SURGICAL ANATOMY OF THE MASTOID AREA
The mastoid antrum. At birth the mastoid antrum is almost fully developed. In infancy it is situated superficially and at a much higher level in relation to the auditory canal than in the adult. In the infant, also, the petro-squamous and the squamo-mastoid suture are still patent. As the mastoid cells develop, the antrum gradually becomes more deeply placed, so that in the adult it is from half to three-quarters of an inch from the surface.
Its roof, the tegmen tympani, is continuous with that of the attic. Anteriorly it is separated from the external auditory meatus by the posterior wall of the auditory canal, whose innermost margin forms the outer wall of the aditus. On its inner wall lie the semicircular canals, whilst posteriorly the lateral sinus is separated from it by an intervening layer of mastoid cells or compact bone. Between the semicircular canals and the lateral sinus is a small area composed of a thin layer of bone, separating the antrum from the posterior fossa of the cranial cavity.
The mastoid process. In the infant this is undeveloped and is merely represented by a small bony protuberance. By the fourth year it has practically reached the adult type.
Anatomically the mastoid process can be subdivided into three chief types: (1) the pneumatic, in which the cells are few and large; (2) the diploic, containing numerous small cells; and (3) the compact, in which the bone is extremely dense. Mixed types are frequently found, the cortex, as a rule, being more dense than the deeper portion. Occasionally it is uniformly sclerosed, almost of the consistence of ivory, but in these cases the condition is usually pathological, the result of chronic inflammation of the mastoid process.
The mastoid cells converge towards the antrum and may be divided into two groups: (1) those extending vertically downwards to the tip of the mastoid process; and (2) those lying between the antrum and the sigmoid process of the lateral sinus. In addition to these two groups, it must not be forgotten that cells may extend in other directions; for instance, (a) anteriorly, along the root of the zygoma; (b) posteriorly, communicating with the cells of the occipital bone; (c) inferiorly, between the floor of the tympanic cavity and the jugular fossa; (d) internally, spreading inwards towards the apex of the petrous bone and surrounding the labyrinth; or (e) enveloping the orifice of the Eustachian tube.
The facial nerve, after dipping beneath the external semicircular canal, passes vertically downwards through the mastoid process to emerge at the stylo-mastoid foramen. Entering this foramen and running along the canal are the stylo-mastoid branches of the posterior auricular artery. These vessels, if cut through by the chisel, may bleed in a marked manner, thus drawing the attention of the operator to the fact that he is in close proximity to the facial canal and nerve.
Surface anatomy. Although it is impossible to foretell with certainty before operation what the anatomical structure of the mastoid process may be, yet some information may be gathered from the formation of the skull.
In the dolichocephalic type, the mastoid process is broad and frequently contains large cells, especially at its tip and round the lateral sinus, which is usually deeply placed. In the brachycephalic type, on the other hand, there is a greater tendency for the mastoid process to be narrow and to consist of dense bone, for the middle fossa to extend low down and to overlap the outer wall of the antrum, and for the lateral sinus to project forward and superficially, even to within 2 or 3 millimetres of the posterior border of the external meatus.