672 DISSECTION OF THE EAR. 



by means of a vertical cut of the saw through the root of the zygoma and 

 the Glaserian fissure, the tympanum will be brought into view by cutting 

 away with the bone forceps some of the upper surface of the petrous portion, 

 and the anterior part of the rneatus auditorius. 



In the recent bone prepare the dissection as above, but without doing 

 injury to the membrana tympani, the chorda tympani nerve, and the chain 

 of bones with its muscles. 



Form. The cavity of the tympanum has the form of a slice of a small 

 cork, about a quarter of an inch thick, the outer and inner boundaries 

 being flattened and the circumference circular. Its size is greater across 

 the space, than from without inwards ; in the former direction it measures 

 about half an inch, but in the latter not more than a quarter of an inch. 



The inner boundary (fig. 239) is of greater extent than the outer, and 

 on it the following objects are to be noticed. Occupying nearly the whole 

 surface is the large projection of the promontory (a) ; this is pointed pos- 

 teriorly, and is marked by two or three minute grooves (d) which lodge 

 the nerves in the anastomosis of Jacobson. Above and below the nar- 

 rowed part of the promontory is a large aperture ; both lead into the 

 labyrinth. 



The upper opening (i), semicircular in shape, with the convexity placed 

 upwards, is named fenestra ovalis: into it the inner bone (stapes) of the 

 osseous chain is fixed. The lower aperture (c) -fenestra rotunda, is 

 rather triangular in shape, and leads into the cochlea ; it is situate within 

 a hollow, which is somewhat semi-elliptical in form. In the recent state 

 it is closed by a thin membrane, the secondary membrane of the tym- 

 panum. 



The outer boundary of the cavity is formed by the membrana tympani 

 (fig. 238, c), and the surrounding bone. Above and in front of the mem- 

 brane, is the Glaserian or glenoid fissure, which is occupied in the fresh 

 condition by the long process of one of the small bones (malleus) ; by a 

 small muscle, the laxator tympani ; and by tympanic vessels. Crossing 

 the membrane towards the upper part is the chorda tympani nerve, which 

 issues through the Glaserian fissure. 



The circumference of the tympanum is circular, and is rough and 

 uneven on the surface here and there : round it the student may observe 

 the following facts. 



The roof is flattened, and consists of the thin osseous plate separating 

 the tympanic cavity from the cranium. The floor is narrower than the 

 roof, and is curved over the subjacent jugular fossa ; it has more or less 

 of an areolar condition, and some small apertures communicate with the 

 fossa beneath. 



At the posterior part of the circumference, towards the roof, is one 

 large with other small apertures, leading into the mastoid cells (fig. 239). 

 Below those apertures, but near the inner wall and on a level with the 

 narrowed part of the promontory, is the small conical hollow projection 

 (e) of the pyramid; this is perforated by an aperture, and contains the 

 stapedius muscle ; a minute canal connects its cavity with the aqueduct of 

 Fallopius : sometimes a small round spiculum of bone attaches the pyramid 

 to the promontory. In a line with the pyramid, and arching upwards 

 above the fenestra ovalis, is a bony ridge (f) marking the situation of the 

 aqueduct of Fallopius. 



The front of the tympanic cavity corresponds with the carotid artery, 

 only a thin piece of bone intervening. In it are the apertures of two 



