-^" THOMAS D WIGHT ON 



with each two transverse processes and two ribs, while they are single on the other. What 

 has occurred is a unilateral error of segmentation, explain it as one may. 



It is self-evident that the intercalation theory, or the better one of an error of seg- 

 mentation, in no way excludes Rosenberg's. The migration of a developing ilium is not 

 in the least inconsistent with a cervical vertebra too few or too many, depending on other 

 causes. 



The Vertebra Fidcralis. — There is one important point in the discussion of the rival 

 theories, which was raised long ago by Welcker, and which needs to be settled. Accord- 

 ing to what principle are the vertebrae of one spine to be compared with those of another? 

 Do, for instance, the 19th and 24th vertebrae of one spine correspond to those of the same 

 number in all others; or does the last praesacral always correspond to the last praesacral, 

 though it be the 23d, the 24th, the 25th, or even the 26th? According to Rosenberg, and 

 probably most biologists, the former view is to be adopted. On the other hand, in my 

 opinion, if it can be shown that a certain vertebra has a distinct teleological significance, 

 it is an excellent starting point. Welcker's ("81) idea of a vertebra fulcralis is a very 

 valuable one. It is the vertebra, normally the 25th, which has the most to do in support- 

 ing the ilium, as is shown by its forming a larger part of the auricular surface than any 

 other. According to Welcker this vertebra is always to be compared to the one having 

 a similar function, be the number what it may. He goes even further and would have 

 not only the whole praesacral region of one spine correspond with that of another, but he" 

 would have its component regions correspond respectively with those of other spines. 

 While I fully adopt the former view, I am not sure that it is wise to attempt to carry it 

 into such details. There is, however, one serious difficulty. The fact that a vertebra is the 

 fulcralis, is established by its being the one that forms the largest part of the auricular 

 surface, thus bearing more of the ilium than any other, not by its general shape. This is 

 particularly insisted upon by HoU, who adopts the idea. It is common enough for it to be 

 the 26th instead of the 25th, in which case there is rarely any difficulty in deciding which 

 vertebra is the fulcralis . HoU declares he never has known the 24th to be the fulcralis, 

 though he has seen it so sacralized that at first he was inclined to think so. In the present 

 series there are several in which it is certain that the 24th is the fulcralis, but there are also 

 some in which it is very difficult or, rather, nearly impossible to decide whether the 24th 

 or the 25th be the fulcralis. Does this difficulty do away with the conception of a verte- 

 bra fulcralis altogether? I think not. There is nothing in the human body that is not 

 variable, and why should we exact absolute stability from the vertebra fulcralis F 



The Vital Princi2ole. — My reasons for accepting the idea of the vertebra fulcralis are 

 the following : Every living organism has the tendency to develop in a certain way to 

 adapt itself to certain purposes. The fact that under changed circumstances there may 



