PLEXUS AND SACRAL VERTEBRAE OF LIZARDS. 529 



Birds answer to the sacral vertebra? of mammals, which, are preacetabular ; while the post- 

 acetahular and most postaxial of the vertebra? giving out sacral roots in Birds answer to 

 the sacral vertebrae of Reptiles, which are postacetabnlar in position. But, as has been 

 said, in some Birds the posterior parapophysial vertebrse abut beside the acetabulum ; 

 and there is so little difference in this region between Man and the Peacock that it 

 would seem to be very unreasonable not to consider the more preaxial of the posterior 

 parapophysial vertebra? of that bird as being the homologues of the anterior sacral ver- 

 tebrae of Man — and this the more so, seeing that the difference is bridged over by the 

 Tortoises and Crocodiles, which have their sacral vertebra? beside the acetabulum. May 

 we not, then, take the nervous conditions as our guide, and say that every vertebra which 

 has a sacral root coming forth, either pre- or postaxiad of it, is " sacral " ? But then the 

 last two lumbar vertebrse of at least many mammals would be sacral ; while if we deter- 

 mine that only those vertebrse shall be sacral which give forth from their postaxial end 

 a root going entirely to the sacral plexus, then the last lumbar of Man and the Cat would 

 be " sacral," as also the first presacral vertebra of Terns and Agama, while the second 

 sacral vertebra of no Lizard we have examined would be " sacral " at all ! 



If we were to add to our definition of a sacral vertebra, as proposed for Mammals, Rep- 

 tiles, and Batrachians, the words " together with any or every vertebra without parapo- 

 physes and interposed between vertebrse abutting against the ilium, and having a root 

 of the sciatic plexus coming forth either immediately preaxiad or postaxiad of it," we 

 should have a definition which would include all the vertebra? which might be considered 

 " sacral " in most Birds ; but it would leave out in an arbitrary and unnatural manner the 

 vertebrse in such forms as Buceros and Pica, which vertebra? are plainly homologous with 

 those vertebra? which in most Birds are without parapophyses, but are interposed between 

 vertebrse which have parapophyses. Neither, moreover, would it apply to Fregata. 



According to Professor Gegenbaur, the most preaxiad root of the sacral plexus is, in 

 some species at least, the fifth spinal nerve in front of the posterior parapophysial ver- 

 tebrse ; and if he has correctly determined his vertebrse x a and h in Pica and Buceros, 

 then the vertebra which he considers the fourth presacral will alone enter into even our 

 enlarged definition of a sacral vertebra as above given, and which will therefore be a 

 manifestly incomplete one for birds. The same would be the case with those Parrots 

 which have no parapophysial vertebra? in this region, save greatly preaxial to the aceta- 

 bulum ; while in those which have such vertebra?, five or six vertebrse would be included 

 under it. 



On the whole, then, seeing, on the one hand, the manifest homology between the 

 sacral vertebra? of Man and Lizards (by the help of those of Crocodiles and Tortoises), and, 

 on the other hand, the manifest homology between the sacral vertebra? of Lizards and 

 the posterior parapophysial vertebra? of most Birds, we think it better to consider the 

 latter vertebrse in Birds as alone truly sacral, and to regard such forms as Buceros, Pica, 

 and certain Parrots as differing from the rule of the class by the suppression of their 

 parapophysial processes, and Fregata as differing from the same rule by the development 

 of parapophyses in all the vertebra? of this region. 



1 L. c. plate vii. figs. xsis. and xxs. 



