742 B. \V. KUNKEL 



medial to the rami maxillaris and mandibularis of the nervus 

 trigeminus and lateral to the ramus ophthahnicus. As develop- 

 ment proceeds this process is reduced in size, becoming replaced 

 by the very variable epipterygoideum of the adult. 



The quadratum stands in very close relationship topographi- 

 cally with the columella auris as has already been shown, the 

 posterior margin being deeply incised for the accommodation of 

 the slender stalk of the columella as it passes horizontally from the 

 otic capsule as far laterally as the outer wall of the quadratum 

 where it is expanded to form a mushroom Uke plate which par- 

 tially fills the cup-Uke ventral half of the quadratum. 



COLUMELLA AURIS 



The columella auris in the embryo modelled consists of a single 

 cartilaginous rod in which only a trace of its origin from two cen- 

 ters is preserved in the fact that the chondrification of the ex- 

 ternal end is very distinctly less advanced than of the rest. In 

 earlier stages, however, the stapes and extracolumella, which 

 together form the columella auris, are very evidently distinct. 

 Topographically are to be distinguished the foot plate, stalk, and 

 insertion piece, of which the first two belong to the stapes and the 

 last to the extracolumella. 



The foot plate is triangular and fits closely into the fenestra 

 vestibuli, resting with its ventral margin upon the planum basale. 

 The stalk arises from near the anterior angle of the foot plate, 

 gradually assuming its slender, cylindrical form and extending 

 at right angles to it in a lateral direction. It is shghtly sigmoid, 

 with its lateral extremity further forward than its median extrem- 

 ity. It extends laterally across the cavum tympanicum ventral 

 to the vena capitis lateraUs and nervus facialis and resting in the 

 incisura columellae of the quadratum as ah-eady described. It 

 passes into the median surface of the insertion piece somewhat 

 dorsal to the center of the latter. The insertion piece is mush- 

 room-shaped with its medial surface flat and lateral aspect con- 

 vex. Its plane is vertical and continuous with the lateral wall 

 of the quadratum. It aids in closing the cavum tjTiipanicum 

 laterally. 



