PRIMORDIAL CRANIUM OF THE CAT 317 



interna. Small tributaries enter from the connective tissue fill- 

 ing the foramen jugulare spurium; some of these are emissary 

 veins. Behind the pars canaUcularis the sinus is joined by a 

 small vein formed by tributary systems, one being the plexus of 

 veins in the mesenchyma which fills the vacuity of the massa 

 angularis (p. 309). A plexus of veins in the facial sulcus empties 

 into a large vein which, running backward in company with the 

 facial nerve, joins the internal jugular. An extensive network 

 of veins within the mesenchyma of the cavum cochleae is drained 

 in part by two veins which pass to the vena jugularis interna by 

 way of the aquaeductus cochleae. 



Vestiges of a stapedial artery are present in the embryo of 23.1 

 nrni., perforating the stapes (fig. 21). These can be followed an- 

 teriorly toward the terixdnal branch of the internal maxillary ar- 

 tery and posteriorly toward a plexus of small vessels on the ven- 

 tral sm'face of the cochlear capsule, which is connected with an 

 offset from the internal carotid artery. In an embryo of 15 mm. 

 the stapedial artery is a relatively large branch of the A. carotis 

 interna, extending forward through the mesench>^nal beginning 

 of the stapes to the semilunar ganghon whence it continues as the 

 mandibular artery. 



The internal acustic artery arises from the basilar and enters 

 the internal acustic foramen together with the acustic and sev- 

 enth nerves, giving off several branches to the membranous 

 labyrinth. 



Orhito-temporal region. Embryo of 23.1 mm. 



The basis cranii (figs. 1, 2, 3) of the orbito-temporal region is 

 an unbroken bar of cartilage continuous with the basal plate of 

 the otic region posteriorly, and with the nasal septum of the eth- 

 moidal skeleton anteriorly. The extracranial surface, strongly 

 convex from side to side looks toward the pharynx, from which it 

 is separated by an interval filled by wide-meshed mesenchyma. 

 The intracranial surface presents in its posterior one-third the sella 

 turcica with the root of the ala temporalis on. each side, and in 

 front of this, in successively higher planes, the two roots of the 

 ala orbitahs, the optic foramen intervening. At the level of the 



