Plica 



Semiluna 



^^^.^^ 



Fig. io. — 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 drawingjs 

 under this name. In the case of the shark (GaUus) the muscular mechanism i» 

 shown as dissected. 



