The outer and posterior extremity of the petrous is confounded with the mastoid and squamous portions; the inner and anterior is open, and the bone is so much removed at its upper part (to allow the carotid artery to pass upon the body of the sphenoid) that it there appears more like a deep groove than a tube. This is filled up in the recent subject by a plate of cartilage, but in the dried skull, when this cartilage has been removed, there is found an opening, between the sphenoid bone and this extremity of the temporal, which is called theforamen lacerum anticum.
The mastoid portion is situated at the outer end of the petrous, and behind and below the squamous. It is of a. nipple-like shape, with an upper horizontal denticulated border, with which the posterior inferior angle of the pari etal bone articulates ; and with a posterior semi circular border which is joined to the occipital : in both directions it is overlapped by the bones to which it is joined, except at the lower part, where it is applied to the occipital by a sort of harmonic suture.
On its inner surface there is a deep, semi circular sulcus (the concavity looking back wards) which traverses its entire length ; it receives the lateral sinus from the parietal bone and transmits it to the lower part of the occi pital : there is generally observed in it a fo ramen (the Ibramen mastoideum), through which a vein of the scalp communicates with the sinus.
Its outer surface is roughened and gives attachment to the sterno-cleido-mastoideus, and sometimes to the trachelo-mastoideus ; it terminates below in the mammillary eminence, called the mastoid process, behind and to the inner side of which are two grooves—the one nearest to the process (the sukus digastricus) very evident, for the attachment of the digas tricus ; the other nearly on the articulating edge (sulcus occipitalis), less distinct, for the occi pital artery.
The squamous portion rises upwards from the mastoid, and part of the outer border of the petrous portions ; it has a semicircular mar gin which embmces the parietal and sphenoid bones.
Its internal surface, which is concave, con tributes to form the middle fossa of the cra nium, and exhibits strongly the depressions and elevations which correspond to the con volutions of the brain, and to the spaces between them. At its anterior part, .and com mencing at the angle between it and the pe trous process, there is a groove which runs upwards and divides into other grooves, some of which pass backwards; these are formed by the middle meningeal artery and its branches.
The external plate of its border is prolonged upwards, in such a manner that this surface is surmounted by a rough articulating line, of considerable breadth, which is applied on the outside of the parietal and partly on the sphe noid bone.
The external surface is slightly convex, is smooth, and there may be often seen indica tions of deep branches of the temporal artery having passed over it. It forms in part the temporal fossa, and the temporal muscle is attached to it. At its lower part, a process (the zygomatic process) passes transversely outwards, and is then twisted on itself in a di rection forwards, after the fashion of the ribs at their angles ; so that the surface of the process which would have been superior becomes internal, and that which would have been in ferior becomes external. This process has two roots, an anterior or transverse and a pos terior or longitudinal. The former is a convex elongated eminence, situated transversely and in front of a fossa (the fossa articukris), in which the condyle of the lower jaw is placed. This root is the eminentia articularis, on which the condyle, with its inter-articular cartilage, is thrown when the jaw is depressed. The pos terior root has itself two origins, which cir cumscribe the external auditory foramen ; and it flows into and joins the anterior, just when that root is altering its direction. Be tween the squamous process, and that part of the zygomatic process which is between the two roots, there is a groove in which play the posterior fibres of the temporal muscle. The fossa articularis, which is between the roots, ,is bounded behind by the Glasserian fissure before mentioned; it forms, with the adjoining fossa parotidea, the glenoid cavity. The zygomatic process extends forwards about an inch from its anterior root ; being, therefore, convex externally and concave internally. Its upper border gives attachment to the temporal fascia; its inferior (which is about half the length of the superior) to the masseter muscle. Its external surface is covered by the integu ment, and its internal forms the outer boun dary of the temporal fossa, in which is situ ated the temporal muscle. The extremity of the zygomatic process forms a point, on account of the under margin being bevelled and den ticulated to articulate with the malar bone.