REPTILIA 



237 



Fig. 196. Illustrations of the nictitating membrane in the various 

 animals named, drawn from nature. The letter N. indicates the membrane 

 in each case. In man it is called the plica semilunaris, and is represented 

 in the two lower drawings under this name. In the case of the shark 

 (Galeus) the muscular mechanism is shown as dissected. (Romanes.) 



