88 



APPLIED ANATOMY. 



paracentesis or puncture is resorted to. The preferable spot is the posterior lower 

 quadrant. Paracentesis of the membrane should be done by beginning the incision a 

 little above and behind the centre of the tympanic membrane, which slopes downward 

 and forward at an angle of 140 to the upper wall, and cutting downward to its 

 lower edge. One must avoid the long handle of the malleus, which extends directly 

 upward from the centre of the membrane. In the upper posterior part are the incus 

 and stapes, therefore this portion should be avoided; and running across the upper 

 edge beneath the mucous membrane is the chorda tympani nerve. Division of this 

 nerve is said to be a matter of not much account. Incision through the anterior part 

 is not considered suitable for drainage. 



Perforations frequently occur through Shrapnell's membrane on account of its 

 not having any fibrous layer; thus the pus does not go through the tympanic mem- 

 brane proper. If perforation with a purulent discharge has existed for a long time 

 granulations come through the opening, forming an aural polyp. To remove these 

 a snare is used or caustic is applied. 



The Tympanum or Middle Ear. The tympanic cavity is flat and narrow 

 and is situated directly behind and also above the membrane. It has a floor and 

 roof, and external and internal walls. It is divided into the portion behind the mem- 

 brane and the portion above the membrane called the attic. The floor is narrower 

 than the roof and is formed by the tympanic plate, which separates it from the jugular 



Aditus 



/Facial nerve 

 Oval window 

 Mastoid antrum / / / Canal for tensor 



ni muscle Carotid canal 



Eustachian tube 



Tympanic cavity 

 'Chorda tympani nerve 

 Round window 



FIG. 103. Right temporal bone : The outer surface has been cut away, exposing the tympanic cavity, its inner wall, 



the mastoid antrum, Eustachian tube, etc. 



fossa containing the commencement of the internal jugular vein. The bone forming 

 the floor is more difficult for pus to perforate than is that of the roof, so that exten- 

 sion of middle-ear disease is less frequent through it. The roof is comparatively 

 thin and formed of cancellous tissue with a thin and weak outside compact layer; 

 therefore it is a somewhat common site for pus to perforate and thereby obtain 

 access to the middle fossa of the skull. The distance from the floor to the roof is 

 approximately 15 mm. (f- in.); half is behind the membrane and the rest forms 

 the attic above. 



The external wall is formed below by the tympanic membrane and above by 

 the bone. As the membrane is the weakest portion of the walls, collections of pus 

 in the middle ear most often find a vent through it. Immediately behind the mem- 

 brane are the lower portions of the ossicles, and above is the chorda tympani nerve. 



The internal wall is formed of bone and is from 2 to 4 mm. (y 1 ^ to \ of an 

 inch) behind the membrane. It is so close that in doing the operation of para- 

 centesis care must be taken not to thrust ^ knife too deeply. In it are the oval 

 and round windows (Fig. 103). 



There is no well-defined anterior or posterior wall. The anterior portion of the 

 cavity is continued forward into the Eustachian tube; the canal for the tensor 

 tympani muscle is immediately above it. Posteriorly the cavity of the attic is con- 

 tinuous through the aditus with the mastoid antrum and the cells beyoad. Posterior 



