630 REPTILES 



idophis, Charina, and Epicrates have it as in the colubrids; but in Python 

 it is located closer to the outer surface of the cornea than to the inner, 

 and its connections are to the conjunctival veins. 



The iris is fairly thick, with a highly irregular anterior surface. Its 

 stroma contains not only melanophores as usual, but often guanophores 

 and lipophores as well. The circulation is totally different from that in 

 lizards (p. 624) , consisting of a plexus occupying the whole iris, in which 

 the small vessels cannot be identified as veins or arteries. The striated 

 iris musculature is entirely mesodermal, and derived phylogenetically 

 and embryologically from the ciliary region. Most of the fibers are 

 circular in direction, and most of these are gathered into two accumu- 

 lations, one near the pupil and serving as the sphincter pupillae, and the 

 other toward the root of the iris and acting as the muscle of accommo- 

 dation. Some of the fibers of this latter muscle may be pressed back into 

 the ciliary roll. The dilatator fibers underlie the sphinctral masses, and 

 may also occasionally reach back into the ciliary roll (e.g., in Acanth- 

 ophis) . In diurnal snakes the pupil has a peculiar, special duty owing to 

 the absence of movable lids : during sleep it constricts, to facilitate visual 

 unconsciousness. 



The lens, unlike that of lizards, has sutures; and it lacks the ringwulst 

 of other sauropsidans. Consequently it is not much flattened, and is 

 helped toward its subsphericity (flatness index 1.1 — 1.25) by an 'anterior 

 pad' except in Eryx and Charina (and perhaps all other boids, or at least 

 the fossorial ones) . Like a ringwulst, the anterior pad is simply a region 

 of the lens epithelium in which the cells are extremely tall instead of 

 cuboidal. Except in water snakes, whose lenses deform somewhat during 

 accommodation as well as moving forward, the lenses of snakes are 

 firmer than those of lizards or turtles. 



The zonule is peculiar, and perhaps variable, in organization. In its 

 fullest development it consists of two radially-fibrous membranes. One 

 of these arises from the front of the ciliary roll and passes along the 

 back of the iris and over the face of the lens. The other is essentially 

 equivalent to an anterior hyaloid membrane of the vitreous, and arises 

 from the back of the ciliary roll and surrounds the back of the lens. 

 This posterior 'leaf is readily seen in some snakes (e. g., Bitis, Coronella, 

 Arizona), but in many it can be made out with difficulty or not at all. 

 The anterior 'leaf would appear to function chiefly in hauling the lens 

 promptly backward to its resting position upon the relaxation of accom- 



