368 



THE EYE IN EVOLUTION 



lying naso-ventrally provided with a single duct (Loewenthal, 1935-36 ; 

 Schwarz-Karsten, 1937 ; Bellairs and Boyd, 1947-50). The lacrimal 

 gland, however, is absent in the chameleon and many geckos. The 

 naso-lacrimal duct enters the nose within the accessory olfactory 

 vomero-nasal organ of Jacobson. The nictitating membrane is pulled 

 across by a tendon-like cord arising from its free edge and attached to 

 the dorsal wall of the orbit, its movements being controlled by a 

 special arrangement of muscles behind the eyeball (Fig. 442). 



In addition to the rectus muscles and a well-formed retractor 

 bulbi, two extra muscles are inserted into the posterior aspect of the 

 globe, both supplied by the Vlth cranial nerve (Fig. 443). The first, 

 the BUiiSALis (quadratus) muscle, is inserted into the sclera near the 



Fig. HI. — The Orbit of the Monitor, 

 Varan us. 



With the eye removed showing the 

 nictitating membrane with its tendon 

 looping through the bursahs muscle (after 

 Bland- Sutton). 



Fig. 443. — The Posterior Aspect of 

 the Globe of Lacerta. 



B, bursalis muscle ; N, the tendon of 

 the nictitans ; ON, optic nerve ; R, 

 retractor bulbi muscle ; RB, retractor 

 bursalis muscle (after Franz). 



optic nerve and round it the tendon of the nictitating membrane loops 

 so that the latter is drawn taut when the muscle contracts ; from it a 

 muscular slip runs upwards to be inserted more dorsally in the sclera, 

 the RETRACTOR BURSALIS, wliicli acts by bracing the bursalis so that 

 the muscular apparatus and the looped tendon are kept away from the 

 optic nerve when contraction occurs. In most lizards ocular move- 

 ments are sluggish or occasionally absent, a marked and extraordinary 

 exception being the insectivorous chameleon ^ : in it the extra-ocular 

 muscles are very fully developed (Leblanc, 1925). 



The orbit of lizards is open and fenestrated, a peculiarity being 

 that the optic nerves pass through several openings in the endocranium ; 

 the posterior bony wall is very deficient to allow room for a wide 

 gape of the jaws. 



THE CHELONIAN EYE 



THE TORTOISES AND TURTLES are the most ancient of surviving 

 Reptili'^s 2 — sluggish animals encased in a dorsal and ventral bony cara- 

 pac( ' f) the shelter of which the head as well as the limbs and tail can 



1 p. 604. 2 p. 234. 



