190 SENSORIAL FUNCTIONS. 



to the experiments of others, its 'transmission through 

 water is more than four times more rapid than it is through 

 the air. These facts assist us in understanding why it Is, 

 that no part is required in aquatics, like the tympanum 

 and little bones, to increase the sonorous undulations, and 

 also why these organs, in other respects, may be re- 

 duced to their utmost simplicity, since the water 'in which 

 they live transmits sound with so little diminution of its 

 intensity. 



588. Hearing in the Lobster. The simple auditory ap- 

 paratus, as it is found in the lobster, is represented by 

 Fig. 117. It consists of a vestibular cavity at v, con- 

 Fig. 117. 



taining a membranous sac, g, which is furnished with the 

 filaments of the auditory nerve. This vestibule is. pro- 

 tected on all sides by solid matter (as the same is by bone 

 in the human ear), except at one part, e, where it is 

 closed by a membrane, like the fenestra ovalis, to which 

 part, therefore, it corresponds. The water coming in 

 contact with this membrane, the sound is transmitted 

 through it to the nerves of the vestibule, and so to the 

 brain. 



589. The mollusca appear to be entirely destitute of the 

 sense of hearing, except, perhaps, in the highly organized 

 cephalopoda, or cuttle-fish tribe. In these, there exists a 

 tubercle containing two membranous sacs, which corre- 

 spond to the vestibules of other animals. 



590. Hearing in the Frog. In the frog, the ear is en- 

 tirely closed on the outside by a membrane, situated -over 

 a little cavity on each side of the head, but on a level 



Why is it supposed unnecessary that the vibratory apparatus should 

 exist in aquatics ? 



