74 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



cavity by means of auditory ossicles which extend from the 

 tympanic membrane to the fenestra ovalis. In the anura and 

 sauropsida there are two of these ear bones, the stapes, situ- 

 ated in the fenestra ovalis, and the columella, extending from 

 the stapes to the tympanic membrane. In the mammals the 

 columella is replaced by two bones, the incus and the malleus, 

 neither of which can be homologized with the columella. 



FIG. 79. Diagrammatic section of human ear, from Martin after Czermak. A, 

 auditory nerve ; a, ampulla ; B, b, semicircular canal ; G, external meatus ; k, carti- 

 lages ; M, concha ; o, fenestra ovalis ; /*, tympanic cavity with chain of bones ; Pt, 

 scala tympani, r, fenestra rotunda; K, Eustachian tube; S, cochlea; Vt, scala 

 vestibuli. 



The stapes arises as a chondrification, and, later, ossifica- 

 tion of the membrane closing the fenestra ovalis; the columella 

 is post-spiracular, and may in part correspond to the hyoman- 

 dibular; the incus is apparently the quadrate of the lower ver- 

 tebrates ; while the malleus is the proximal end of Meckel's 

 cartilage (? os articulare) which becomes cut off from the rest. 1 



In all anura and in many reptiles the tympanic membrane is 

 on the outer surface of the body, but in higher groups the mem- 



1 There is great uncertainty upon some of these points, different students having 

 different ideas of the homologies. The view given here is based upon personal studies. 

 Further details are given in the section dealing with the skeleton. 



