The Senses ZS 
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. 
Fic. 156.—Vestige of nictitating membrane in a human eye. 
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 
