io8 The Life Story of the Fish 



remained, but equilibrium was lost, if the utriculus and the 

 three semi-circular canals were excised. 



These parts, then, compose the true equilibrating mecha- 

 nism. Like the sacculus and lagena, they contain a fluid 

 called endolymph, and are surrounded by the fluid of the 

 auditory capsule. The three canals being at right angles to 

 each other, a turn in any direction brings about a movement 

 of the endolymph in at least one of them through force of 

 inertia, which is registered for the brain by appropriate 

 nerves. Further, there are tiny bulbs made of bone, which 

 are free to move in the endolymph, and are surrounded by 

 nerve endings. They indicate by the particular nerve endings 

 on which they rest at any moment whether the body is up- 

 right, upside down, or on its side. 



The utriculus and its connecting canals are also involved 

 in maintaining the muscular tonus of the fish. The muscles in 

 all normal living creatures are never completely relaxed j 

 they are always, even when at rest, alert and ready for action. 

 But if the nerves coming from the upper inner ear are cut, 

 the fish's muscles become flabby — ^the tonus is lost. 



The whole equilibrating machinery is identical with ours. 

 What was good enough for the fish is good enough for vis. 

 It may even be too good for us, for our problem is easier 

 than theirs. What we see helps to keep us going straight and 

 right side up. A tuna or a swordfish, in mid-ocean and far 

 enough down to be out of sight of the surface, sees nothing 

 but uniform water. It has no landmarks to steer by. It must 

 have the automatic control. To the fresh- water fishes, which 

 are usually in sight of bottom or shore, this is not so essential. 

 Remove the machinery from such a fish, and it at first 

 wobbles, but soon learns to handle itself. Blindfold, now, 

 that same fish, and it is again out of control — proof that the 

 eyes alone are capable of doing the work, provided they have 

 something to guide on. This suggests the thought that it is 



