376 MR. A. H. G. DORAN ON THE MORPHOLOGY 



a flat elliptical disk on a narrow pedicle. This is the " os orbiculare," better termed the 

 " Sylvian apophysis" *. Among anatomists, Sappey gives reasons for considering it a 

 separate bone ; but as it is now recognized as being formed on the proximal end of the 

 hyoidean arch, and, on the other hand, is separate from tbe long cms in an early stage of 

 development, it would appear correct, on such premises, to consider it an epiphysis of 

 the incus. 



The Stapes. — Whilst the ossicles already described take, in Homo, a rather central 

 type, this little bone assumes in the Bimana an extreme and highly developed form. 

 The beautiful bicrurate stapes of Man differs strikingly from the simple columelliform 

 representative of that ossicle in the Omithorhynclms ; and the study of the types inter- 

 mediate between those two extreme forms is highly instructive. 



The " head " is broad and shallow ; its free aspect is elliptical and a little concave, to 

 articulate with the os orbiculare. The crura are long and widely divergent, so as to 

 leave a large aperture between them and the base. The anterior crus is the more 

 slender and the straighter of the two ; it is widely grooved towards the aperture in its 

 upper half; but tbe channel generally narrows close to the base. The posterior crus has 

 a curve or shoulder, often very marked, not far from the head ; it is deeply channelled 

 towards the intercrural aperture by a groove almost as deep near the base as near the 

 head. This sulcus joins, under the head, that which exists on the opposite crus. 



Sometimes the anterior crus is more curved than usual, but seldom so sharply as the 

 opposite limb of the stapes. In such cases it is still the more slender, and the groove is 

 less marked near the base than it is on the posterior crus. I have examined several 

 specimens of short, stumpy stapedes, from both infants and adults, where it was found 

 very hard to distinguish the anterior from the posterior crus by their own characters 

 alone t- 



The base, or foot-plate, is reniform, its upper border being a wide arch, its lower 

 slightly concave in the middle. Its posterior extremity is always well -rounded off; and 

 its anterior is usually much sharper ; but this distinction is often the least marked where 

 the crura are most alike. 



The base is slightly convex towards the vestibule. I have never seen a true umho 

 like that which is found on the stapes of many monkeys. On the tympanic aspect it is 

 formed into a shallow tray by the prolongation along its margin of the high edges of the 

 channel within the crura. This condition is very frequent in animals where that ossicle 

 is well developed and lightly made. A bony ridge %, traversing the long diameter of 

 the tympanic aspect of the foot-plate, is sometimes seen, but is not constant. The aper- 

 ture between the crura of the human stapes is larger, when measured vertically as well 



* A beautiful description of this structure and the incudo-stapedial joint is to be found in EyselFs "Eeitrage zur 

 Anatornie des Steigbiigels," Arch. f. Ohrenheilkunde, vol. v. 



t But the straightness of the anterior crus is generally much more apparent in the stapes of an infant than in that 

 of an adult. A specimen from an adult Bushwoman in the College collection is of a very marked infantile type 

 in the form of its crura. This agrees with the characters of the rest of her skeleton, described in Flower and Murie's 

 " Account of the Dissection of a Bushwoman," Journal of Anatomy and Physiology, vol. i. 



\ The " crista stapedis." See Eysell, he. cit. 



