CHAP, iv.] HEAEING. 189 



has its long diameter, about 3 mm., placed horizontally, the vertical 

 diameter being about 1'5 mm. It corresponds in form, but is 



ST 



FIG. 172. THE STAPES IN POSITION. Much magnified. (Schwalbe.) 



1. The end of the shaft of the incus. 2. Its expansion or os orbiculare. 2'. The 

 articular cartilage of the same. 3. The capitulum of the stapes ; 3'. Its 

 articular cartilage. 4. The hoops of the stapes. 5. The foot-plate of the 

 stapes. 5'. Its articular cartilage. 6. The membrane of the fenestra ovalis. 



ST. The tendon of the stapedius muscle attached to the capitulum of the stapes. 



rather smaller than the fenestra, and between its cartilaginous 

 rim and the cartilage-lined rim of the fenestra is attached a 

 ring-shaped membrane (Fig. 172, 6), consisting of radially disposed 

 bundles of connective tissue with which many elastic fibres are 

 mixed. The ring though slightly broader in the front part than 

 in the hind part of the oval is very narrow, at most 100 //, ; hence 

 the movements of the stapes within the fenestra are very limited 

 in extent, and probably do not exceed a small fraction ( -^ to -^3) 

 of a millimeter. The tympanic surface of the fenestra and included 

 stapes is covered with a continuation of the mucous membrane 

 which covers the tympanic membrane, and which not only lines 

 the whole tympanic cavity but is also reflected over the whole 

 chain of ossicles ; the other surface of the stapes, that which forms 

 part of the perilymph space of the labyrinth, is lined like the rest 

 of the cavity with a lymphatic epithelium. 



815. The chain of ossicles, thus jointed together, attached 

 to the tympanic membrane at one end, and to the fenestra ovalis 

 at the other, and secured by ligaments, may be regarded as a lever. 

 Observations and experiments shew that the end of the short 

 process of the incus serves as the fulcrum, the power being 

 applied at the umbo in which the handle of the malleus ends, and 

 the effect being brought to bear on the end of the long process of 

 the incus attached to the stapes. In thus acting as a lever the 

 heads of the malleus and incus rotate round a horizontal line 

 drawn through them in the direction of the line ax in Fig. 171. 

 Such a lever may be represented by the line xx' in Fig. 173. 



