390 THE SKIN AND THK SENSE-ORGANS. 



thelium, a continuation of the mucous membrane lining- the tympanic- 

 cavity. 



b. The tympanic cavity (Fig- 242 II) is seen, after removing- 1 1n- 

 tympanic membrane, as a flattened, funnel-shaped cavity. The 

 walls of the cavity are lined with a pigmented mucous membrane, 

 under which the cartilaginous ring- (annulus membranae tympan'i} 

 extends internally to form the greater part of the wall of the cavity. 

 The cavity is an elongated oval slit (Fig. 242 II, ct\ with its longer 

 axis directed from above and in front, downwards and backwards, 

 and leading inwards ; it is bounded above, below, and in front by the 

 squamosal, behind by the soft parts ; above in the roof is the cartila- 

 ginous part of the columella. The inner or deeper portion of the 

 tympanic cavity is bounded in front by the squamosal, and by the 

 squamous process of the prootici (jproc. squamoms prootici) in front 

 and above ; internally by the cartilage (primordial-cranium) between 

 the prootic and the exoccipital ; behind by the M. depressor maxillae. 

 This part of the cavity is rounded and covered with mucous mem- 

 brane, and has in its roof the bony part of the columella, which 

 covers the foramen ovale (fenestra ovalis) by its oval, widened end- 

 piece. This deeper portion of the cavity communicates by a short, 

 wide Eustachian tube with the pharyngo-oral cavity; the tube is 

 wide and is of a rounded oval form in section : anteriorly, externally, 

 and internally it is bounded by the pterygoids, posteriorly by soft 

 parts, in which is embedded the styloid process. 



The tympanic cavity can be examined from without after re- 

 moving the tympanic membrane, or from below by means of the 

 Kustachian tube. 



c. The columella auris (Figs. 12, 243) is described by Retzius 

 as. consisting of three portions, of which the middle is bony, the 

 external and internal cartilaginous. Parker divides it into four 

 parts (see pp. 25, 26). 



The extrastapedial (Fig. 243 a', a"} is attached to the middle of 

 the tympanic membrane by the oval surface opposite a', and is 

 attached by the process a'" to the annulus tympanicus ; this process 

 of Retzius is the suprastapedial of Parker. 



The mediostapedial (Parker), or middle bony piece of Ret/- ins 

 (Figs. 12 a', 243 ,'), is narrow externally but widens internally 

 to articulate with the interstapedial. Just before reaching the 

 latter it gives off a process ('), to which are attached a few fibres of 

 striated muscle (M). 



