OSS A TEMPORUM. 71 



an important part of the nervous plexus of the tympanum. — 

 Before this spine, or partition, is the orifice of the second aque- 

 duct of the ear, the aqueduct of the cochlea, through which 

 passes a vein from the cochlea to the internal jugular, and in 

 which is lodged a process of the dura mater. This jugular 

 fossa is at the termination of the groove, in the internal surface 

 of the bone, made by the lateral sinus. At a small distance 

 before the jugular fossa is the commencement of the carotid 

 canal, which makes a curve almost semicircular, and then pro- 

 ceeds in a horizontal course to the anterior extremity of the 

 bone : through this winding canal passes the carotid artery, and 

 the filaments from the fifth and sixth pair of nerves, which are 

 the beginning of the intercostal or sympathetic nerve. 



Between the carotid canal and the cavity for the condyle of 

 the lower jaw, at the junction of the anterior part of the 

 squamous portion with the petrous portion of this bone, is a 

 very rough aperture, the bony margin of which appears broken ; 

 this is the orifice of the bony part of the Eustachian tube, or 

 passage from the throat to the ear. This canal is divided 

 lengthwise by a thin bony plate ; the upper passage contains 

 the internal muscle of the malleus bone of the ear (tensor 

 tympani) ; the lower and largest canal is the bony part of the 

 Eustachian tube. 



The external passage to the ear, called Meatus Auditorius 

 Externus, is situated between the zygomatic and the mastoid 

 processes. The orifice is large and smooth above, but rough 

 below, and is surrounded by a rough lip called the auditory 

 process. The direction of the canal is obliquely inward and 

 forward. 



— Angles of the bone. The superior angle of the bone which 

 separates the anterior and posterior faces, is sharp and gives 

 attachment to the tentorium cerebelli. It is slightly grooved 

 for the lodgment of the superior petrous sinus, and near its 

 extremity is marked by a smooth notch, upon which rests the 

 ' fifth or trigeminus nerve. The anterior angle which separates 

 the anterior from the inferior or basilar surface of the bone, is 

 grooved for the Eustachian tube, and forms the posterior 



