i 4 4 THE POPULAR SCIENCE MONTHLY. 



bending of the trunk impossible, so that an almost universal union of 

 the trunk or body segments ensued, owing to this structurally enforced 

 loss of mobility between the vertebral elements. As a consequence, 

 the centra or bodies of the segments disappeared, or were atrophied, 

 leaving only their trough-like plates about one-fourth of an inch thick, 

 formed of the degenerate and united central bodies. This trough, with 

 the united rib-bearing arches that arose from its edges, formed a tube 

 for the lodgment and protection of the spinal or nervous cord. Un- 

 like all vertebrates, except turtles, this tube in that portion over the 

 lungs is perforated at intervals on each side at points about midway of 

 the length of each one of the several united segments to give egress to 

 the spinal nerves. 1 The points of egress for the spinal nerves are usu- 

 ally between the spinous processes in other orders of vertebrates. 



In living armadillos the centra of the trunk vertebrae still remain 

 as more or less depressed cylinders of bone, or at least they are dis- 

 tinguishable as centra, from which arise the rib-bearing arches, which do 

 not completely unite, leaving lateral inter-spinous openings so as not to 

 entirely close over the nervous cord, as happens in fossil forms. The 

 reason why the vertebras remained separated in recent species is un- 

 doubtedly because of the mechanical conditions to which these parts of 

 their skeleton were subjected. Here the carapace was jointed and flex- 

 ible ; hence the need of flexibility in the spinal column. In the extinct 

 species, as in turtles, the degeneration of the centra into mere conduits 

 for the nervous cord is one of the many contrivances the origin and 

 ideological significance of which can only be explained by a mechanical 

 theory. The vertebral column in both was similarly conditioned with 

 respect to strains, mostly transverse hence the similarity of structure ; 

 which it must be borne in mind is, however, no indication of zoological 

 affinity. 



Beginning with the homogeneous notochord or continuous rod-like 

 axis of some such form as Amphioxus, Mr. Spencer points out how, 

 as this axis became bony with the assumption of the characters of 

 the higher fishes, the alternate pressure and tension incident to the 

 flexures of this axis during locomotive acts would tend to differ- 

 entiate the vertebral segments ; for it is obvious that, in order to be 

 flexible and at the same time bony, the vertebral axis must become 

 segmented. The mechanical conditions under which vertebral axes are 

 placed would indicate that the segmentation took place from within out- 

 ward, which is in accordance with observed facts. It is also obvious, in 

 view of the premises, that, in the absence of flexures or bendings of the 

 vertebral axis, we should have a return to the homogeneous struct- 

 ure, such as we actually find to result in the two cases under consid- 

 eration, and as happens in a few of the posterior trunk-segments (sacral) 

 of birds and mammals. Embryology and phylogeny both bear out these 



1 In birds, as, e. g., the common fowl, the first segments of the sacrum, the centra of 

 which are similarly atrophied, are perforated laterally in the same situation. 



