290 



THE EYE IN EVOLUTION 



The orbit is cartilaginous and usually very incomplete ; in it the 

 eye lies in a bed of gelatinous connective tissue rich in blood sinuses. 

 The extra-ocular muscles are simple — four recti form a cone inserted 

 into the globe about its equator wliile the two obliques, arising close 

 together, sweep round the anterior part of the globe in front of the 

 recti and are inserted in common with the vertical recti. These muscles 

 may be enormously developed in the larger sharks ; in the basking- 

 shark, Selache, for example, they are as thick as the biceps of the 

 average man. The most characteristic structure in the orbit, however, 

 is the peculiar optic pedicle, a prop-like cartilaginous structure which 

 runs from the cranium to the posterior pole of the eye which it receives 



in an expanded cupped head, thus 

 forming a simple ball-and-socket 

 joint (Figs. 290 and 298). The 

 globe in its cartilaginous sclera 

 thus receives a firm support. 

 Sometimes the pedicle is firm and 

 stiff ; in some sharks and rays it 

 is slender, bending when the extra- 

 ocular muscles contract, 

 straightening and proptosing the 

 eye when these relax. Sometimes 

 it is incomplete, either not reach- 

 ing the eye or the cranium (in the 

 elongated orbit of the hammerhead 

 shark, Sphyrna zyqcena) (Fig. 387), 



Fig. 320.— The head of the rabbit-fish, • i , i i i • /xi, 



Chimcera monstrosa (Bland-Sutton's O^ mdeed, may be lacking (the 

 Lectures and Essays, Heinemann). spotted dogfish, ScylUorMnus) . 



THE HOLOCEPHALIAN EYE 



THE HOLOCEPHALiANS are represented today only by the Chimseras 

 (rabbit-fishes or ghost-sharks), somewhat shark-like fish of wide distri- 

 bution and very primitive in type (Fig. 320) ; they are all deep-sea 

 bottom fishes, and their eyes, which are of the same type as the 

 selachian eye, are remarkable for their adaptation to the dim illumina- 

 tion of the ocean depth. For this reason the pupils are large, round 

 and almost immobile, a tapetum is lacking, and the retina has an 

 unusually dense population of rods summated by an unusually small 

 number of ganglion cells (100,000 rods per sq. mm. and 600 ganglion 

 cells, Franz, 1905) — a ratio not exceeded amongst Selachians except 

 in the abyssal forms such as the luminous shark, Etmopterus. The 

 shape (f the eye is the typical ellipsoid of the selachian eye but, 

 curious • the sclera is thin, sometimes apparently discontinuous. 



