THE MASTOID ANTRUM. 



11 



the external auditory meatus ; and the coronal plane in which the opening is 

 contained, in consequence of the forward inclination of the bony meatus, is placed a 

 little (about one-fourth of the horizontal diameter of the meatal opening) in front 

 of the posterior margin of the external orifice of that canal. The epitympanic 

 recess is situated above the anterior three-fourths of the orifice. Behind the 

 entrance the floor of the antrum sinks, forming a hollow which does not usually 

 extend below the level of the centre of the auditory meatus. The cavity is, however, 

 continued into the mastoid cells, which are often of large size, and then as a rule 

 reach to the tip of the mastoid process. 



Superiorly, the antrum is separated from the middle fossa of the base of the skull 

 by a thin plate of bone which continues backwards and upwards the tegmen 



masCcid. cetie 



Fig. 7. RIGHT TEMPORAL BONK, FROM WHICH THE SUPERFICIAL PORTION OF THF. MASTOID DIVISION 

 HAS BEEN REMOVED, EXPOSING THE MASTOID ANTRUM. Natural size. (From a photograph by 

 G. W. B. Waters.) (G. D. T.) 



The broken line indicates the position of the lateral sinus. 



tympani. This sometimes presents small deficiencies, in which there is only a 

 slender fibrous layer between the mucous lining of the cavity and the dura mater ; 

 and these two membranes are always united by connective tissue and vessels passing 

 through the petro-squamosal fissure, as well as through minute apertures in the 

 tegmen. In position, the roof of the antrum corresponds as a rule to the supra- 

 mastoid crest externally, but not unfrequently it rises somewhat above that level, 

 and in that case the upper part of the antrum may be overlapped by the lateral 

 margin of the cerebral hemisphere, the inferior temporal convolution of which is 

 received at this spot into a slight groove between the prominent tegmen internally 

 and the lower border of the squamous temporal externally. 



From the communication with the epitympanic recess the antrum extends back- 

 wards and outwards, so that it comes nearer to the surface behind than in front. 

 Anteriorly, there is only a thin bony wall between the cavity and the deep part of 



