[ 513 ] 



XX. On the Sacral Plexus and Sacral Vertebrce of Lizards and other Vertebruta. 

 By St. George Mivart, Sec. L.S., and the Rev. Robert Clarke, F.L.S. 



(Plates LXVL, LXVII.) 



Bead May 3rd, 1877. 



Introductory Remarks. 



IT has of late been recognized that in any attempt to answer the question, which 

 vertebra of any lower animal answers to the first sacral vertebra of Man, the nervous no 

 less than the osteological relations of the parts should be carefully investigated ; and it 

 has been considered that the nervous rather than the osteological relations should be 

 deemed the more important — in fact it has been sometimes assumed that the nerves 

 must be taken as the fixed points, and that the bones must rather have their homology 

 decided by the nerves than vice versa. Should it be possible to show that in any group 

 of reptiles both the nervous and the osteological relations of any vertebra constantly 

 agree with the nervous and osteological relation of Man's, first sacral vertebra, the 

 homology between two such parts may well be taken as thereby established; but if 

 either of these sets of relations exhibit discrepancy, then of course such homology 

 cannot be considered satisfactorily determined. Nor can we justly set aside osteo- 

 logical in favour of nervous resemblances if it should turn out that the nerves 

 themselves exhibit notable variations of condition as we pass from one allied form to 

 another — a fortiori if there should be variations in this respect even within the limits 

 of a species. It might surely be anticipated that more or less variation would be found 

 to exist in nervous as well as in skeletal structures; and in the event of such 

 anticipations being justified, the determination of sacral homology must depend upon a 

 comparison of the values of the conflicting claims of differing degrees of resemblance in 

 both the osseous and nervous systems — unless we prefer to consider the osteological 

 sacrum and the nervous sacrum as distinct structures, which may or may not com- 

 pletely coincide, and may or may not widely diverge. 



Professor Gegenbaur, in his "Beitrage zur Kenntniss des Beckens der Vogel" 1 , has 

 come to the conclusion 2 that there is such a complete agreement between the anterior 

 of those two Saurian vertebras the expanded and elongated transverse processes of which 

 abut against the ilium and the first sacral vertebra of Man. In both such transverse 

 process abuts against the ilium, and in both (as he represents it) such vertebra is that 

 one which is directly postaxial to the issuing forth of the most preaxial of those nerves 

 the fibres of which go to the sciatic plexus and not at all to the lumbar plexus. 



With respect to the second vertebra abutting against the ilium, however, he represents 



1 See the ' Jenaische Zeitschrift,' Band vi. Heft 2. 



2 As shown by his diagram fig. 2, at page 201. 



