530 MESSRS. MIVART AND CLARKE ON THE SACRAL 



The sacral vertebrae in Birds may be denned, tben, as " vertebrce having one of the 

 more postaxial roots of the sciatic plexus coming forth either immediately preaxiad or 

 postaxiad, having par apophysial transverse processes abutting against the ilium, and placed 

 immediately postaxiad to vertebrce which are devoid of such parapophyses, or else being 

 the homologues of vertebrce so conditioned in most other Birds." By the combination of 

 these two definitions, the former one for Mammals, Reptiles, and Batrachians, and this 

 latter one for Birds, it seems that the sacral vertebrae may be defined in all Vertebrata, 

 above Fishes, which have pelvic limbs. 



What, then, are we to consider those vertebrae of Birds to be which are preaxial to the 

 sacrum — those, namely, which are generally more or less entirely devoid of transverse 

 processes ? It appears to us that in Birds this region of the spine is really augmented in 

 extent and in the number of its vertebrae in unison with the augmentation in the number 

 of the sciatic nervous roots. Such vertebras may, of course, be considered as corre- 

 sponding with the human lumbar vertebras ; but inasmuch as they are so constantly 

 developed in Birds, and are so peculiar in that class, it may be as well to distinguish 

 them by a distinct appellation, and to call them, as one of us has elsewhere 1 before sug- 

 gested, " lumbo-sacral vertebrae." 



A.t first sight it might be well supposed that these vertebras answer to the sacral ver- 

 tebrae of Man ; but it is evident which of the vertebrae of Beptiles answer to the human 

 sacral vertebrae ; and we agree with Professor Gegenbaur in thinking that the sacral ver- 

 tebrae of Birds may be determined by the sacral vertebras of Reptiles. Upon this view the 

 normal type must be taken as existing in Beptiles and Batrachians, compared with which 

 we have in Man a multiplication of vertebras, postaxial to the true sacral vertebras, giving 

 their nerves to the sacral plexus, while in Birds we have a similar multiplication of the 

 vertebrae which are immediately preaxial to the sacrum. The sciatic plexus in Man is 

 mainly furnished from the true sacral vertebrae and the posterior vertebrae which have 

 coalesced with them : it is famished mainly from the presacral (lumbo-sacral) vertebrae 

 in the Bird. That there should be this difference is, however, very little surprising when 

 we consider that even in the Cat it is the presacral and not the sacral vertebrae which give 

 forth the greatest part of the fibres of the sciatic plexus. Yet no one would dream of 

 calling the last lumbar vertebra of the Cat " sacral " on account of such increase in 

 volume of the presacral part of its sacral plexus. As to augmentation in the number 

 of roots, the variations we have found to exist in Lizards afford abundant ground for 

 declining to change the designation of a vertebra from " lumbar " to " sacral," or vice 

 versd, on this account alone. 



1 Trans. Zool. Soc. vol. x. p. 9. 



