NUMERICAL VARIATION IN THE HUMAN SPINE. 309 



tliau its follow, or even replaced by a pretty typical transverse process; and these con- 

 comitant variations may extend even further so as to include the sacralization of one side 

 of the last lumbar, or even the absorption of one side of the atlas into the occiput. In the 

 same way we may have on one side the spread of the transverse processes of the lumbar 

 vertebrae so modified that the largest (which is that of the od under normal circum- 

 stances) should be higher on one side than on the other, so as to be three places distant 

 from the last rib and the more sacralized half of the last lumbar. These phenomena have 

 been pointed out and commented on in the description of the spines, so that it is enough 

 here to give the numbers of the spmes in which they occm-.^ 



Spines 1, 729, 306, H-3, have the thorax moving up practically symmetrically on 

 both sides. A-oO, 349, 202, show the movement on one, or chieiiy on one, side. In 

 G-22 there is something of a tendency for the thorax to move down. In 561 and X 

 there is a tendency to asymmetrical shifting of several regions without the thorax being 

 involved as a whole. 



How are these thino-s to be accounted for? As to the original cause of the variation 

 I have not the faintest idea, but it is clear that the vertebrae at the jimctiou of regions 

 are particularly variable, and it seems hard to doubt that errors of segmentation may 

 occur. The original error having occurred, there seems to be a tendency in the organism 

 to reproduce the type as nearly as may be under the changed conditions ; to make as nor- 

 mal a series of regions as circumstances will permit ; and that this tendency manifests itself 

 to some extent independently in the two halves of the spine. It is remarkable that the 

 action of this tendency is not limited to the morphology of the particular vertebi'ae, but 

 to some extent maintains the relative size of the several regions. The most striking 

 example of this is in 267 in wliich the 11 thoracic vertebrae are so large as to make that 

 res-ion, iudgino- from the bones alone, even rather lonscer than usual. 



The Vital PrhuAph. — Early in this paper it was stated that the vital principle was 

 accepted as a factor in biology. Evidently I rely upon it to account both for the normal 

 development and for the tendency to correct as much as may be the results of certain 

 errors in development. Spine 267 is an admirable illustration. First, as to the atlas. 

 The anterior ai'ch is altogether wanting. Its place is supplied by two ligaments passing 

 one from each side of the odontoid to the occipital condyle, by which the safety of the 

 neck is secured. As this arrangement does not allow the freedom of motion of the normal 

 articulation there is a want of union of the sides of the posterior arch, so that to a slight 



' The last case of rudimentai-y 1st rib seen at this writing, is tliat of Low (1900). Tliere are 25 praesacral vertebrae, of 

 wliich l:-! are thoracic. The last rib on the side of the rudimentary 1st one is very much larger than its fellow. The last 

 three lumbar vertebrae seem normal, except for some sacralization on one side of the last. It is on the same side as the rudi- 

 mentary rib. There is an extra vertebra in the thorax. 



