SKULL OF A HUMAN FETUS OF 40 MM. 359 



men tympani as they do in the rabbit. In the oldest rabbit 

 skull examined by Voit the lateral portion of the tegmen had 

 become quite a prominent plate, covering the ossicles, and reach- 

 ing out toward the lateral wall of the middle cranial fossa. It 

 appears evident that the tegmen tympani is rudimentary in man. 



The crista parotica (figs. 2, 3 and 6) forms a conspicuous ob- 

 ject upon the border between the ventral and lateral surfaces. 

 It is narrower, as well as more prominervt, below than above, and 

 its edge shows younger cartilage than the adjacent regions. 

 The cartilage of the incus, though in the model it appears to be 

 attached, is really quite separate from that of the ear capsule, 

 there being an intervening sheet of perichondrium. 



The lowermost part of the ventral surface lies in a somewhat 

 more posterior plane than the upper, and forms the dorsal wall 

 of a small recess, open in front and below, bounded laterally by 

 the crista parotica and medially by the interfenestral septum 

 of the vestibular portion of the parscochlearis(or promontorium). 

 In the upper and medial part of this recess appears the lower 

 portion of the fenestra vestibuli, while in the lateral portion, 

 sheltered by the lower part of the crista, the facial nerve is to 

 be found, this region becoming later the lower part of the facial 

 canal or aqueduct of Fallopius. The proximal end of the cartil- 

 age of Reichert may be seen just medial to the lower extremity 

 of the crista (fig. 2). 



Just below the crista, and separated from it by a small notch, 

 there appears, on the right side, a short, free, anteriorly pro- 

 jecting conical spur of cartilage, slightly younger in character 

 than that of the adjacent otic capsule, and representing the 

 mastoid process of the adult condition (fig. 6). Its substance 

 is directly continuous with that of the ear capsule dorsally, but 

 medially it is separated therefrom by perichondrium. On the 

 left side the same formation is to be seen, except that a portion 

 of the intervening sheet of perichondrium is, near the point of 

 the process, replaced by cartilage. Immediately medial to each 

 process is to be seen the origin of the stapedius muscle. 



A brief word as to the course of the facial nerve may here be 

 in place. After entering the internal acoustic meatus it traverses 



