5<5 



THE AUDITORY APPARATUS 



the periosteum, from the outer surface of the mastoid part of the temporal 

 bone, and identify (i) the supra-meatal triangle and the supra-mental 

 spine, which he 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, posteriorly and upwards, above the supra-meatal triangle. 

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

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

 one, is avoided. The tympanic antrum lies at the level of the supra- 

 meatal triangle, that is above and posterior to the external meatus, and 



Lateral semicircular canal 

 Remains of posterior 

 wall of external meatus 



Posterior semicircular canal 

 / Temporal line 



Wall of groove for 

 sigmoid part of 

 transverse sinus 



Tympanic pi 



Styloid process 



Mastoid 



Wall of canahs notch 



facialis 



FIG. 245. 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. 



about half an inch from the superficial surface of the temporal bone. 

 The dissection should be commenced above, below the temporal 

 line, and should be carried, anteriorly 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 cancellous tissue of the anterior part of the mastoid area must be 

 gradually removed till the more medially situated and more compact 

 bone is exposed. When this stage of the dissection is completed, the 

 dissector should note the following points: (i) In the anterior boundary 

 of the exposed area is the compact posterior wall of the bony part of the 

 external meatus. (2) Posteriorly is a broad projecting ridge indi- 

 cating the position of the groove which lodges the sigmoid part of the 

 transverse sinus. (3) At the upper and deeper part of the area are the 

 medial wall of the tympanic antrum and the aditus leading into the tym- 

 panic cavity. (4) The intermediate area is occupied by the remains of 

 the mastoid air-cells, which may extend downwards to the tip of the 

 mastoid process. They are continuous above with the cavity of the tympanic 





