304 ROBERT J. TERRY 



plete lateral wall of the meatus. The primary floor of the 

 meatus appears in the model as an extension from the roof of the 

 anterior part of the cochlear cavity. It can be followed laterally 

 from the dorsal part of the cochlear capsule at the medial mar- 

 gin of the fenestra acustica to pass into the first turn of the sep- 

 tum spirale. This part is the forerunner of the longitudinal 

 bony ridge in the floor of the meatus of the adult, the crista 

 transversa of human anatomy, which separates the superior 

 cribriform area and entrance to the facial canal above from the 

 middle cribriform area and the spiral tract of foramina below 

 (Jayne '98, p. 200). Anteriorly it continues as the floor of the 

 prunary facial canal and at the same time the roof of the fore 

 part of the cochlear cavity. Lateral to the facial foramen the 

 primary fccr of ihe meatus passes into the floor of the promi- 

 nentia utJ'icalo-r.-T.pullaiis superior and forms the ventral bound- 

 ary of the superior acustic foramen. The posterior free edge 

 of the floor constitutes the cephalic margin of the inferior acustic 

 foramen. The lateral wall of the meatus is formed mostly of 

 precartilage. Its free ventral margin bounds the acustic fissure 

 dorsally. The wall itself covers the medial aspect of the sac- 

 cule. The medial edge of the meatus, derived from the roof of 

 the cdchlear capsule, stretches from the suprafacial commis- 

 sure to the posterior wall and is very low. The anterior and 

 posterior walls, on the contrary, tend to become very high 

 toward the vestibular part of the capsule, and are formed re- 

 spectively by the suprafacial commissure and an elevation made 

 at the junction of the basivestibular commissure with the promi- 

 nentia ampullaris inferior and cochlear capsule. 



Within the meatus acusticus internus are found, anteriorly, 

 the nerve of the vestibule and its ganglion, together with the 

 facial nerve, and, posteriorly, the nerve of the cochlea. 



In the model of the chondrocranium, there is a small flat- 

 tened surface of the cochlear capsule, lateral to the suprafacial 

 commis.sure (figs. 3, 20). This, the planum supracochleare of 

 Voit, supports the seventh nerve as it passes out of the foramen 

 faciale, the geniculate ganglion and the beginning of the great 

 superficial petrosal nerve. Upon it rest also the caudal part 



