418 BIOLOGY. [Book vi. 



have a small pyriform sac. Deserving of remark is this last fact, 

 completely in contradiction with the modem theories of the office 

 of the cochlea in man, in whom it is supposed to have the ap- 

 preciation of musical sound for function. Such an organ ought to 

 be extremely developed in singing birds. There is no cochlea in 

 the internal ear of reptUes and of fishes. The most inferior fishes 

 have not even any semicircular canal, and consequently their 

 ear has much affinity with that of the invertebrates. 



From this rapid anatomical summary it manifestly results, 

 that the truly essential part of the organ of hearing is the 

 internal ear, that is to say a vesicle full of liquid holding in 

 suspension otoliths, and into which lead the terminal threads of 

 the auditory nerve. The shock of the aerian sonorous waves puts 

 in movement the liquid of the auditory ampulla, and the vibra^ 

 tions of the molecules of this liquid agitate and impress the threads 

 of the auditory nerve. In the superior maromifers and in man 

 these threads are very numerous; also the auditory ampulla has 

 grown complicated ; it is furnished with appendices, with circular 

 canals, and especially with the labyrinth, that is to say, with a 

 tubular cylindrical 'prolongation, rolled on itself spirally, and the 

 windings whereof go on gradually decreasing (Fig. 77). The 

 nervous auditory threads penetrate into the stem of this cochlea, 

 and come forth therefrom' successively, spreading themselves in 

 the partition which separates from each other the spiral windings. 

 These nervous threads of the cochlea are composed of nervous 

 fibres, which, at first white, and with double contour, have con- 

 tinuance with the ganglionary cells, become afterwards fine 

 and pale, and end their course in small cells emitting each an 

 extremely fine filament, a sort of auditory bacillum. 



As the spiral windings go on decreasing, the le^igth of the 

 terminal nervous threads diminishes also gradually, and their 

 regular arrangement recalls that of the strings of a harp or of a 

 piano, with which they have often been compared. An accessory 

 organ, the organ of Corti, lodged in the spiral partition itself, 

 has been compared to the fingerboard of this living piano. But 



