336 



THE EYE IN EVOLUTION 



Fig. 397. — Diagram of an Anuran Ete. 



Ch, choroid ; MV, nieinbrana vasculosa retinae ; ON, optic nerve ; 

 PL, protractor lentis muscle ; PN, pupillary nodule ; S, scleral cartilage ; 

 Sc, sclera ; TC, tensor choroidese ; V, hyaloid vein ; VS, ciliary venous 

 sinus ; Z, zonule. 



Fig. 3'JS. — [Section through the Head of the Frog to show Both 

 Eyes ( x 20) (Norman Ashton). 



meridional smooth fibres running backwards to be inserted into the 

 choroid. This muscle is discontinuous and is absent in the lateral 

 segments of the globe. In the same two regions a ciliary venous sinus, 

 reminiscent of a canal of Schlemm, forms two discontinuous crescents, 

 a dorsal and a ventral, situated between the sclera and the ciliary 

 body, connecting the veins of the iris with the subconjunctival veins 

 (Tretjakoff, f906). Also in the same two regions there are two pro- 

 tractor lentis muscles (Tretjakoff, 1906) of mesodermal origin 

 supplied by a branch of the 3rd nerve. Each, the dorsal and the 

 ventral, arises from the corneal margin, enters the ciliary triangle and 

 is inserted into the hypertroplued ciliary processes. These by traction 

 on th 'lular fibres 23ull the lens forward in accommodation, approxi- 

 mating is structure to the cornea as in Selachians (Figs. 400-1). 



