ACCOMMODATION IN SNAKES 283 



shape (water snakes excepted) and moves forward, adjusting the eye for 

 nearer objects. The physiological indentation of the anterior scleral re- 

 gion is so pronounced that the sclera may even have a couple of perma- 

 nent meridional furrows anteriorly, in readiness for their further deep- 

 ening during accommodation. 



Beer found that in a snake eye in which the posterior sclera had been 

 cut away, and in which the accommodatory action was evoked electri- 

 cally, the lens moved backT^ard. This was due to the (unbalanced) rise 

 of anterior-chamber pressure, which in the intact eye is far exceeded by 

 the rise in vitreous pressure, so that the lens has to move forward. Ac- 

 cording to Beer, various snakes are anywhere up to nine diopters hyper- 

 metropic, but most have more than enough accommodation to overcome 

 their refractive error. 



In some snakes, particularly those with a fovea {Dryophis and The- 

 lotornis) , there is a nasad component of the forward motion of the lens. 

 This is exactly equivalent, in its optical consequences, to the nasad move- 

 ment of the lens of a teleost when the accommodation is relaxed to adjust 

 for near objects. The same basis obtains in the two cases: a strongly 

 temporal position of the area centralis in the retina (see Fig. 79, p. 186; 

 cj. Fig. 77, p. 185, and Fig. 105d, p. 261). 



Mammals — As with peoples and their governments, vertebrate eyes get 

 the kind of accommodation they deserve. The degree of 'eye-mindedness' 

 in the subphylum sinks from the higher jfishes to the amphibians, rises 

 sharply in the reptiles, still higher to a peak in the birds and falls off 

 woefully again in the mammals — with some recovery in the highest forms 

 and a very considerable one in the squirrels and simians, to be sure. The 

 engineering efficiency of the accommodatory apparatus runs exactly par- 

 allel with this variation in the value set upon vision. 



The mammals originated as small-bodied, small-eyed, forms which 

 were almost certainly nocturnal. Within the marsupial and placental 

 series, parallel evolution has culminated in the production of swift, large- 

 bodied, large-eyed types (the kangaroos on the one hand, the ungulates 

 on the other) , adapted to open country, where good vision is more valu- 

 able than to a forest animal. Such animals have expanded their visual 

 capacities to twenty-four-hour performance and some have gone on close 

 to diurnality, with a steady increase in visual acuity. The more eye- 

 minded forms, with much sharper vision than their primitive relatives, 

 may also have much more extensive accommodation. But since their 

 evolution passed through the bottle-neck of the monotremes, opossums. 



