THE NICTITANS; ADNEXA IN REPTILES 421 



lower lid contains a tough tarsal plate which stiffens it and makes it slide 

 smoothly. The nictitans contains a supporting cartilage, and its free 

 edge continues ventrally as a cord or tendon around to the back of the 

 eye (Fig. 143 b). Here it is attached to the retractor bulbi, hence is pulled 

 upon whenever that muscle shortens. The cord continues to an attach- 

 ment on the dorsal wall of the orbit. The horizontal nasolacrimal duct 

 has two openings or punctae lacrimalia, on the lower-lid margin, one at 

 the nasal end and the other several millimeters laterally from that point. 



Fig. 143 — Musculature of the nictitating membrane in various vertebrates. After Franz. 



a, frog, h, Sphenodon. c,\izatd {Lacerta). d, alligator, e, turtle, f, bird. B,B(Q)- 

 bursalis or quadratus muscle; B. r.- retraaor of bursalis; n- tendon to nictitans; N- optic 

 nerve; p. «.- tendon to lower lid; Pyr- pyramidalis muscle; R. b.- retractor bulbi muscle. 



Crocodilians — The crocodiles and their allies have gone back into the 

 water, but they had previously developed a full panoply of terrestrial 

 ocular accessories. They are exceptional among the reptiles in having the 

 upper lid the larger and the more mobile, as it is in mammals. Corres- 

 pondingly, the upper lid usually contains a (bony) tarsus. A cartilage- 

 like one is present in the nictitans, but there is none in the lower lid. The 

 large nictitans has developed a 'pyramidalis' muscle in its own tendon 

 (Fig. 143 d), which inserts on the back of the eyeball itself instead of on 

 the orbital wall as in Sphenodon — a change which keeps the nictitans in 

 a more nearly constant relation to the eyeball during eye movements. 



