THE CRANIUM. 1169 
The mastoid antrum is to be considered, developmentally as well as anatomically, 
as an extension upwards and backwards of the tympanum. Its anatomy and 
relations will be best understood by studying it in the child, in whom it is 
relatively larger than in the adult. Situated above and behind the tympanic 
cavity proper, its owter wall is formed by-a triangular plate of bone which descends 
behind 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 birth, and remains of it may frequently be 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 by a plate of bone, 
from $ to } in. in thickness, which occupies the triangular and somewhat depressed 
area between the ridge extending backwards and shehtly upwards from the 
posterior root ot the zygoma (Ssupra-mastoid portion “of temporal 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 shght depression 
immediately behind the auricle, and immediately below the ridge ‘formed by the 
supra-mastoid crest ; below the depression i is the prominence corresponding to the 
insertion of the sterno-mastoid muscle. Trautmann 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 vestibule and the groove for the sigmoid portion of the lateral sinus, and which 
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 unfavourably 
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 the 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 backwards into the occipital bone. Pus retained within 
the “ border-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 process is grooved upon its inner surface by the sigmoid portion of 
the lateral sinus. The 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 5 mm. of the meatus. The 
average minimum distance of ‘the lateral sinus from the outer surface of the 
mastoid is about 1 cm., but when the sinus is large and far forward the thickness 
may be reduced to 1 or 2 mm. 
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