XXV] EAR-OSSICLES 645 



The apparent absence of the quadrate bone in the upper and of 

 the articular in the lower jaw has given rise to much speculation as 

 to what has become of these elements, which are so constantly 

 present in Aves and Reptilia and are distinctly represented by 

 cartilage even in Amphibia. For a long time the favourite theory 

 was that they had been metamorphosed into the so-called ossicula 

 auditus or bones of hearing. In Anura, Reptilia, and Aves sound 

 is conveyed from the ear-drum or tympanic membrane to the wall 

 of the auditory capsule by a single rod, called the columella auris. 

 In Mammalia however the connection is effected by a chain of three 

 small bones called the malleus (Lat., hammer), incus (Lat., anvil) 

 and stapes (Lat., stirrup) respectively, the last named being 

 apposed to a membranous spot in the auditory capsule, called the 

 fenestra ovalis, while the malleus is in contact with the ear- 

 drum, and this theory has been practically proved to be true by the 

 extensive series of discoveries of extinct Theromorphous Reptiles 

 made in the last ten years. Some of these Reptiles have become so 

 mammalian in their general appearance that their isolated teeth and 

 bones would certainly be regarded as mammalian remains if they 

 were found separated from the rest of the skeleton. In these 

 Reptiles the dentary bone of the lower jaw becomes progressively 

 larger and the other bones, including the articular, become smaller 

 and crowded into the hinder angle of the lower jaw. This enlarged 

 dentary acquires an articulation with the squamosal and the 

 quadrate bone becomes reduced in size, till it is no larger than 

 the columella auris with which it articulates. The stapes, since it 

 rests against the fenestra in the peri-otic capsule, evidently repre- 

 sents the columella auris of Birds and Reptiles. The reduced 

 quadrate represents the incus and the articular becomes the 

 malleus. The tympanic bone, which encircles the outer ear and 

 to which the ear-drum is attached, is not represented by cartilage, 

 but is a dermal bone. It is believed to be the representative of the 

 supra-angular bone which lies on the outside of the Reptilian lower 

 jaw above the angular (Fig. 318). 



This view of the homology of the ear- ossicle, which is deduced 

 from the comparative anatomy of the Theromorpha is strongly 

 supported by the developmental history of these ossicles in the 

 higher Mammalia. The malleus is for a considerable time a block 

 of cartilage, which is part of the rod forming the cartilaginous lower 

 jaw and only becomes separated from it comparatively late in 

 development, whereas the incus is segmented from the upper half 



