"There is no external sign to indicate the situation of the fossae of the skull. In general, however, it may be said that the anterior fossa extends as far back as the anterior end of the zygoma; that the middle fossa lies between this and the mastoid process, and the posterior includes all the base behind the process" (Eisendrath).
The mastoid antrum may be exposed by trephining Fig. II, 12a. in Macewen's suprameatal triangle, a space which (Fig. II, 12.) is bounded above by the backward continuation of the upper root of the zygoma (the supramastoid crest), behind by a vertical line drawn upwards from the posterior border of the external auditory meatus, and below and in front by the suprameatal spine, a prominent bony process which assists in the formation of the posterior superior quadrant of the external auditory meatus. In this triangle there is usually a well-marked depression, the suprameatal fossa. The supramastoid crest not only indicates the uppermost possible limit of the mastoid antrum, but, as has already been stated it corresponds also to the level of the base of brain in this situation. The crest, therefore, represents the level of the tegmen antri,(Fig. II, 13.) and, in mastoid exploration the scene of operation must be confined to an area below this crest. In the adult the antrum usually lies at a depth of 1/2 to 3/4 inch from the surface.
The lateral sinus lies posterior and nearer to the surface (Fig. II, 9.) whilst the facial nerve pursues its course in front Fig. II, 12b. and on a deeper plane.
The parotid gland occupies the space which is bounded above by the zygomatic arch, behind by the (Fig. III, 5.) auricle and the mastoid process, and below by a line drawn from the angle of the jaw to the apex of the mastoid process. In front, the gland extends a variable distance over the anterior surface of the masseter muscle. This muscle passes downwards and backwards from the lower border of the zygomatic arch to be attached to the outer surface of the descending ramus and angle of the lower jaw. When the teeth are clenched, the anterior border of the muscle is easily defined, a well-marked line of demarcation being so formed between the masseter muscle behind and the buccinator in front.
Stensen's duct, the duct of the parotid gland, corresponds to the middle third of a line drawn from the (Fig. III, 7.) lower border of the tragus of the ear to a point situated half-way between the ala of the nose and the red line of the upper lip. At the anterior border of the masseter muscle the duct dips inwards, through the buccinator muscle, to open on the buccal mucous membrane, opposite the second molar tooth of the upper jaw.
The transverse facial artery, a branch of the superficial temporal, runs inwards parallel to and immediately below the zygoma, (Fig. III, 6.) lying above the level of Stensen's duct.
The facial nerve, after emerging from the stylo-mastoid foramen, curls round the condyle of the jaw, and traverses the substance of the parotid gland, in which part of its course it divides into numerous branches. The general transparotid course of the nerve and the direction of its buccal branch may be indicated by a line drawn forwards parallel to and below Stenson's duct from the lobule of the ear.
The inferior dental nerve may be exposed by trephining over the ascending ramus of the lower jaw, midway between the anterior and posterior borders, and on a level with the last molar tooth. In this manner, the nerve, accompanied by the corresponding artery, is exposed as it enters the inferior dental canal. (Fig. IV, 2.)
The tonsil corresponds in position to a point situated just above and in front of the angle of the lower jaw. (Fig. IV, 3.)
The following bones assist in the formation of the orbital margin: Above, the frontal bone.
Externally, the external angular frontal process and the malar bone.
Below, the malar bone and the superior maxilla.
Internally, the nasal process of the superior maxilla and the internal angular frontal process.
By alternate forcible closure and opening of the lids, the internal tarsal ligament, or tendo oculi, (Fig. II, 17.) can be felt passing to its insertion into the nasal process of the superior maxilla. Immediately below the tendon, at the junction of the inner and lower walls of the orbital cavity, (Fig. II, 18.) is the depression for the lachrymal sac, which sac narrows below into the nasal duct.
The duct passes downwards, backwards and slightly outwards, to open into the anterior part of the inferior meatus of the nose under cover of the inferior turbinated bone. The duct is about 1/2 inch long.