The Senses 



215 



although usually much less perfectly developed than it 

 is in birds. Alligators, however, have it fully functional. 

 In the inner corner of our own eyes we may detect a trace 

 of it, useless to us, but showing that far back in dimly 

 imaginable geological epochs our forebears had need of a 

 third eyelid. 





FIG. 156. Vestige of nictitating membrane in a human eye. 



i 



The Sense of Hearing 



"The Gauls," says Livy, " having discovered that the 

 rock Carmentalis was accessible, one night when it was 

 pretty clear, sent a man to examine the way, without 

 his arms which were afterward handed to him. Others 

 followed, lifting and assisting each other, according to 

 the difficulties which they encountered in the ascent, till 

 they reached the summit. They proceeded with so much 

 silence that neither the sentinels nor even the dogs, ani- 

 mals usually so vigilant as to be aroused by the slightest 

 noise, took any alarm. They did not, however, escape 

 the notice of the geese, which, being sacred to Juno, 

 had been fed by the Romans notwithstanding the famine 

 caused by the siege. This saved the capitol; for, by 



