78 A MANUAL OF ANATOMY 



forms the posterior and inner walls, and it presents the mastoid 

 antrum. 



The borders are anterior, superior, posterior, and inferior. The 

 anterior border separates the superior from the anterior or tympanic 

 surface. It is very short, and forms an acute angle with the 

 squamous portion within which the posterior pointed extremity 

 of the great wing of the sphenoid is received. This angle presents 

 an opening, called the Eustachian orifice, and the canal to which 

 it leads is divided into two compartments by a thin transverse 

 shelf of bone, scooped out on its under surface, called the processus 

 cochleariformis. The upper small compartment lodges the tensor 

 tympani muscle, and the lower large one forms the osseous 

 part of the Eustachian tube. Both of these canals lead upwards 

 and backwards to the anterior part of the tympanic cavity. 

 The superior border, which is the longest, separates the superior 

 from the posterior surface. It gives attachment to the tentorium 

 cerebelli, and is grooved for the superior petrosal venous sinus. 

 The inner part of this border frequently presents a process, which 

 projects over the upper end of the groove for the inferior petrosal 

 venous sinus, and gives attachment to the petro-sphenoidal ligament. 

 This ligament connects it with the lateral border of the dorsum 

 sellae of the sphenoid (which may present a superior petrosal 

 process), and, if it ossifies, it bridges over a foramen through 

 which the inferior petrosal sinus and sixth cranial nerve pass. 

 The posterior border separates the posterior from the inferior 

 surface. Its outer part, opposite the jugular fossa, enters into the 

 formation of the jugular foramen, and its inner part presents a 

 groove for the inferior petrosal sinus, and articulates with the side 

 of the basilar portion of the occipital. In line with the opening 

 of the internal auditory meatus it presents a triangular depression, 

 which encroaches on the inferior surface and leads to a small canal, 

 called the aqueductus cochleae. This aqueduct transmits a small 

 vein from the cochlea to the inferior petrosal sinus, and also a com- 

 munication between the perilymph of the scala tympani and the 

 subarachnoid space, sometimes called the ductus perilymphaticus. 

 The inferior border separates the inferior from the anterior surface, 

 and coincides with the line of contact of the tympanic plate with 

 the petrous portion. 



The apex of the petrous portion presents on its anterior aspect 

 the Gasserian depression superiorly, and the outlet of the carotid 

 canal inferiorly. 



The base is the part of the petrous portion which appears on 

 the external surface, and it presents a large opening leading into 

 the external auditory meatus. This opening is oval, its long axis 

 lying vertically, and it is bounded above by the posterior root of the 

 zygoma, whilst the remainder of its circumference is formed mainly 

 by the external auditory process of the tympanic plate. The external 

 auditory meatus is formed chiefly by the tympanic and squamous 

 portions, but also slightly by the mastoid portion. Its direction 



