4— ON THE HOMOLOGY OF THE MAMMALIAN 

 ALISPHENOID BONE. 



By R. Broom, M.D., D.Sc, C.M.Z.S. 



By the older writers, the Alisphenoid bone was looked upon as 

 serially homologous with the exoccipital and orbitosphenoid, forming 

 the principal part of the neural arch of the basisphenoid segment or 

 vertebra, as these others do of the basioccipital and presphenoid 

 segments. Though the old theory of Owen and others was pretty 

 effectually disproved by Eluxley and Parker, it affords such a con- 

 venient way of looking at the mammalian skull at least, as admitted 

 even by Huxley, that in all our text books it seems to be tacitly 

 assumed that the alisphenoid bone is a lateral outgrowth from the 

 basisphenoid, formed to support part of the brain. Even a study 

 of the development does not at first sight seem to be against such a 

 view. The back part of the parachordal forms at its lateral deve- 

 lopment behind the auditory capsule the exoccipital, and in front of 

 the capsule there seems to be formed in a similar way the alisphenoid. 



When we turn to comparative anatomy, we find a rather remark- 

 able circumstance in that, while in the bird there seems to be a well 

 developed alisphenoid, there is no trace of an alisphenoid in most 

 reptiles. In' the crocodile, the bone appears to be present in a fairly 

 well developed condition ; in chelonians there is a very small bone 

 which some regard as an alisphenoid. and others as a columella cranii, 

 but in the lizard and Sphenodon, there is no trace. In snakes there 

 is a small alisphenoid bone. In amphibians no alisphenoid can be 

 identified. In bony fishes a distinct alisphenoid is believed to be 

 present, but, as with a number of the other cranial bones, the 

 homology with the bone called alisphenoid in the mammal is 

 extremely doubtful. 



It will thus be noted that in most reptiles and amphibians, there 

 is no trace of an alisphenoid, but that it appears in the more special- 

 ised forms, such as crocodiles, snakes, chelonians, and in birds and 

 mammals. And the question arises : how did it originate? 



If we look at the Sauropsida alone, we observe the very curious 

 circumstance that almost invariably there is present either a columella 

 cranii or an alisphenoid, but never both. And this holds good for 

 extinct reptilian orders, as well as recent. The lizard has a colum- 

 ella cranii, but no alisphenoid ; the allied snake has an alisphenoid, 

 but no columella. The crocodile has only an alisphenoid ; Sphenodon 

 has only a columella. The chelonian has a bone which might 

 equally well correspond to either. One might, therefore, readily 

 suspect that the columella cranii is the homologue of the alisphenoid, 

 or in other words, that the alisphenoid is a modified columella cranii. 



This view, which I have held for a good many years, receives 

 very strong confirmation from the condition of affairs in the mammal- 

 like reptiles. In the very primitive reptiles, of which Procolophon 

 may be taken as a tvpe, we .have a lizard-like columella cranii. In 

 the early types, which have specialised along the mammalian line, 

 such as the Therocephalians, we still have a columella cranii. In 



