QUADRATE AND INCUS 



matter. But the liall-mark of truth is not always simplicity ; 

 indeed the converse appears to be frec|uently the case. And 

 on the whole this view does not commend itself to zoologists 

 at present. For it mnst he liorne in mind tliat the lower jaw of 

 the mammal is not the precise eijuivalent of that of the reptiles. 

 Apart from the membrane bones, which may be collectively the 

 e^nivalents of the dentary of tlie mammal, there is the eartilagiuous 

 articular bone to be considered, which forms the connexion 

 between the rest of the jaw and the quadrate in reptiles. Even 

 in the Anomodontia, whose relations to the j\lammalia. are con- 

 sidered elsewhere, there is this bone. But in these reptiles the 

 articrdar bone articulates not only with the quadrate, but also t(j 

 a large extent witli the squamosal, the (j^uudrate shrinking in 

 size and develo])ing processes whi(di give to it veiy much the 

 look of either the incus or the malleus of the mammalian ear. 

 In fact it seems on the whole to fit in witli the views of the 

 majority, as well as with a fair interpretation of the facts of 

 embryology, to consider that the chain of ear bones in the 

 mammal is not the equivalent of the columella of the reptile, 

 but that the stapes of the mammal is the columella, and that 

 the articulaie is represented by the malleus and the quadrate 

 by the incus. It is very interesting to note this entire change 

 of function in the bones in question. Bones which in the reptile 

 serve as a means of attaeliment of the lower jaw to the skull are 

 used in the mammal to conv^ey the waves of sound from the 

 tympanum of the ear to the internal organ of hearing. 



Another important and diagnostic feature in tlie mammalian 

 skull is that the first A'ertebra of the vertelnal column ah\ays 

 articulates with two separate occipital condjdes, which are borne 

 by tlie exoc(dpital Ijones and formed mainly though not entirelv 

 liy them. Certain Anomodontia form the nearest a]>Y)i(jacli to 

 the mammals in this particular. "J'he two condyles of Amphibia 

 are purely exoccipital in origin. 



In the ]\lammalia, unlike what is found in lower Yertelirates 

 (but here again the Anomodontia form at least a partial exception), 

 the jugal arch does not connect the face with the cjuadrate, for, 

 as already said, that bone does not exist, in the Sauropsidan 

 form, in mammals. This arch passes from the squamosal to tlie 

 maxillary, and has Ijut one separate bone in addition to those 

 two, viz. the jugal or malar. 



