70 INNERVATION. [CHAP. XVIII. 



by a twig from the portio dura of the seventh pair. At the base of 

 the pyramid is an aperture through which the chorda tympani, m, 

 enters the tympanum. Thence this nerve passes forwards, between 

 the handle of the malleus and the long arm of the incus, and emerges 

 through a canal close to the Glaserian fissure. Above the pyramid 

 an arched prominence, e, indicates the course of the aqueductus 

 Fallopii, close to the tympanum; and behind this is the free com- 

 munication with the mastoid cells, f. 



The anterior part of the tympanum presents above the canal for 

 the tensor tympani muscle, and below the orifice of the Eustachian 

 tube. The former, i, is chiefly formed by a curled plate of bone, 

 the processus cochleariformis, g, ending in a kind of perforated sum- 

 mit, that some have termed, anterior pyramid. This is a little above 

 the fenestra ovalis, and gives passage to the tendon of the tensor 

 tympani, which becomes attached to the short process of the mal- 

 leus. The Eustachian tube, about one inch and a half in length, 

 leads from the tympanum downwards, forwards, and inwards to its 

 orifice in the pharynx, w 7 hich is seen as a slit with an elevated edge 

 close behind the inferior turbinated bone of the nose (see fig. 106, t, 

 p. 8). By its straight, but inclined course, the passage of mucus 

 from the tympanum is facilitated. Its upper extremity for more 

 than half a inch is bony, while in the rest of its extent it is carti- 

 laginous. It dilates at each end, especially the lower, where the 

 cartilage is thickened and everted. It forms a passage for the air 

 in and out of the tympanum. It exists in all animals in which a 

 tympanum is found, but in many, the tubes of opposite sides have a 

 common outlet on. the pharynx. External to the opening for the 

 Eustachian tube is the opening for the anterior muscle of the malleus 

 (Glaserian fissure) and that for the escape of the chorda tympani. 



The ossicles of the tympanum are three, the malleus, the incus, 

 and the stapes (fig. 133). The malleus (hammer) has a large ex- 

 tremity above, termed the head, m, bounded by a constriction or 

 neck, from which the handle (manubrium), h, passes down, imbedded 

 in the membrana tympani, as already described. Its concavity di- 

 rected outwards explains the similar inequality of that membrane. 

 The short process is a slight conical projection from the neck, which 

 receives the insertion of the tensor tympani muscle : the slender 

 process (p. gracilis), g, also passes from the neck, but forwards and 

 outwards, to enter the Glaserian fissure. On the back of the head and 

 neck an articulation is formed with the incus. The incus (anvil), is 

 shaped not unlike a molar tooth. It articulates with the malleus by 

 the anterior surface or summit of its body, and has two processes, a 



