causing complete paralysis of the face on the same side (contrast with effect of pressure on face centre given above).
It is thus a matter of great importance to be able to go down upon the mastoid antrum, and give exit to the pus contained within it, without damaging the very important structures lying upon all sides of it. This, fortunately, is a comparatively easy matter owing to the constant relationship between the mastoid antrum and the surface markings to be described. The posterior root of the zygoma passes nearly horizontally across the superior border of the external auditory osseous meatus, and extends backwards beyond it, forming an angle with the posterior wall of the osseous meatus. This angle can always be made out on the surface of the bone, and is frequently occupied by a very well-marked depression in the bone. The angle may be converted into a triangle by supplying an imaginary third or posterior side formed by drawing a tangent upwards from the posterior part of the osseous auditory meatus, and is called the suprameatal triangle (Macewen).
An aperture made straight in through this triangle, keeping at right angles with the surface of the bone, will strike the antrum, without damaging the sigmoid sinus, provided that the aperture made be not large. The spine of Henle sometimes given as a guide to the antrum, is generally only present in well-marked skulls, in which the suprameatal triangle is indicated by a distinct depression in the bone. When present, the spine is situated on the margin of the posterior wall of the osseous meatus. Such an aperture would enable pus to escape, but would not, as a rule, cure the condition, a radical mastoidectomy performed with proper instruments being necessary for this purpose. In performing the radical operation much care is required. When working backwards the sigmoid sinus may be exposed at any moment, and as its position cannot be determined by landmarks, the operation can only be safely performed by an instrument which will cut away the diseased bone gradually without damaging the soft parts (such an instrument is the surgical bur, when properly made). In the radical operation also it is usual to lay the middle ear and mastoid antrum into one, by cutting away the bridge of bone which, forming the external wall of the iter, separates the two. Here again care must be exercised, as if the bone on the inner side of the iter be cut, both the facial nerve and the external semicircular canal may be damaged. When operating with a surgical bur, the facial canal, even in very extensive dissections, can be recognized and avoided, owing to its being encased in dense bone, which contrasts with the softer bone surrounding it. The facial nerve enters the internal auditory meatus along with the eighth nerve, and at the bottom of the internal meatus enters the aqueduct of Fallopius, the canal of dense bone already mentioned. In this it first runs outwards and forwards until it reaches the inner wall of the tympanum, when it suddenly bends backwards and downwards first to the inner side and then to the floor of the iter. Finally, it descends almost vertically just in front of and on the deep surface of the antrum, to the stylomastoid foramen. The ossicles are frequently involved in disease, and are generally removed in a radical operation, with the exception of the stapes, which is fixed in the foramen ovale. It is worthy of note that hearing is often much improved after the radical operation, partly due, no doubt, to the removal of the granulation masses, which previously blocked up the parts, and also possibly to the freeing of the stapes.
The mastoid process varies much in structure. Sometimes, and especially after long-standing disease, it is hard and dense, and contains few cells. At others it is honeycombed with intercommunicating cells, one of which may open at the tip, forming a ' Bezold's aperture.' Where mastoiditis occurs in such cases, the pus may escape by this aperture and present in the neck, generally behind the sterno-mastoid. In most cases the pus, when it escapes, does so by the Eustachian tube, or, once perforation of the drum has occurred, by the external ear. It is important to remember, however, that active mischief may be going on, although the drum is intact. Sometimes the pus finds its way to the surface of the mastoid bone, under the periosteum, by a process of caries, or through remains of the masto-squamosal suture, and in such cases the periosteum, carrying with it the pinna, is raised from the bone, so that the patient presents the appearance of having a very prominent ear, which also is projected forwards. A simple incision into this superficial abscess is known to aurists as Wilde's incision. It cannot, of course, cure the condition. In some cases, where such an aperture exists on the surface of the mastoid, the patient may possess the power of inflating the superficial tissues by forcing air through the Eustachian tube. Such tumours are known as pneumatoceles. Apart from disease due to pathogenic organisms, the mastoid antrum is sometimes occupied by an epithelial tumour, composed of pearly masses of epithelium (cholesteatoma).
In such cases the antrum is generally very large, extending deeply beneath the sigmoid sinus, and profuse suppuration appears to occur readily on the invasion of organisms, and is, of course, extensive. Sometimes pathogenic processes extend inwards and affect the internal ear. In some of these the semicircular canals may be slowly affected (Meniere's disease), and even the whole osseous labyrinth, consisting of cochlea, vestibule, and canals, has been found lying detached in the ear as the result of suph processes.