Base Of The External Surface

External to the stylo-mastoid foramen is the auricular fissure, for the auricular branch of the pneumogastric, bounded behind by the mastoid process. Upon the inner side of the mastoid proc­ess is a deep groove, the digastric fossa; and a little more internally the occipital groove, for the occipital artery. At the base of the internal pterygoid plate is a large and somewhat triangular aperture, the foramen lacerum medium, bounded in front by the great wing of the sphenoid, behind by the apex of the petrous portion of the temporal bone, and internally by the body of the sphenoid and basilar proc­ess of the occipital bone: it presents in front the posterior orifice of the Vidian canal; behind, the aperture of the carotid canal. The basilar surface of this open­ing is filled in the recent state by fibro-cartilaginous substance f across its upper or cerebral aspect passes the internal carotid artery. External to this aperture the petro-sphenoidal suture is observed, at the outer termination of which is seen the orifice of the canal for the Eustachian tube and tLat for the Tensor tympani muscle. Behind this suture is seen the under surface of the petrous portion of the temporal bone, presenting, from within outward, the quadrilateral, rough surface, part of which affords attachment to the Levator palati and Tensor tympani muscles; exter­nal to this surface the orifices (If thecarotid canal and the aqureductus cochlere, the former transmitting the internal carotid artery and the ascending branches of the superior cervical ganglion of the sympathetic, the latter serving for the passage of a small artery and vein to the cochlea. Behind the carotid canal is a large aperture, the Jugular foramen. formed in front by the petrous portion of the tem­poral, and behind by the occipital; it is generally larger on the right than on the left side, and is divided into three compartments by processes of dura mater. The anterior is for the passage of the inferior petrosal sinus; the posterior, for the lateral sinus and some meningeal branches from the occipital and ascending phar­yngeal arteries; the central one, for the glosso-pharyngeal, pneumogastric, and spinal accessory nerves. On the ridge of bone dividing the carotid canal from the jugular foramen is the small fora.men for the transmission of Jacobson's nerve; and on the wall of the jugular foramen, near the rootof the styloid process, is the small aperture for the transmission of Arnold's nerve.