Page:Popular Science Monthly Volume 13.djvu/156

144 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. Unlike 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. The points of egress for the spinal nerves are usually between the spinous processes in other orders of vertebrates.

In living armadillos the centra of the trunk vertebræ still remain as more or less depressed cylinders of bone, or at least they are distinguishable 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 vertebræ remained separated in recent species is undoubtedly because of the mechanical conditions to which these parts of their skeleton were subjected. Here the carapace was jointed and flexible; 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 differentiate 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 outward, 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 structure, such as we actually find to result in the two cases under consideration, and as happens in a few of the posterior trunk-segments (sacral) of birds and mammals. Embryology and phylogeny both bear out these