PRIMORDIAL CRANIUM OF THE CAT 307 



Vestibulum. A frontal section through the middle of the 

 vestibular region reveals a space within the cartilage stretching 

 antero-posteriorly and lying nearer the medial than the lateral 

 margin of the section (fig. 10). This space, a part of the future 

 vestibule, is narrowed in its middle by the encroaichment of a 

 massive projection of the lateral wall and is expanded rather 

 widely in front and behind this. The anterior wide room, con- 

 taining in the section the utricle and recessus utricuU and the 

 a,mDullae of the anterior and lateral semicircular ducts, will be 

 described under the name of the cavum vestibulare anterius. 

 The posterior room includes, in the section, the posterior part of 

 the utricle, the ventral part of the ductus endolymphaticus, the 

 beginning of the canalis utriculi-saccularis, the sinus inferior, 

 ampulla posterior and the crus simplex of the ductus semicircu- 

 laris lateraUs. This room will be further considered under the 

 name of the cavum vestibulare posterior. The parts of the mem- 

 branous labyrinth mentioned do not fill completely the space 

 limited by the cartilaginous walls of the pars vestibularis, but 

 leave an interval filled by mesenchyma representing the future 

 perilymphatic space. 



Cavum vestibulare anterius (figs. 10, 11). This large, irregu- 

 larly shaped cavity occupies the dorsal part of the vestibular 

 region. Its contents have been noted. Its anterior wall swells 

 out as the prominentia utriculo-ampullaris superior. This cor- 

 responds to an anterior extension of the cavity called recessus 

 ampullaris anterior containing the ampulla anterior. The floor 

 is broad, slopes ventrad in a medial direction, then drops abruptly 

 to continue into the lateral cochlear wall. At this spot is located 

 the fenestra vestibuh (figs. 19, 20). The medial wall bulges 

 slightly into the cranial ca\dty forming the prominentia utricu- 

 laris, in conformity with the subjacent surface of the utricle and 

 saccule. The ventral free edge of this wall forms the dorsal 

 boundary of the superior acustic foramen as already described 

 (p. 304). The lateral wall presents a dorso- ventral ridge which 

 projects into the cavity, fitting into the angle between the am- 

 pulla laterahs and ampulla anterior. The considerable space 

 behind the ridge, containing the greater part of the lateral am- 



