526 MESSRS. MIVART AND CLARKE ON THE SACRAL 



of the first sacral vertebra ; the third (the smallest hut one) comes out (29) from the 

 most preaxiad sacral foramen ; the last (the smallest) comes forth from (29 a) the second 

 sacral foramen. Thus here the sciatic nerve is formed from nerves issuing out of the 

 sacrum in a less proportion than in Man, and more by nerves issuing preaxiad to the 

 sacrum — not that the number of the latter is greater than in Man, but that their relative 

 size, compared with those from out the sacrum, is greater. Compared with Lizards, it would 

 seem, with respect to nerves, that the first and second sacral vertebrae of the Cat answer 

 very well to the two vertebra? with enlarged transverse processes of Lizards, while osteo- 

 logically they, of course, also answer very well to them. There can be little doubt, how- 

 ever, but that the two first sacral vertebrae of the Cat are to be considered homologous 

 with the anterior human sacral vertebra? ; and therefore it would seem that the two 

 ilium-joining vertebra? of Lizards should be considered homologous with the anterior 

 human sacral vertebra?. 



In Man, the Cat, and also in other mammals, down to the Echidna, the transverse 

 processes abutting against the ilium are parapophysial, like those of the other classes 

 before described— viz. Reptiles and Batrachians. In all mammals which we have ex- 

 amined, however, the junction of the sacral transverse processes with the ilia is pre- 

 acetabular, although that junction is much less preacetabular in position in Man than it 

 is in most mammals. 



Altogether, from the osseous and nervous conditions considered jointly in the groups 

 hitherto referred to, we would venture to propose the following definition of a " sacral 

 vertebra " in Mammals, Reptiles, and Batrachians : — "A vertebra with parapophysial 

 transverse •processes which abut against the ilium preaxiad, or postaxiad, or opposite to the 

 acetabulum, and having a root of the sciatic plexus coming forth either immediately pre- 

 axiad or postaxiad of it" This definition will exclude from the sacrum of Man, as not 

 abutting against the ilium, the more posterior vertebra? called " sacral " in anthropotomy. 

 But in the lower mammals (even already in Apes) the number of so-called " sacral " 

 vertebra? augments more or less with age by the ankylosis of the sacral vertebra?, so as 

 to render the extent of the " sacrum " very variable. It would surely be well, then, to 

 distinguish the human sacral vertebra? (like the ribs) into true and false, those being the 

 true sacral vertebra? which abut against the ilium. 



The Sacral Region in Birds. 

 The determination of the homological relations of the different parts of the postdorsal 

 part of the spinal column of Birds is a matter of much difficulty. It might be antici- 

 pated, however, from the close relations and probable genetic affinity between Birds and 

 Reptiles, that the true sacral vertebra? of the former class might be readily determined 

 by the aid of reptilian characters. But however close may be the morphological affi- 

 nities, they are so strangely disguised by great physiological differences, that the results 

 of comparison are much too unsatisfactory to justify the anticipation above referred to. 

 The enormous forward prolongation of the iliac bones and the great power and activity 

 of the erect and vigorous legs of birds, compared with the backwardly extending ilia and 

 the sprawling and feebly pushing legs of Saurians, combine to produce in Birds a redun- 



