THE EYE OF SPHENODON 619 



run roughly radially; but these could hardly be called iris folds (e.g., in 

 the anuran sense). 



The bridge-membrane, besides contributing to the anchorage of the 

 lens, probably helps to hold the iris against the periphery of the anterior 

 surface of the lens during accommodation — if any — by using the iris as 

 a third-class lever, thus confining the accommodatory deformation of the 

 lens surface to the portion behind the pupil (see footnote, p. 614). 



Gross dissection reveals what is apparently a transversalis muscle; but 

 this lacks histological confirmation as yet. 



In the loose meshwork of the ciliary body, the most conspicuous struc- 

 ture is the enormous canal of Schlemm, which lies at the inner side of 

 the sclera just behind the thickening at the iris root. Toward its posterior 

 side there is a large annular nerve, as in most lizards. The canal is sup- 

 posed to be lacking in Sphenodon (but Osawa was looking for it in the 

 sclera, where it seldom lies in reptiles; and his drawing shows it plainly 

 — unlabelled — in its true location) . The ciliary muscle is relatively weak. 

 It does not commence at the anterior end of the ciliary zone, but about 

 a quarter of the way back. Its fibers originate partly upon scleral tissue 

 lying behind the iris-root thickening, partly from the inner surface of the 

 sclera paralleling the posterior half of the ciliary body, and insert into 

 the connective tissue of the orbicular base-plate and on the outer surface 

 of the glass membrane in that region, a very few of them all the way 

 back to the ora terminalis. In horizontal sections of the eye, the anterior- 

 most ciliary-muscle fibers on one side of the eye are seen to be cut in 

 cross-section (c/. such lizards as Seps and Lacerta; p. 624). 



According to Ida Mann the iris is covered anteriorly by a layer of 

 chocolate chromatophores, through breaks in which some coppery and 

 silvery-buff patches of deep-lying iridocytes can be seen in the living 

 animal. The blood vessels form a system of arcades aimed inward toward 

 the pupil, and many of them form loops which burst free of the iris 

 surface into the anterior chamber. This iridic circulatory pattern re- 

 sembles those of crocodilians and geckoes about equally well.* Muscle 

 fibers with a sphinctral function are evenly distributed throughout the 

 stroma from pupil to iris root; but they are concentrated toward the 

 periphery since the iris is thickest here and thins gradually toward the 

 pupil. The dilatator fibers lie against the epithelial retinal layers. It is 

 clear that they are direct derivatives of the sphincter (as perhaps in all 



* Unfortunately, the vascular pattern of the whole eye of Sphenodon has never heen worked 

 out. 



