345 



^ould account for both. Now in several cases in this series, and there 

 are plenty like them in the literature, we see a tendency sometimes 

 for the whole thorax to move forward by cervical ribs associated with 

 absence of the last thoracic ones or with their existence in a rudi- 

 mentary condition. We also see cases in which, when the cervical rib 

 on one side is distinctly larger than its fellow, the last rib on that 

 side is either correspondingly smaller than its fellow, or even replaced 

 by a pretty typical transverse process; and these concomitant 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 3d under normal circumstances) 

 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. 



How are these things 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 junction 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 normal a series of regions as circum- 

 stances 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 vertebrae, but to some extent maintains the relative size 

 of the several regions. The most striking example of this is in 267 

 in which the 1 1 thoracic vertebrae are so large as to make that region 

 judging from the bones alone, even rather longer than usual. 



The Vital Principle. 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 arch is altogether wanting. Its place is sup- 

 plied 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 extent each half may move indepen- 



