PRIMORDIAL CRANIUM OF THE CAT 303 



is situated upon the medial aspect of the ear capsule, partly in 

 the wall of the cochlea, partly in that of the vestibule. Its 

 plane faces dorsad and mesad, the lateral margin made by the 

 prominentia utricularis being considerably higher than the 

 medial cochlear edge. The fenestra leads into a short, imper- 

 fectly formed meatus acusticus internus ; within is an incomplete 

 floor presenting two openings, one transmitting the facial nerve^ 

 the other the branches of the acustic nerve. The former, which 

 lies anteriorly, is the beginning of the primary facial canal. It 

 will be recalled that this short canal passes beneath the supra- 

 facial commissure and ends in the facial foramen outside th^i 

 chondrocranium. The opening for the acustic nerve is an irregu- 

 lar gap ; for convenience of description it will be referred to as the 

 acustic fissure. It is shaped Uke a V with apex caudad, one 

 limb lying almost wholly in the cochlear, the other in the ves- 

 tibular region. The latter division, widened in the form of an 

 oval notch, lies anterior and slightly dorsal to the cochlear di- 

 vision. It transmits the nerves to the utricle, and to the 

 ampullae of the anterior and lateral semicircular canals. Its 

 position and relations to the structures just named identify it 

 with the superior acustic foramen of other mammahan embryos. 

 The larger cochlear portion of the acustic fissure hes caudad, 

 medialward and somewhat ventrad to the superior acustic 

 foramen. It gives passage to the nerve of the cochlea and the 

 nerve to the saccule, and is identified with the inferior acustic 

 foramen. In the dorsal caudal corner of the meatus acusticus 

 internus is a notch opening into the cochlear part of the fissure 

 and leading to a groove, lodging the nerve to the ampulla pos- 

 terior. The notch is the forerunner of the foramen singulare 

 and will be designated the incisura singularis. 



The position and relations of the facial and acustic foramina 

 having been noted, it now remains to describe the walls of the 

 internal acustic meatus. Figures 13 and 14 show the meatus 

 from the medial aspect. Of the stretches of cartilage inter- 

 vening between the facial and acustic foramina two are of special 

 interest: one, constituting the primary floor of the meatus, sepa- 

 rating the two named foramina; the other forming the incom- 



