PRACTICAL, CARP CULTURE. Si 



it were in the liquid that fills the chamber. These are distributed on the 

 walls of the semicircular canals, and of the spiral cavity, and are ex- 

 ceedingly numerous. Their ends are excited by the vibration of the 

 liquid and the nerve currents thus produced are transmitted along the 

 nerve trunk to the brain. 



In addition to the curious and complex mechanism already described 

 there is another very singular portion whose exact duty is but ill under- 

 stood, but which appears to be all important if we may judge from the 

 fact that it is constantly present even among animals very low in the scale- 

 There is in the labyrinth a variable number of small masses of limestone 

 resembling bone in appearance. These ear-stones or "otoliths" seem to 

 produce or to increase the effect of the vibrations of the liquid on the nerve 

 ends, and in this way perhaps enhance the keenness of the sense. 



The reader will now be able to follow the application of the above de- 

 scription to the special case of the fish, and to appreciate the value of the 

 evidence deduced therefrom in relation to the power of hearing possessed 

 by these animals. We have already said that the complexity of the ear 

 grows less and less as we descend in the animal scale. Part after part dis- 

 appears. Important though non-essential accessory organs cease to be 

 present, and the ear is reduced at length to its most rudimentary form, 

 namely, a simple chamber filled with liquid and containing the ends of 

 the auditory nerve and some otoliths. Such an ear is found in the fresh 

 water mussel of our rivers and ponds. 



In illustration of this statement I may here state that the external ear 

 is found in all mammals, in most birds and in some reptiles. In the am- 

 phibians, as in the frog and toad, it is absent and the drum head of the mid- 

 dle ear is visible on the outside. In fishes a still lower grade is found. 

 Here the middle ear or drum is also absent. There is no drum head, no 

 chain of small bones, and therefore no Eustachian tube. The inner ear is 

 now the only portion left. 



The ear of the fish accordingly corresponds only to the inner portion 

 or labyrinth of the human ear, and even this in an incomplete form, for 

 the spiral portion the cochlea is absent. The drum and outer ear being 

 both undeveloped, there are no membranous partitions, the two openings 

 in the bone do not exist, and the whole organ is shut up close within the 

 bony cavity of the skull. It has no direct channel of communication with 

 the world outside. No trace of it can be seen on the surface, and some 

 care and pains are necessary to demonstrate its existence. Yet every part 

 that is essential to hearing is present and may be found. la the cavity of 

 the skull and near the base on each side is a recess, and in this the ear is 

 situated. It is in actual contact save for some delicate, separating mem- 

 brane, with the brain itself. This recess, or alcove, a8*"it may well be 

 called, extends from the top of the skull to a point a little below its base 

 and is lined with a layer of liquid called the lymph of the ear. 



line ends of the auditory nerve remain undamaged and in connection. These are the trans- 

 lators of the sound-waves into nerve vibrations, and where they are present hearing in some 

 degree is possible. 



