PRIMORDIAL CRANIUM OF THE CAT 309 



where the floor is wanting anteriorly there is a wide communi- 

 cation with the ca\iim cochleae. 



The semicircular canals (figs. 2, 9, 10, 11, 19) are cyUndrieal 

 tunnels of a diameter from two to two and one-half times that 

 of the contained membranous ducts. The future periljanphatic 

 space between the ducts and walls of the canals, is filled with 

 mesenchyma. Within the compass of the canals is the body of 

 cartilage to which Gaupp has given the name massa angularis. 

 The lateral semicircular canal lies in a frontal plane; its floor, 

 lateral and medial walls go into the cartilage of the crista parotica ; 

 its roof is formed by the crista inter vestibularis. The plane of 

 the anterior semicircular canal is dorso-ventrad inclining latero- 

 mesad from before backward. Within its concavity is that part 

 of the massa angularis which forms the roof of the cavum vestib- 

 ulare anterius and which corresponds to the fossa subarcuata of 

 the intracranial vsurface of the pars canalicularis. The posterior 

 semicircular canal also runs in a dorso-ventral plane, which, 

 however, is inclined from behind and laterally, forward and medi- 

 ally. It includes within its concavity that part of the massa 

 angularis forming the roof of the caM.im vestibulare posterius 

 and which corresponds on the intracranial surface of the pars 

 canahcularis to a fos.sa. From the bottom of this fossa, which 

 is situated behind the prominentia cruris communis, a canal, 

 transmitting some blood vessels, leads to a great vacuity within 

 the angular mass. 



The crista intervestibularis (fig. 11) is a projection of the base 

 of the massa angularis. It begins just dorsad of the fenestra 

 vestibuli as a ridge lying between the ampulla lateralis and the 

 saccule ; it reaches its maximum height opposite the ductus utric- 

 ulo-saccularis. 



The massa angularis (figs. 2, 10, 11), filling the space between 

 the three semicircular canals and the vestibule, is composed of 

 hyaline cartilage, fully formed. Its relation to the walls of the 

 canals and vestibule have already been described. Ventrally 

 the mass becomes continuous with the crista intervestibularis. 

 The angular mass is not continuous thi'oughout, being broken 

 in its central portion by a space occupied by connective tissue 



