SKULL OF A 43-MM. HUMAN FETUS. 



73 



form of the cavity, modeled as a solid, will be 

 helpful in gaining a true concept of it. 



Cochlear space. — The cochlear space begins at 

 the foramen perilymphaticum and proceeds 

 forward in the trough-like inclosure for the com- 

 mencement of the first turn of the cochlear duct 

 and spiral ganglion. Here it is partially overhung 

 by the low ridge already described, which projects 

 outward from the cranial wall of the cochlea and is 

 walled in laterally by the medial part of the spiral 

 septum and outer wall of the capsule behind this. 

 Continued forward, upward, and finally backward 

 and downward, the space is seen to suggest 

 strongly the spiral character of the adult bone. 

 The spiral cochlear duct contained in this space 

 makes one and three-quarter turns, as seen in 

 figures 19 and 22. 



Vestibular space. — The vestibular portion of the 

 space is very capacious. There is but little evi- 

 dence of the sacculation which is present in the 

 osseous condition. Just above the superior 

 opening of the medial acoustic meatus (represent- 

 ing the future superior cribriform area) there is a 

 faintly marked depression for the recessus utriculi 

 of the utriculus, the representative of the future 

 recessus ellipticus. Of the recessus sphericus, 

 there is little evidence. The foramina which 

 pierce the wall have been mentioned. 



The semicircular canal spaces are all large and 

 all show dilatations for the ampulke. They bear 

 the usual relationships to each other and to the 

 vestibular cavity. The plane of the lateral canal is 

 very nearly parallel with that of the basal plate. 

 In its course it circles around a short trabecula 

 (cut in fig. 25), which joins the angular mass with 

 the floor of the capsule just behind and lateral to 

 the fenestra vestibuli. The caudal non-ampul- 

 lated end opens into the vestibule immediately in 

 front of the trabecula inclosed by the posterior 

 canal. 



Membranous labyrinth.— But little need be said 

 as to the membranous labyrinth (figs. 19 and 22), 

 since its features are familiar. It approximates in 

 form the mature condition. All the parts are 

 represented. As in the adult, it occupies but a 

 comparatively small volume of the space contained 

 within the otic capsule. The cochlear duct does 

 not show such a distinct projection of the apex of 

 the coil from the base as in the adult condition. 

 Lying within the coil is the spiral ganglion, which 

 ends in a slight enlargement, just medial to the 

 smallest coil of the duct; this enlargement shows a 

 short spur, directed toward the medial portion of 

 the middle meatus. The ganglion ends dorsally 

 at the angle between the dorsalmost straight 



portion of the duct and its basal limb, and thus 

 does not reach the vestibular cascum. 



The ductus endolymphaticus is long and slender 

 and crosses the crus commune in a direction from 

 above downward and forward. Its saccus is 

 broad, flattened latero-medially, and of oval out- 

 line, and it lies medial to the transverse sinus. Its 

 dorsal extremity is a very slender filamentous 

 process containing an almost imperceptible 

 lumen. 



Stapes. — The stapes (figs. 31, 34, 44) is a ring of 

 cartilage with a thick cellular perichondrium. A 

 section of the ring, representing the future foot- 

 plate, is in tne membrane filling the vestibular 

 window. The ends of this primitive foot-plate 

 are indefinitely marked off from the fenestral 

 margins; indeed, anteriorly there is a union by 

 young cartilage with the anterior edge of the 

 fenestra, shown as a blunt spur in figure 20. This 

 primitive foot-plate presents a marked bowing 

 inward and upward, and pushes in the membrane 

 filling the oval window so that it bulges into the 

 vestibular cavity (fig. 15). 



The section of the ring opposite to the foot- 

 plate is bent a little to form a rounded angle which 

 articulates laterally with the crus longum of the 

 incus. Here the tendon of the stapedius muscle is 

 inserted (fig. 44). The stapes makes with the crus 

 longum of the incus an angle of about 90°. 



Incus. — In form the incus approximates the 

 mature bone, presenting a body and two crura. 

 The body is separated from the adjoining head of 

 the malleus by membrane, but as yet there is no 

 joint cavity. There is a well-marked notch on the 

 cranial aspect of the body for the cog-tooth of the 

 malleus. A distinct spur of cartilage, tipped with 

 young cartilage, marks the cranio-lateral limit of 

 the body and projects forward to the upper 

 extremity of the malleus. The crus breve points 

 backward, with a curve of its tip downward 

 (fig. 6) to apply the medial surface of its ex- 

 tremity (fig. 31) to the wall of the otic capsule just 

 lateral to and a little below the anterior limb of the 

 lateral semicircular canal — the site of the future 

 fossa incudis (figs. 16, 20). This connection is 

 accomplished by young cartilage. In la it was 

 membranous. This represents the sole connec- 

 tion of the otic capsule with the first visceral arch. 

 The crus longum, underlying the facial nerve, 

 points inward, downward, and a little backward. 

 It is parallel with the handle of the malleus, but 

 is considerably shorter than this. The repre- 

 sentative of its future processus lenticularis is 

 edged with young cartilage. 



Malleus. — The head of the malleus (fig. 6) is 



