PRIMORDIAL CRANIUM OF THE CAT 301 



cavnm tympani. In it lies also the vena capitis lateralis. In 

 the model the expanded posterior part of the groove com- 

 municates freely with the great foramen cochleae, opening at 

 its medial side. 



The intracranial surface of the pars canalicularis is depressed 

 below the general surface of the cranium and presents an uneven 

 contour (fig. 1). Ventrally it passes into the pars cochlearis. 

 At the place of transition a great quadrilateral fenestra acustica 

 transmits the divisions of the N. acusticus. A ridge, eminentia 

 cruris communis, produced by the crus commune of the anterior 

 and posterior semicircular canals, subdivides the intracranial 

 surface into anterior and posterior faces. The eminence pre- 

 sents the foramen endolymphaticum and inferiorly passes into 

 the prominentia utricularis. The latter, corresponding to the 

 utricle and saccule, reaches below to the acustic window. The 

 posterior surface of the pars canalicularis looks caudad, mesad 

 and slightly ventrad, contrasting in its position therefore with 

 the caudally and ventrally directed corresponding .surface in 

 mammals generally. It passes caudally into the medial surface 

 of the lateral occipital arch and ventrally into the prominentia 

 utriculo-ampullaris inferior. The posterior surface is hollowed 

 to form the sigmoid sulcus, occupied by the sinus transversus. 

 The anterior surface of the pars canalicularis presents the fossa 

 subarcuata anterior limited anteriorly and dorsally by the 

 prominentia semicircularis anterior. This prominence begins 

 at the dorsal pole of the pars canalicularis and curves anteriorly 

 and ventrally to the prominentia ampuUaris superior. On this 

 surface, also, the stout commissura suprafacialis^ is found, con- 

 necting the medial wall of the prominentia utriculo-ampullaris 

 superior with the anterior part of the roof of the pars coch- 

 learis, and participating in the ventro-caudal circumference of 

 the spheno-parietal fenestra. It is continued dorsally into a 



''■ In van Wijhe preparations of embryos 17 to 30 mm. long, the suprafacial 

 commissure appears deeply stained and separated from the pars canalicularis 

 by a zone of lightly stained cartilage. It continues directly into the com- 

 missura orbito-parietalis where this is united with the otic capsule (commissura 

 parieto-capsularis) (fig. 9). 



