340 THE EYE IN EVOLUTION 



myo-epithelial cells will absorb a considerable amount of light-energy ; 

 indeed, contraction occurs in the excised eye (Brown-Sequard, 1859 ; 

 Steinach, 1890 ; Magnus, 1899 ; Guth, 1901), the effect being most 

 marked in the blue region of the spectrum (Weale, 1956, in Rana 

 temporaria but not in Xenopus). Ordinarily, however, this action is 

 masked by the nervous reflex action which originates from retinal 

 activity. 



Amphibians are the first among Vertebrates wherein the movements of the 

 pupil are mediated by a neural mechanism, but although their pupils are more 

 actively motile than those of Fishes, their movements remain sluggish for the 

 sphincter muscle is still comparatively weak. Thus the oval pupil of the common 

 frog requires the stimulus of an increase of light -intensity of 200 times to induce 

 a contraction to ^ of its size from full dilatation. 



The le7is in the tadpole, like that of Fishes, is spherical and 

 approximates the cornea ; in the adult frog it moves posteriorly 

 leaving a deep anterior chamber and becoming somewhat, flattened in 

 an antero -posterior direction (axial : equatorial diameter, 1: 1-3, Rabl, 

 1898) (Fig. 398). The large nucleus is dense and the periphery soft 

 and elastic, the internal structure and the epithelium conforming to 

 the usual vertebrate plan with a vertical suture anteriorly and a short 

 horizontal suture in the posterior part, as in selachian fishes (Fig. 314). 



The blood supply to the eye has several points of interest and has 

 been studied by H. Virchow (1881), Tretjakoflf (1906), Grynfeltt (1907), 

 and Kutsukaka (1952). It is derived from the ophthalmic artery, a 

 branch of the internal carotid. From this artery two posterior ciliary 

 branches are given off which enter the eye posteriorly just above the 

 disc and diverge to run forwards in the choroid on its nasal and temporal 

 sides to supply the choriocapillaris. From this layer blood is gathered 

 into the central venous plexus of Gaupp, the flat vessels of wliich run 

 in a general vertical direction in the midst of the choroid ; these 

 converge to leave the globe — dorsally as two veins which imite to form 

 the superior bulbar vein, ventrally as a single vein which enters the 

 jugular vein. After giving off the ciliary branches the main trunk of 

 the ophthalmic artery enters the globe on its under aspect in the region 

 of the foetal fissure and runs forward to the ciliary region ; here, at 

 the mid-ventral pomt, it sends off two branches which rim circum- 

 ferentially round the ciliary body as an arterial circle. From this the 

 superficial arteries of the iris emerge ; the corresponding veins lie 

 more deeply and drain partly into the venous plexus of the ciliary 

 body and thence to the choroidal veins, partly through the two 

 crescentic segments of the ciliary venous sinus into the subconjunctival 

 veins. After it has given off the ciliary arterial circle, the ophthalmic 

 arter lirns backwards on itself as a " hyaloid artery " and almost 

 immc: ly divides into two branches, a nasal and temporal, wliich 



