282 ADAPTATIONS TO SPACE AND MOTION 



that of any other breed of pigeon, has been found experimentally to 

 have a near-point at 40 centimeters. This implies that there is a consider- 

 able hypermetropia in the resting eye. 



Snakes — Turning to the snakes, we find that all sauropsidan rules are 

 off. As is explained fully in Chapter 16, the snakes seem to have origi- 

 nated as animals whose way of life was such as to allow the eye to degen- 

 erate extensively. Among the parts lost from the equipment handed on 

 to them by their good-eyed lacertilian ancestors were such items as scleral 

 cartilage, scleral ossicles, ciliary processes, annular pad, and (the eye 

 being very badly off indeed for a time!) iris muscles. In modem snakes, 

 the sclera is fibrous as in higher mammals, the eyeball consequently 

 spherical. 



The snakes eventually had to make good all of these losses as best they 

 could. The ciliary body being far out of contact with the lens, and with 

 its proper musculature stolen by the iris to become a revamped pupillo- 

 motor apparatus, it is quite out of the picture of accommodation. The 

 only intra-ocular muscles are the mesodermal ones of the iris, which have 

 been taken into the iris secondarily from the ciliary body. These muscles, 

 along with their new job of operating the pupillary aperture, have had to 

 retain the function of accommodation which they had when they were in 

 the ciliary body, but perform that function in an entirely new way : 



The iris is pressed forward into a strongly conical shape by the spher- 

 ical lens (Fig. 154, p. 456). At its root there is a powerful aggregation of 

 sphinctral fibers — those which have moved least from their old position in 

 the ciliary body. When these fibers contract, they draw in the sclero- 

 corneal junction and put a pressure upon the vitreous which it in turn 

 communicates to the back of the lens. The main body of the sphincter, 

 near the pupil, and the more-or-less radially disposed iris fibers also 

 contract simultaneously. The conical iris tries to flatten back into a 

 plane, augmenting the backward pressure upon the vitreous. The end 

 result is that the firm lens moves bodily forward, without appreciable 

 change in shape, a third to a half of the distance from its resting position 

 to the cornea. The cornea may also move forward a little, due to an 

 elongation of the eyeball which compensates for a reduction in its equa- 

 torial diameter by the pull of the iris. Accommodation in the snake eye 

 is thus accomplished essentially as in the eye of the squid (in which, 

 likewise, the intra-ocular pressure is raised in accommodation), and re- 

 sembles that of only the elasmobranchs and amphibians among the 

 vertebrates; and even there only to the extent that the lens is fixed in 



