OF THE MAMMALIAN OSSICULA AUDITUS. 375 



Rodentia, &c. As for its comparison with the malleus of the Anthropomorpha, such a 

 consideration may be more conveniently deferred until the latter have been described 

 individually. The same remark applies to the other ossicula. 



The Incus. — In the embryo this little bone holds a very prominent position, as the 

 proximal end of the hyoidean arch. In comparative anatomy it has hardly as yet 

 received the attention it deserves, on account of its having been supposed to vary less 

 in different mammals than do the other ossicles. My own observations lead me to con- 

 sider that in reality the chief objection to the incus as a distinctive feature in the skeleton 

 is the freqiiency with which it varies in form in different individuals of the same species. 

 Still it possesses important distinctive features in many orders of mammals. The incus 

 of Homo is of even more central type than the malleus. 



The " body " is well developed both in length and in height, and is considerably com- 

 pressed laterally. Its posterior part, or that which lies over the processus brevis, is more 

 developed than the anterior. The articular surface looks upwards and forwards, and is 

 shaped to fit that of the malleus ; its more external limi t is on the posterior segment of 

 the body, which is higher and more external than the anterior, in accordance with the 

 high position of the external extremity of the opposing surface on the malleus, already 

 described. On the other hand, that boundary of the surface on the incus opposing the 

 inner, which is the much lower extremity of the corresponding area on the malleus, lies 

 upon the lower and more internally placed anterior segment of the body. Into the 

 deep groove between the two extremities fit the convexities of the two facets on the 

 malleus. The very oblique position of the incus renders any purely verbal explanation 

 of the relations of this surface somewhat confusing ; and the difficulty is increased in 

 this case chiefly because, in describing the malleus, I have acted in conformity with its 

 condition in the lower animals, where, as I have already remarked, that which is almost 

 the upper end of the articular surface in Man is in them rather the outer extremity, the 

 boundary which lies almost quite superiorly in most mammals being almost internal in 

 our species ; hence the homologue of their upper facet in Homo has an interno-superior, 

 and that of their lower facet an externo-inferior position. 



Around the inner margin of this articular surface of the incus is a distinct groove, like 

 that which is often so strongly developed in the Seals (especially in Cystophora and 

 Phoca) ; and there is a distinct concavity on the inner surface of the body, at the bottom 

 of which is a minute nutrient foramen, often, however, filled up, even in the incus of a 

 new-born child. 



The "processus brevis" of a human incus is, as a rule, not much shorter than the so- 

 called " processus longus." It may, with more reason, be called the " posterior crus." 

 In Mau, however, it lies as much superior to the rest of the ossicle as posterior ; and in 

 some animals (the Dugong, &c.) it is almost absolutely superior ; but the term given 

 above is less clumsy than " postero-superior crus." This posterior crus is broad at the 

 base and blunt at the point, where, on the inner aspect, is a minute pit or concavity, 

 generally admitted to be a true articular surface. It is moderately divergent from the 

 body.- The processus longus, or stapedial crus, is slender and slightly convex inwards at 

 first, then forms a gentle curve, suddenly turning inwards at the extremity, which bears 



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