The Senses cmd the Nervous System lOi 



there exists a kind of eyelid, called a nictitating membrane, 

 which contracts into the forward angle of the eye when not 

 in use. The little red triangle in the inner corner of the 

 human eye is a vestige of the same apparatus. Just what the 

 shark does with it is something no one seems to know. 



HEARING 



For a long, long time, people looked at fish, saw no ears, 

 nodded their heads, and repeated, "Fish cannot hear." Then 

 a Benedictine monk at a place called Krems, in Austria, 

 started keeping trout in a pond. Being of a romantic nature, 

 he rang a dinner-bell when he fed them. Before very long 

 he found out that, whenever he stood at the edge of the 

 pond and swung his bell, the fish gathered for their meal. 

 Experimental evidence of the simplest and seemingly most 

 conclusive sort that fish could hear. Then came a skeptic. 

 He stood on the bank, swung his arm in the air, but left the 

 dinner-bell in the house. The hungry fish gathered for their 

 meal just the same. Experimental evidence of the simplest 

 and seemingly most conclusive sort that it was the sight of a 

 man on the bank swinging his arm, and not the sound of the 

 bell, which attracted the fish. And ever since, people have 

 been debating whether fish hear. 



The differences of opinion have been due to differences 

 in structure. Man indisputably has an ear, a visible feature 

 on the side of his head which acts as a funnel to collect 

 sound-waves coming through the air. This is called the outer 

 ear. From it a passage leads to the middle ear. Here are lo- 

 cated the ear-drum, which is set in vibration by the sound- 

 waves, and the ossicles, the three little bones poetically 

 named anvil, hammer, and stirrup, which transmit those 

 vibrations to the inner ear. The inner ear is the indispensable 

 part of the machine. It is a capsule filled with fluid and 

 surrounded by bone. It contains two very different mecha- 



