302 ROBERT J. I'ERRY 



prominent ridge which joins the orbito-parietal commissure 

 (figs. 1, 3). The suprafacial commissure constitutes the roof of 

 the short first portion of the facial canal, the floor of which is 

 made by a cartilaginous septum between it and the cavum 

 cochleae. The canal leads from the meatus acusticus internus 

 to the foramen faciale. The dorsal circumference of the pars 

 canaUcularis is partly free, partly continuous with the cranial 

 wall. Posteriorly it passes into the occipital wall; anteriorly it 

 is connected with the commissura parieto-capsularis. Between 

 these connections the dorsal margin is free, forming the ventral 

 boundary of the foramen jugulare spurium. Upon the medial 

 side of the dorsal circumference is a groove occupied by the 

 transverse sinus and its main anterior tributary. 



Pars cochlearis 



The most conspicuous object of the cranial floor as presented 

 by the model is the huge, rounded bulging of the pars cochlearis 

 of the auditory capsule (figs. 1, 2). The strong walls of this 

 shell enclose the cochlear canal and the saccule. The form of the 

 cochlear capsule is not quite round, but longer in a direction 

 from behind, forward and medialward. Upon the ventral, 

 extracranial surface a shallow groove, sulcus septalis, extends 

 from behind and laterally, medially and anteriorly. Upon the 

 caudal aspect is the large foramen cochleae. Where the cochlear 

 and canalicular parts come together are three openings: the 

 fenestra vestibuli, appearing outside the cranium, the fenestra 

 acustica, within the cranium and the foramen perilymphaticum, 

 appearing partly inside and partly outside the skull. The pars 

 cochlearis shows no sharp boundary toward the pars canaUcu- 

 laris ; as stated above, the two are continuous in the region of 

 the vestibule. On the surface, this common vestibular zone 

 corresponds to the intermediate stretch which includes the sulcus 

 facialis laterally, and the region of the fenestra acustica, medially. 

 The connections between the cochlear capsule and the floor of 

 the cranium have already been described (p. 288). 



Foramina acustica (figs. 10, 11). A large quadrilateral 

 opening, fenestra acustica, the future porus acusticus internus, 



