538 COMPARATIVE ANATOMY. 



bony labyrinth, unite to form the apparatus of the auditory ossicles; 

 the homologies of which have not yet been definitely made out for 

 the different classes. The first portion is formed by an ossicle 

 (operculum) which closes the fenestra ovalis ; in the Urodela this is 

 either flat, or provided with a stalk-like process. Sometimes it is 

 cartilaginous and its stalk ossified (Siredon) ; sometimes the reverse 

 is the case (Menopoma). In the Ccecilias they are both ossified. 

 The same arrangement obtains in the Ophidii (Eurystomata), where 

 a small jnece of bone (columella) reaches to the quadrate. 



When there is a tympanic membrane present the columella is 

 connected with it; for its cartilaginous end, which has often a 

 peculiar form given to it by processes, sinks into that membrane. 

 The lining tissue of the tympanic cavity then surrounds part of the 

 columella, and causes this bone to appear to be more or less placed 

 in the tympanic cavity. These arrangements are first seen in the 

 Anura, and are still further developed in the Saurii, Chelonii, Cro- 

 codilini, and Aves. The process of the columella is in some Birds 

 (Dromams) connected with its plate by two limbs; in other cases it 

 is simple, or is connected with the plate by one enlargement only. 



Just the same relations are seen to obtain in the columella of the 

 Mammalia ; with this modification, however, that it is never directly 

 attached to the tympanic membrane. It is converted into the stapes, 

 the form of which, in the Monotremata and many Marsupials, calls 

 to mind the columella. In the monodelphous Mammalia it is 

 ordinarily divided into two limbs, which carry the plate. The 

 other auditory ossicles are the incus, which is connected with the 

 stapes, and the malleus, which is attached to the tympanic mem- 

 brane by a styliform process. A connection between the tympanum 

 and the fenestra ovalis, which was previously effected by a single 

 bone — by the columella alone — is now effected by it, and two other 

 bones. This " chain " of auditory ossicles is, for the most part, at 

 any rate, placed in the tympanic cavity, for it is covered by the 

 mucous membrane which is continued into that cavity from the 

 pharynx, through the Eustachian tube. The tympanic cavity itself 

 has, however, another relation, for it is principally formed by the 

 tympanic bone, in addition to the boundaries provided by the wall of 

 the labyrinth ; this tympanic bone commences as the framework of 

 the tympanum. 



§ 403. 



The external ear is derived from a prolongation of the edges 

 of the first branchial cleft. In the Amphibia, Reptilia, and Aves, 

 these parts are either altogether absent, or are only present in 

 individual cases, where they have been developed in consequence of 

 various kinds of adaptive changes. Thus, in the Crocodilini a fold 

 of the integument forms an operculum above the tympanic mem- 

 brane, and in the Owls there is a movable membranous valve. 

 As early as the Saurii the tympanic membrane is removed 

 some way from the surface, so that there is a short " external 



