ix.] THE NERVOUS SYSTEM. 395 



In osseous Fishes a yet further segmentation occurs, as we 

 find in addition a third bone, called the meta-pterygoid. 



Tnc. 



FIG. 346. THE MEMBRANE OF THE DRUM OF THE EAR, seen from the inner 

 fide, with the small bones of the ear ; and the walls of the tympanum, with 

 the air-cells in the mastoid part of the temporal bone. 



MC t mastoid cells ; Mall, malleus ; Inc, incus ; St, stapes ; a b, lines drawn 

 through the horizontal axis on which the malleus and incus turn. 



The incus becomes relatively smaller as we descend to the 

 Sauropsida, and the stapes appears as a little straight ossicle 

 (the columella auris] coming to the incus from the fenestra 

 ovalis as usual. In Fishes, however, where there is no fenestra, 

 there is no such ossicle, and the representative of the incus 

 (i.e. the highest part of the hyoidean visceral arch) is a very 

 large bone termed the hyo-mandibular. 



Thus, if this interpretation be correct, it is plain that our 

 comprehension of man's auditory ossicles would be very im- 

 perfect if studied only in man himself. 



The three little auditory bones, the malleus, the incus, 

 and the stapes, instead of being all similar in nature as in 

 function, are singularly different. 



The malleus is, as it were, the upper part of the lower jaw, 

 separated, reduced in size, and taken into the ear through the 

 Glasserian fissure, and applied to the assistance of a special 

 sense. 



The incus is the extreme summit of the anterior cornu 

 of the os hyoides, separated and also applied to a similar 

 purpose. 



The stapes is quite different in nature, being a small por- 

 tion of the cranial wall which has grown out, become 



