The outer end of the articular eminence is roughened by the attachment of the external lateral ligament of the joint, and the temporal fascia is fastened to the upper border of the free process and along the supramastoid crest.
The prominent mastoid process projects downwards and forwards immediately behind the meatus : between it and the tympanic plate is the point of emergence of the posterior auricular branch of the vagus. The process is made by a mass continuous with the petrous, with a part of the squama covering its upper and front area externally, so that a petro-squamous suture may be apparent on this surface of the bone. Beside this it presents : (a) attachment of Retrahens aurem ; (b) some fibres of origin of Occipito-frontalis ; (c) attachments of Sterno-mastoid, Splenius, and Trachelo-mastoid ; (d) a mastoid foramen near its posterior border, opening internally into the groove for the lateral sinus ; (e) digastric groove, etc., already considered. The process is not present at birth, may owe its existence to the pull of the muscles inserted into it, and is usually better developed in men than in women.
The upper part of the region between the process and the external meatus exhibits several vascular foramina and may present a small pos -meatal spine projecting from the squama : this depressed area (vascular spot) is of surgical importance, as it marks on the surface the position of the mastoid antrum.
On the front border the squama articulates with the great wing of the sphenoid and with the parietal in the upper and back part. The meeting of these two surfaces is marked on the articular bevelled area of the squama by a definite ridge (A in Fig. 175). In front of this ridge the sphenoidal surface extends to the apex of the petrous, while behind it the parietal extends to the prominent posterior angle on the mastoid margin. The occipital articulates along the lower and inner margin from this angle to the apex of the petrous. The malar joins the zygomatic process, and the lower jaw articulates with the squama. The occipital articulation is largely through the medium of a thin layer of fibro-cartilage : in this are sometimes found small ossifications, the ossicles of Riolan.
The outer side of the petrous is covered by the other elements of the bone and cannot be seen without removing these : this is impossible without the saw in the adult bone, though easily effected in the young specimen. A semi-diagrammatic view of the outer side is given in Fig. 178. The tympanic cavity is exposed by this removal of the squama and tympanic plate : the styloid process is left and its upper end is seen embedded in the posterior wall of the tympanum (tympano-hyal). The tympanic plate covered this in ; its area of articulation is shown. The rest of the cut surface represents the extent to which the squama rested on the petrous.
The cavity of the bony Eustachian tube is opened, leading to the tympanum, and behind this again a narrower adilus opens into the mastoid antrum. Evidently these cavities are covered in externally by the squama and tympanic plate ; the latter forms the outer wall of the tympanum-The plate meets the jugular plate below and thus completes the tympanic floor : it also forms the floor of the bony tube. The canal for the Tensor tympani is above the tube and also leads to the tympanum. Mastoid air-cells are seen behind the antrum : these are very variable in extent of development and may be found throughout the whole mastoid region, communicating with the antrum.
The inner ear is close to the inner wall of the tympanum, and the first turn of the cochlea causes a bulging, the promontory, on the inner wall of the tympanum : immediately behind this are the fenestra ovalis, for the footpiece of the stapes, and the fenestra rotunda, which is closed by membrane : behind and below these is a depression, the sinus tympani. The facial nerve, having run outwards from the inner meatus to the point X in the figure, turns back sharply here along the upper part of the inner wall, in the bony canal known as the " aquaeductus Fallopii," to the lower and inner side of the aditus ad antrum, where it turns down along the posterior wall to emerge at the stylo-mastoid foramen. Outside the descending part of the canal is the " pyramid," a small bony eminence containing the Stapedius muscle, whose tendon emerges from its apex : to the outside of this and a little lower the iter chorda posterius allows the chorda tympani to leave the facial canal. The tympanic branch of the ninth nerve comes through the iloor just below the promontory, on to which the nerve runs and ramifies.
The promontory marking the position of the cochlea, it follows that the fenestras must be in the region of the bony vestibule, into which they open, and the semi-circular canals must lie behind this. These canals cause prominences on the surfaces of the bone, the ampulla of the external canal showing in the aditus, above and behind the facial canal, the upper curve of the superior semi-circular canal making the arcuate eminence, and the upper arc of the posterior one causing a rounded elevation between the sabarcuate fossa and the aquerdactus vestibuli on the inner aspect of the bone.
An incomplete bony septum separates the bony tube from the canal of the Tensor tympani, ending behind in a hook-like process round which the tendon of the muscle turns.
At birth the bone (Fig. 214) is practically in the condition shown schematically in the fifth and sixth drawings in Fig. 172. The four elements of the bone are easily separated, the styloid process being as a rule cartilaginous, although its embedded part is bony. The tympanic plate is represented by the incomplete ring, so that there is no bony meatus, but the membrane is practically on the surface of the bone. It is unnecessary to give a further account of the origin of the four parts of the bone already illustrated in Fig. 172, and it only remains to deal with the ossification : this occurs in membrane in the squama and tympanic ring, but in cartilage in the petrous and styloid.
One centre, above the root of the zygoma, about the seventh or eighth week : some observers maintain that there are two additional centres, one above and one behind this.
One centre, appearing in the middle of the third month in the thick tissue by the wall of the cavity and extending from this backward and upward to make the ring.