220 THE SENSES OF ANIMALS 



the skull close to the cochlea, the part of the ear concerned 

 with hearing, which is connected to one of the bags. All are 

 filled with fluid which is continuous throughout the canals, 

 the bags and the cochlea. 



Inside each of the semicircular canals and near one end 

 there is a pimple covered with elongated cells bearing 

 filaments like hairs which project into the fluid. In each of 

 the little bags there is a similar mound, but suspended among 

 the hairs there are many grains of limy matter. The last are 

 the gravity-receptors, and the pressure of the limy grains 

 on the hairs stimulates the cells into sending messages to the 

 brain along the nerve-fibres that ramify among them. Any 

 change of the body position moves the grains so that they 

 stimulate cells in different parts of the mound and enable the 

 animal to keep its balance. In the bony fishes the grains are 

 not separate, but form a comparatively large chip of bone, 

 the otolith, which differs in shape in different kinds of fish, 

 so much that it is not difficult to learn to what kind of fish 

 an otolith may have belonged. Fish otoliths are rather hard 

 and are not quickly digested by animals that feed on fish — 

 birds, dolphins and other fishes; they often remain in the 

 stomach long after all the rest has been digested and so it 

 is possible to find out what kind of fishes an animal may 

 have been eating from an examination of the otoliths in its 

 stomach. There is another use that the naturalist can make 

 of otoliths. The otoliths increase in size as the fish grows, 

 but the fish grows fastest during the summer abundance of 

 food, and consequently the otolith is marked with rings 

 somewhat like those in the trunk of a tree. By counting the 

 number of rings in the otoliths it is possible to know the age 

 of the fish from which it was taken. 



The patches of sensory hair-cells in the semicircular canals 

 are stimulated when the fluid flows over them. The fluid 

 flows along the canals whenever the head moves ; if the head 

 is moved to one side the fluid flows away from the cells on 

 that side of the head and towards them on the other. The 

 changes in pressure stimulate the hair cells, which start the 

 appropriate messages on their way to the brain. 



One of the semicircular canals lies in the horizontal plane, 

 the other two in the vertical plane but at right angles to 



