528 



MESSRS. MIVART AND CLARKE ON THE SACRAL 



Fig. 9. 



this region in such forms as Buceros and Pica and the Parrots ? Osteologically they 

 are indeterminable, but they do not stand alone; the similar vertebrae of the Diver 

 {Colymbus) are also osteologically indeterminable from their ankylosis and lateral com- 

 pression, and the corresponding vertebrae of the Penguins can hardly be discriminated. 



Quite osteologically indeterminable, again, is such a sacrum as that of the Prigate Bird 

 (Fregata), in which none of the vertebrae of this region are devoid of parapophyses, and 

 in which there is therefore no differentiation into anterior and posterior parapophysial 

 vertebrae within the pelvic part of the vertebral column. 



"We must turn, then, for aid to the nervous conditions of the parts. 

 Now in birds (fig. 9) the sciatic plexus and nerve has (as repre- 

 sented by Professor Gegenbaur) as many as six roots 1 ; and if 

 we are to take nervous conditions as our test, and if we are to 

 consider the most preaxial parts of the sciatic nervous mass 

 in Birds as corresponding with the most preaxial parts of the 

 sciatic mass in Lizards, then we must consider that some or ci 

 other of the avian vertebrae without transverse processes (an- 

 terior to Gegenbaur's vertebrae a and b) must answer to those 

 vertebrae of Lizards which have enlarged transverse processes 

 to abut against the ilium. 



But we have seen that the amount of variation in Lizards 

 is such that, if nerves be taken as our test, we must deny that 

 the ilium-joining vertebrae of one species answers to the ilium- 

 joining vertebrae of other species — nay, even that, in indi- 

 viduals of the same species, we ought (according to this view) 

 to call the ilium-joining vertebrae "sacral" in one and 

 "lumbar" in another. So to do, however, would surely be 

 most absurd. 



If Professor Gegenbaur is, as he doubtless is, right as to 



the sciatic nerve-roots in Birds, then, as there are six such 



roots, and the most preaxial of them comes forth in front of 



the most postaxial of those vertebrae with parapophyses which 



are preaxiad to the vertebrae without parapophyses, it follows 



that such vertebra would be included within that definition 



of " sacral " which we have above proposed as applicable to 



Mammals, Reptiles, and Batrachians. But, at the same time, 



at least the first of the posterior parapophysial vertebrae (i. e. the first vertebra postaxiad 



to those without parapophyses) would also enter into that definition ; and thus in most 



birds we should have sacral vertebrae separated from each other by interposed vertebrae 



not sacral, which would be a very forced and unnatural conception. 



But it might, perhaps, be suggested that as the ilia are so antero-posteriorly extensive 



in Birds, the preacetabular and most preaxial of the vertebrae giving out sacral roots in 



Nerves of Birds (from 

 , Gegenbaur). 

 a, b, vertebrae with elongated 

 transverse processes abutting 

 against the ilium, opposite 

 the acetabulum ; cr, crural 

 nerve; obt, obturator nerve ; 

 is, ischiatic nerve ; 1-8, ver- 

 tebrae preaxiad to a b ; 

 l'-5', vertebrae postaxiad to 

 a b. 



1 L. c. p. 201. 



