594 AMPHIBIANS 



two bufonids which are hkewise permanently aquatic. The only massive 

 gland present is the Harderian, which, like the other 'terrestrial' features 

 (lids, nasolacrimal duct, flattening of lens, fusion of cornea and skin) 

 develops during metamorphosis, and forms most of the packing for the 

 eyeball in the largely membranous orbit. A broad and powerful retractor 

 bulbi muscle — probably evolved by the bifurcation of the external rectus 

 — is present, along with the six standard eye-muscles and a levator bulbi 



Fig. 172 — The anuran eye in vertical section; semi-diagrammatic; based largely upon 

 the leopard frog, Rana pipiens. x 1 1 1^ . 



ac- area centralis; to- inferior oblique; »V- inferior rectus; //- lower lid; Im, Im- lens muscles 

 (cf. Fig. 173); n- optic nerve; nm- 'nictitating membrane'; pn, pn- pupillary nodules; sc- 

 scleral cartilage; so- superior oblique; st- superior rectus; «/- upper lid; z- zonule. 



Stolen from the chewing-muscles. The retractor is of aid in swallowing 

 food, as well as in the protection of the eyeball. 



TTie eyeball is almost a perfect sphere, and has a deep anterior chamber 

 owing to the arching of the cornea and the recessed position of the lens 

 (Fig. 172). The curvature of the cornea blends smoothly into that of 

 the sclera, but is sharpened at its apex. During or after metamorphosis, 

 the fibrous sclera develops an extensive cup of hyaline cartilage, covered 

 externally by conneaive tissue which reaches forward, beyond the rim of 



