Licence: In copyright
Credit: Text-book of anatomy / edited by D.J. Cunningham. Source: Wellcome Collection.
Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.
1209/1358 (page 1169)
![M THE CRANIUM. 11G9 yjChc mastoid antrum i.s lo be considered, developiiieiitally as well as auatiniiLcally, as an extension upwards and backwards of tlie tympanum. Its anatomy and relations will be best understood ))y studying it in the child, in whom it is relati\'ely larger than in the adult. Situated a))Ove and liehind the tympanic cavity proper, its outer locdl is formed by a triangular plate of bone which descends liehind the external auditory process from the squamous portion. Posteriorly, this triangular plate is separated from the petro-mastoid element by the petro- mastoid suture, which overlies the posterior part of the antrum and transmits small veins to the surface. The suture does not become completely ossified until a year or two after l)irth, and remains of it may frequently l)e detected in the adult bone. The anterior and upper portion of the triangular plate turns inwards at an angle to form the upper and posterior wall of the rudimentary osseous canal, as well as the floor of the tympanic attic. In the adult the outer wall of the mastoid antrum is formed l)y a plate of bone, from h to | in. in thickness, which occupies the triangular and somewhat depressed area l)etween the ridge extending liackwards and slightly upwards from tlie posterior root of the zygoma (supra-mastoid portion of tenaporal crest), and the upper and posterior quadrant of the osseous external auditory meatus; upon the latter is the supra-meatal spine, immediately behind which, upon the floor of the above triangle, is a crescentic depression, the fossa mastoidea. The outer wall of the antrum is felt through the skin as a slight depression immediateh' behind the auricle, and immediately below the ridge formed by the supra-mastoid crest; l)elow the depression is the prominence corresponding to the insertion of the sterno-mastoid muscle. Trautniann has pointed out, however, that the supra-mastoid crest, which varies considerably in its obliquity, is some- times situated a little above the level of the roof of the antrum, and that it is safer, therefore, to take the level of the upper border of the osseous meatus as the guide in order to avoid opening the middle fossa of the skull. In children the supra- mastoid crest is not developed, so that if the operator mistake the posterior root of the zygoma for the crest, he will open into the middle fossa of the skull immediately in front of the attic. The upper and posterior quadrant of the osseous meatus is, therefore, the only reliable guide to the antrum in the child. The inner wall is formed by a thick plate of spongy bone which separates the antrum from that portion of the posterior fossa lying between the aqueduct of the vesti])ule and the groove for the sigmoid portion of the lateral sinus, and whicb contains the posterior semicircular canal. The roof, which slopes downwards and forwards, is formed by the posterior and thinnest part of the tegmen tympani. The jloor is on a lower level than the aditus, and is therefore unfavouralily placed for natural drainage. The mastoid process begins to develop in the second year. As development advances the diploe surrounding the antrum in the child becomes excavated to form the mastoid cells, which radiate from the antrum, and either directly or indirectly communicate with it by small openings. In the pneumatic type of mastoid tlie whole of the process is excavated by these cells, which extend upwards into the squamous portion, forwards to the posterior wall of the osseous meatus (border-cells), and ])ackwards into the occipital l)one. Pus retained withhi the horder-cells may bulge into, and rupture through, the posterior wall of the osseous meatus. Less frequently the mastoid cells are absent, the bone consisting either of osseous tissue similar to that of the diploe, or of dense bone (sclerosed type). The mastoid ])rocess is grooved u])on its inner surface by the sigmoid portion of the lateral sinus. 'I'he average distance of the foremost part of the sinus from the supra-meatal spine is 1 cm. The right sinus usually receives the superior longitudinal sinus, and when this is the case it is larger and farther forward than the left; in extreme cases it may reach to within 2 or ;} mm. of the meatus. The average minimum distance of the lateral sinus from the outer surface of the mastoid is al)out 1 cm., but when the sinus is large and far forward the thickness may ])e reduced to 1 or 2 mm. 74](https://iiif.wellcomecollection.org/image/b21458856_1209.jp2/full/800%2C/0/default.jpg)
