1370 



SUKFACE AND SUKGICAL ANATOMY. 



take the level of the superior 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 epitympanic recess. The upper and posterior quadrant of the osseous meatus 

 is therefore the only reliable guide to the antrum in the child. 



The medial 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 transverse 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 floor 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 



FIG. 1075. FRONTAL SINUSES OF AVERAGE 

 DIMENSIONS, WITH A MEDIAN SEPTUM (Logan 

 Turner). 



FIG. 1076. A LARGE RIGHT FRONTAL 

 SINUS WITH SEPTUM OBLIQUE TO THE 

 LEFT (Logan Turner). 



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 portion of the temporal bone is grooved, upon its medial surface, 

 by the sigmoid portion of the transverse sinus. The average distance of the fore- 

 most part of the sinus from the supra-meatal spine is 1 cm. The right sinus 

 usually receives the superior sagittal 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 

 3 mm. of the meatus. The average minimum distance of the transverse 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. 



The facial nerve, after entering the facial canal at the bottom of the internal 

 acoustic meatus, lies immediately above and behind the fenestra vestibuli, between 

 it and the prominence of the lateral semicircular canal ; thence it descends almost 

 vertically in the mastoid wall of the tympanum in. posterior and medial to the 



