily convince one of this, though the 

 irides of tame birds, such as chickens, may react quite staidly to light. 

 There is thus no inconsistency in the fact that the birds have both iridic 

 and retinal photomechanical changes well developed (Table II, p. 150). 



The behavior of the pupil is influenced more immediately, as well as in 

 in the long evolutionary run, by the presence or absence of retinal migra- 

 tions. The first reaction of a pupil (i.e., partial closure) upon a sudden 

 increase of illumination is not permanent. It gives opportunity for the 

 retina to reduce its sensitivity; and when this has been sufficiently effected, 

 the pupil slowly reopens to a size which is smaller than its original one, 

 and is constant until a further great change in intensity. This physiolog- 



*In the dog, according to Nicolas, the accommodation reflex works backwards — the pupil 

 dilating for near, contracting for distant objeas; and there is no consensual reflex. These 

 peculiarities have yet to be explained. 



