TYMPANIC CAVITY 521 



tympanic cavity should be approached from the postero-lateral 

 aspect. The dissection of the bone should be carried out after 

 the manner adopted by the surgeon when operating for the cure 

 of extensive mastoid and middle ear disease, but, to facilitate 

 the dissection, and to gain better access to the bone, the auricle 

 may be removed by cutting through the cartilaginous part of 

 the external meatus. 



After the auricle has been cut away take all the soft parts, 

 including the periosteum, from the outer surface of the mastoid 

 part of the temporal bone, and identify (i) the supra-meatal 

 triangle and the supra-meatal spine, which lie at the junction 

 of the superior with the posterior border of the bony part of the 

 external meatus, and (2) the temporal line, which passes, 

 backwards and upwards, above the supra-meatal triangle. The 



Lateral semicircular canal Posterior semicircular canal 



Remains of posterior | Temporal line 



wall of external meatus , r 



; / / Wall of groove for 



sigmoid part of 



Tympanic plate 



Styloid process Wall of canalis 



facialis 



FIG. 220. Dissection of the Tympanic Antrum and the petro-mastoid part of 

 the temporal bone from the outer side. The arrow is passing through 

 the aditus from the tympanic antrum into the tympanic cavity. 



objects of the first stage of the dissection are (i) the removal 

 of the outer compact layer of bone ; (2) the opening up of the 

 spongy tissue of the mastoid part of the temporal bone, and 

 the exposure of the mastoid air-cells and the cavity of the 

 tympanic antrum, whilst, at the same time, injury to the posterior 

 wall of the bony part of the external meatus and to the sigmoid 

 part of the transverse sinus, which lies in a groove on the inner 

 aspect of the posterior part of the mastoid portion of the temporal 

 bone, is avoided. The tympanic antrum lies at the level of the 

 supra-meatal triangle, that is, above and posterior to the external 

 meatus, and about 14 mm. (a little more than half an inch) from 

 the superficial surface of the temporal bone. The dissection 

 should be commenced in the supra-meatal triangle, and should 

 be carried, forwards and medially, into the bone, parallel with 

 the posterior wall of the external meatus, until the tympanic 

 antrum is opened into. After the tympanic antrum has been 

 identified, the spongy tissue of the anterior part of the mastoid 

 area must be gradually removed till the more medially situated 



