THE EYE OF SPHENODON 617 



The Eye as a Whole — The adnexa bear closer resemblance to those of 

 lizards than to those of amphibians or other reptiles (see pp. 420-3, Fig. 

 143 on p. 421), The lacrimal gland can be lacking in a lizard (as it is in 

 Sphenodon). The nictitans tendon attaches to the orbital wall in Sphen- 

 odon as in lizards; and the nictitans musculatures are mutually convert- 

 ible. The two-headed retractor bulbi of Sphenodon is the largest of the 

 extra-ocular muscles, and is innervated not only by a branch of the sixth 

 cranial (abducens) nerve but also by a sprig from the ciliary ganglion. 



The eyeball has been described in its entirety only by Osawa (1898), 

 who made certain errors and oversights. It is large for the size of the 

 animal (as compared with a diurnal lizard) , with an equatorial diameter 

 of 17mm. and a slightly shorter axis. A considerable change in surface 

 curvature takes place at the limbus, creating a sclero-corneal sulcus 

 (which, it will be remembered, we have not seen in any forms below 

 Sphenodon, but which we will encounter regularly hereafter). 



The sclera contains a cartilaginous cup, and, including the fibrous 

 layer outside of this, is about as thick as the retina. Anteriorly the carti- 

 lage extends about to the ora terminalis, and is there overlapped slightly 

 (externally) by the circlet of scleral ossicles.* These agree in number 

 (16-17) better with those of lizards (12-15) than with those of turtles 

 (6-11). The cornea is strongly arched, of uniform thickness throughout, 

 and is 9.5mm. in diameter — the same size as that of an Iguana eye of the 

 same diameter, but relatively large as compared with that of such a sun- 

 worshipping diurnal lizard as the deserticolous Uromastix (eye 12mm., 

 cornea 3.4). The cornea has a thin epithelium (consisting of only two 

 layers of cuboidal cells with round nuclei) , Descemet layers, and a thick 

 propria which contains no such vertical fibers as are described by Osawa. 

 The inner layers of its fibers, at the margin of the cornea, blend into a 

 narrow thickened zone of the sclera which lies opposite the iris root. 



The chorioid is especially heavily pigmented on its scleral side. It is 

 thicker than in small-eyed lizards (but no thicker than in, say, Varanus) , 

 and is well vascularized. Grouped and scattered in it are peculiar spher- 

 oidal pigment cells with central nuclei suspended by delicate protoplas- 

 mic strands, as in a brown-fat cell. These cells form a dense aggregation 

 opposite the fovea (not visible, owing to bleaching, in Fig. 82 on p. 189). 

 The glass membrane can be easily followed through the ciliary body 

 (where it is greatly thickened), but not into the iris; and the chorio- 

 capillaris also extends well into the ciliary region. 



*Not obvious (e.g., Fig. 178) unless section passes through center of ossicle. 



