XXVIII.] THE EAR. 295 



B. MAMMAL. 



1. Take the head of a cat 1 , and cut away all the 

 soft tissues surrounding the tympanic bulla, 

 having snipped off the external meatus as close 

 to the skull as possible, look down and observe 

 the tympanic membrane ; it is placed obliquely 

 to the canal and faces forwards, outwards, and 

 downwards. The attached handle of the malleus 

 may be seen shining through it. 



2. Place the head with the lower jaw uppermost, 

 and with a strong pair of forceps break away 

 piecemeal the projecting part of the bulla. The 

 cavity thus exposed has a floor 2 made irregular 

 by projections. Note in the centre a projection 

 of somewhat yellowish bone running in the long 

 axis of the bulla ; this is the promontory of the 

 cochlea : at a lower level in the exterior and 

 posterior part of it will be seen a round depres- 

 sion, the foramen rotundum. 



3. In front and outside the cochlea is a projection 

 of thin bone which prevents the membrana 

 tympani from being seen; it divides the tym- 

 panic cavity into two parts communicating by 



1 The details of arrangement differ considerably in different 

 animals: the following description applies to the cat, the more 

 general features may however be made out in other mammals. 



2 It must be particularly noticed that the words floor, roof, 

 exterior, posterior, etc. are here used with reference to the position of 

 the head during the dissection; the position of the parts when the 

 head is placed in the normal position should also be observed. 



