242 ANATOMY OF VERTEBRATES. 



same accidental relation to the embryonal, cartilaginous, hyoidean, 

 haemal arch described by Huschke ' as extending from the mastoid 

 or petro-mastoid to the upper or short horn of the hyoid (cerato- 

 hyal), and from the outer part of the capsule of which cartilage the 

 styloid process (stylo-hyal) is ossified. The stapes first appears 

 as a compressed pyramidal wart from the petrosal or inner wall of 

 the tympanum, projecting from a depression the bottom of which 

 becomes the fenestra vestibuli : the malleus, according to Rathke 

 and Valentin, 2 projects, somewhat earlier, as a small wart from 

 the back wall of the tympanum. Ossification begins first in the 

 malleal wart by a point at the head, and by a second at the root 

 of the long process. According to Meckel, the rudiment of the 

 stapes has grown, at the third month of the human foetus, to a 

 cartilage representing both stapes and incus, like the columella 

 of Ovipara : as such it is ossified in the Ornithorhynchus. The 

 ossification of the columella begins first in the ' incudeal ' part, 

 extending along the long crus toward the stapes, which is subse- 

 quently ossified, according to Rathke, 3 from three nuclei, one for 

 each crus and one for the base. As regards the vacuity, it does 

 not exist in the cartilage, but is produced by the modelling absorp- 

 tion in the course of the ossification, transitorily representing the 

 characters shown in the porpoise, seal, and bear. Abnormal 

 arrests of development of the stapes in the human subject have 

 been found to represent the imperforate avian columella and most 

 of the above-cited mammalian conditions of the stapes. 



The membrane lining the tympanum, fig. 187, «, invests the 

 small bones and the tendons of their muscles where they run free 

 in the cavity. A fold of it fills up the 

 space bounded by the crura and base of the 

 stapes. The chorda tympani, also, in its 

 passage across the tympanum, is enveloped 

 by it. Lastly, it forms the inner borrowed 

 layer of the membrana tympani, covering 

 and adhering closely to the handle of the 

 malleus. 



The nerve called ' chorda tympani,' fig. 



Tvnu'inum, otostcals and 'chorda 187, C, IS COntillUOUS, as shown at p. 157, 

 tympani -Human, xcvm". fig> ^ yf ^ ^ fa^ ^ superficial 



petrosal nerves : it leaves the facial before the exit of the latter 

 by the stylo-mastoid foramen, ascends in its own osseous canal, 

 enters the tympanic cavity, crossing the inner side of the tympanic 

 bone, as in birds, advances between the handle of the malleus and 



l civ". 2 CXLll", p. 211. 3 CXLlll", p. 120. 



