198 



SCIENCE. 



[N. S. Vol. XIV. No. 345. 



what semi-cylindrical surface which fits 

 into a corresponding groove or cavity in 

 the articular. 



Another set of bones may be mentioned 

 now. These are the bones of the middle 

 ear, the ossicula auditus of anatomists. It 

 seems probable that the sense of hearing 

 appears in the vertebrates only with the 

 assumption of a terrestrial life, and that the 

 so-called ears of the fishes are organs for 

 the maintenance of equilibrium. In the 

 amphibia, then, true hearing appears. In 

 the salamanders an opening occurs in that 

 part of the cranium which surrounds the 

 inner ear. This opening is the fenestra 

 ovale, and is partly closed by a small carti- 

 lage or cartilage bone, the stapes, possibly 

 to be homologized with the hyomandibular 

 already referred to as apparently lacking in 

 the non-piscine vertebrates. This interpre- 

 tation receives some confirmation from the 

 fact that in several salamanders the stapes 

 articulates with the quadrate, a point which 

 is of importance when we come to a con- 

 sideration of mammalian structures. 



In the frogs, reptiles and birds the stapes 

 is a long slender rod,* frequently called the 

 columella, and is in no way connected with 



Fig. B. Diagram of Ear Eegion in a Lizard, s, 

 stapes ; c, columella ; 5, quadrate ; m, lower jaw ; ct, 

 chorda tympani nerve. 



the quadrate. It rather extends straight 

 outwards from the fenestra ovale, across the 

 cavity of the tympanum, or middle ear, to 



* The columella is more than stapes, but for present 

 purposes the details are not necessary. 



reach the tympanic membrane. It thus be- 

 comes a sound-conducting apparatus, con- 

 veying the sound waves across the tym- 

 panum to the inner ear. This columella 

 arises in the posterior wall of the tympanum, 

 and, although it later moves forward into 

 the tympanum, must consequently be re- 

 garded as a structure belonging to the post- 

 tympanic region. 



In the mammals, on the other hand, the 

 sound-conducting apparatus is greatly dif- 

 ferent. Instead of a columella there is a 

 chain of three bones leading from the 

 fenestra ovale to the tympanic membrane. 

 These are called, in order from within out, 

 the stapes or stirrup bone, incus or anvil, 

 and malleus or hammer, the stapes being 

 situated in the fenestra ovale, the malleus 

 being connected with the tympanic mem- 

 brane and the incus intervening between 

 these two. Many attempts have been 

 made to homologize these bones with the 

 columella of lower forms, but none of these 

 attempts have been successful, and it is 

 probable that complete homology does not 

 exist. This is shown almost conclusively 

 by two facts of anatomical relationship. 

 In the first place, the middle member of the 

 mammalian chain — the incus — arises in 

 front of the tympanic cavity, and hence 

 cannot correspond to any part of the 

 columella, which, as we have seen, is post- 

 tympanic in origin. Again, the incus lies 

 in front of that branch of the seventh or 

 facial nerve which is known as the chorda 

 tympani, while the columella lies behind 

 it. Now, nerves are older structures than 

 skeletal elements, and any cartilage or bone 

 placed in front of a major branch of a nerve 

 cannot be homologized with a skeletal 

 element lying behind the same nerve in an- 

 other vertebrate. 



Now, if these ear bones of the mammals 

 are not homologous throughout with the 

 columella, with what structures in the 

 lower vertebrates can they be compared? 



