THE EAR. 



93 



angle, or ridge of bone between the upper side of the bony meatus and epitympanum, 

 or attic, cut away. The antrum is now entered by chiselling away the upper posterior 

 wall and the chiselling away of bone continued until the mastoid cells have been suffi- 

 ciently exposed. The final result of these two methods is the same. The external 



Course of lateral 

 (transverse) sinus 



Position of 

 mastoid antrum 



Suprameatal spine 



Anterior root of 

 zygoma or enii- 

 nentia articularis 



Posterior root 

 of zygoma 



Mastoid process 



External auditory meatus 



FIG. no. Lateral view of the temporal bone, showing the relations of the lateral or transverse sinus and 



mastoid antrum. 



meatus, tympanum, epitympanum, antrum, and mastoid cells are all thrown into one 

 large cavity. Wounding of the facial nerve is to be avoided by first learning its course 

 and then by sponging away the blood and cutting only the structures which are 

 clearly visible. Tracing the facial nerve backward, it is seen (Fig. 103) entering 

 the stylomastoid foramen, 

 passing upward posterior 

 to the tympanic cavity, 

 and crossing at about its 

 upper edge to pass above 

 the oval window. Viewed 

 in Fig. 101, it is seen that 

 the Fallopian canal lies a 

 trifle nearer to the external 

 surface than does the tym- 

 panic membrane, so that 

 in making the opening into 

 the antrum or in connect- 

 ing the mastoid cells below 

 the antrum with the tym- 

 panic cavity, care should 

 be taken to keep a little 

 anterior or superficial to 

 the membrane. 



Relations of the 

 Brain and Lateral Si- 

 nus. In operating on the 

 skull for middle-ear dis- 

 ease, it is desirable to know 

 how to reach and how to avoid the brain and lateral sinus. The lower level of the 

 brain in the region of the ear corresponds to a prolongation directly backward in a 

 straight line of the posterior root of the zygoma. If one keeps below this line, he is 

 not likely to open the brain case. If it is desired to explore the under surface of the 



Jugular foramen 



FIG. in. Transverse section of the right side of the skull just behind the 

 mastoid process ; looking forward. 



