564 



HIGHER FISHES 



The oculomotor muscles of adults are orthodox, though in the embryo 

 (in Squalus, at least) a mysterious extra muscle ('muscle E') appears 

 and then degenerates. In Chimcera, which is primitive in many anatom- 

 ical respects, the internal rectus originates far nasally as in lampreys; 

 but in most elasmobranchs the four recti originate close together, and 

 the orbit ordinarily affords room for them to form a cone as in mammals. 

 They insert at about the equator of the eyeball, the internal rectus how- 

 ever a little behind and the external a bit ahead. The obliques originate 

 close together, far forward, and share insertion-sites with the correspond- 

 ing vertical recti. This arrangement — probably more primitive than that 

 in living cyclostomes — is essentially preserved in higher fishes, and 



Fig. 164 — The eye of a shark, Carchawdon carcharius. xWi. 

 Combined from figures of Franz. 



a, horizontal, b, vertical seaion. c- cornea; c/- ciliary folds, forming anchorage of gelatin- 

 ous zonule; ch- chorioid; ext- external rectus; «'- iris; inj- inferior reaus; int- internal rectus; 

 /- lens; op- optic pedicel; p- lens-muscle papilla (c/. Fig. 166); r- retina; s- suspensorium of 

 lens; /c- scleral cartilage; t/- fibrous portion of sclera; lo- superior oblique; /Mp- superior rectus. 



indicates that the original function of the obliques was to impart 

 compensatory reflex wheel-movements to the eyeball in the plane 

 of its equator (Fig. 163; cj. Fig. 16, p. 37, and p. 303). 



A characteristic structure of the orbit is the cartilaginous optic pedicel, 

 running prop-like from cranium to eyeball. At the eyeball end, it is often 

 expanded and cupped to fit a broad, low boss on the back of the sclera, 

 thus forming a ball-and-socket joint for the rotation of the eyeball. In 

 various genera it may be lacking (Scylliorhinus, deep-sea forms; always 

 through disappearance?), or may not reach to the eyeball, or may even 

 contact the eye but not the cranium (Sphyrna) . In a few forms — sharks 

 as well as rays — it is slender and so bent and elastic that its tendency to 

 straighten itself can proptose the eyeball when the extra-ocular muscles 



