45 



usually splits to give exit to the small deep petrosal (carotico- 

 tympanicus superior) from the tympanic to the carotid plexus. 

 Externally in the ascending part of the carotid canal is a small 

 foramen for the caroticotympanicus inferior, a sympathetic 

 nerve going from the carotid plexus to the tympanic. On 

 the outside of the jugular fossa is a foramen for Arnold's nerve 

 from the pneumogastric; its canal runs through the petrous 

 transversely and out, and splits into two, an inner to meet 

 the facial canal, i inch (5 to 6 mm.) above the stylomastoid 

 foramen, and the other to open at the tympanicomastoid 

 (auricular) fissure. 



Behind the jugular fossa is the jugular facet, for articulation 

 by synchondrosis with the jugular process of the occipital. 

 Externally is the styloid process, enclosed between the layers 

 of the vaginal process. It gives attachment to three muscles 

 and two ligaments. Between the styloid and mastoid processes 

 is the stylomastoid foramen, the end of the aqueductus Fallopii, 

 which passes first out and back over the labyrinth, then in 

 and back, and then down to terminate here; the stylomastoid 

 artery enters and the facial nerve leaves this foramen. 



The anteroexternal surface is free anteriorly for a short 

 distance, and articulates with the greater wing of the sphenoid ; 

 posteriorly it is concealed by the tympanic plate. 



At the angle between the squamous and petrous portions 

 is the opening of a canal, the musculotubarius, incompletely 

 divided into two by a projecting lamella, the cochleariform pro- 

 cess or septum tubas. The upper groove is for the tensor tympani 

 muscle, and the lower is the bony wall of the Eustachian tube. 

 This common canal is covered by the tegmen, its inner wall 

 is the anteroexternal surface of the petrous, and its floor 

 and outer wall are the tympanic plate. The septum tubse 

 rarely reaches the opposite wall, and rises from the anterior 

 wall of the carotid canal. This wall is made of two thin lamellae 

 with diploe between, in which runs the small deep petrosal 

 nerve. The superior border is grooved for the superior petrosal 

 sinus, and gives attachment to the tentorium cerebelli. The 

 posterior border presents on its inner portion a half-groove 

 for the inferior petrosal sinus, and externally the margin of 

 the jugular fossa. From the apex, where a bony projection 

 often overhangs the inferior petrosal groove, a fibrous band, 

 the petrosphenoidal ligament, extends to the side of the dorsum 



