THE EAR OF* MAN. IQI 



scribe the internal ear of one of our American Torpedoes 

 or Electric Rays {Torpedo occidentalis), and bring into 

 comparison with it, part by part, the homologous struc- 

 tures of the ears of Myxine, or the Hag-fish, and Petro- 

 myzon, or the Lamprey Eel, as two representatives of 

 the Cyclostome type ; the Alligator as a representative 

 of the reptilian type, the Thrush {Miimis) to illustrate 

 the avian condition, and the ear of Man as typical for 

 the mammalian group. 



• You will find the vertebrate internal ear described, in 

 the text-books and special memoirs dealing with this 

 organ, as a paired sense-organ, occupying a position on 

 either side of the head behind the eye. It resembles in 

 the oreneral features of its relations to the head and 

 nervous system the other organs of special sense. One 

 prominent feature is its constant position between the 

 roots of the fifth and tenth cranial nerves, which in 

 the higher vertebrates especially, may be said to be 

 pushed cephalad and caudad respectively by the growth 

 of the ear. 



The first trace of the ear seen in the embryo is, as 

 shown in Fig. 7, a simple saucer-shaped depression of 

 the ectodermic epithelium on the dorso-lateral region of 

 the head, not far removed from the gills. This thick- 

 ened saucer sinks into the head, and divides into two 

 parts, a superior and an inferior, or an utriculus and a 

 sacculus as they are called. From the former the semi- 

 circular canals are differentiated, while the latter portion 

 gives off the aqueductus vestibuli, or the endolymphatic 

 duct, and the cochlear canal. These parts by further 

 growth are converted into the auditory labyrinth of 

 the adult. 



