294 



COMPARATIVE ANATOMY 



from the surface, so that a new method for conducting the sound- 

 waves to the internal ear is necessary, and certain accessory 



FIG. 215. DIAGRAM OF THE ENTIRE AUDITORY ORGAN OF MAN. 



External Ear. Mae, external auditory meat/us; M, M, pinna; Mt, tympanic 

 membrane ; O, wall of nieatus. 



Middle Ear. Ct, Of, tympanic cavity; M, fenestra rotunda (cochlea?) ; O 1 , wall 

 of tympanic cavity ; 0", wall of Eustachian tube ; SAp, sound conducting 

 apparatus, drawn in the form of a rod, representing the auditory ossicles ; 

 the point t corresponds to the stapes, inserted into the fenestra ovalis 

 (vestibuli) ; Tb, Eustachian tube ; Tb', its opening into the pharynx. 



Internal Ear, with the greater part of the bony labyrinth (KL, KL 1 ) removed. 

 'a, b, the two vertical canals, one of which (b) is shown cut through : the 

 horizontal canal is seen between 2 and S ; c, Co, commissure of the canals of 

 the membranous and bony labyrinths respectively ; Con, membranous cochlea, 

 which ends blindly at - ; Con\ bony cochlea ; Gp, cavum perilymphaticum ; 

 Cr, canalis reunions ; D.p, ductus perilymphaticus, which arises from the 

 scala tympani at d, and opens at 2).p l ; .S', sacculus ; S'.e, D.e, saccus and 

 ductus endolymphaticus ; the latter bifurcates at 2, where its outline should 

 have been dotted, so as to indicate that it opens on the inner side of the 

 labyrinth ; .S'y and >SV, scala vestibuli and scala tympani, which at * pass 

 into one another at the cupula terminalis (6V). 



structures become developed (Fig. 215). In the first place, a canal, 

 developed in the position of the hyomandibular or spiracular cleft, 1 



1 The cavity of the middle ear is doubtless derivable in the first instance 

 from the spiracular cleft which in Elasmobranchs is in close relation to the ear, 

 and thus seems to be particularly well adapted for conducting sound waves. In- 

 dications of the former relation of the tympanic cavity with the respiratory func- 

 tion are seen in the Frog, in which the cutaneous branch of the pulmonary artery 

 sends a large branch to its mucous membrane. The morphology of the tym- 

 panum itself is not clear, as it is developed secondarily and is apparently not strictly 

 homologous in Anura, Sauropsida, and Mammalia. . 



