Page:Proceedings of the Royal Society of London Vol 69.djvu/12

4 vertebrae presented somewhat more difficulty. Over each centrum in the trunk, and towards its caudal end, issue the dorsal and ventral roots of a spinal nerve the latter being rather more rostrally situated than the former. As the nerve whose ventral root issues between the

Fig. 1. Hinder portion of skull and first few vertebrae. Seen after a nearly median sagittal section.

skull and first vertebra has been taken as :, it follows that the first spinal nerve will issue out over the first centrum, the second nerve over the second centrum, and so on. This relation is always kept up as we pass caudally along the vertebral column, i.e., a nerve of any given serial number always leaves the column over the caudal end of a centrum having the same serial number. When, however, we reach a point on the vertebral column somewhere between the 45th and 50th

Cenbrum48

FlG. 2a.

Gen C mm 43 FIG. 2b.

Two vertebral columns at region where the transition from half- to whole- vertebrae occurs. Seen from the inside after bisection and removal of the dorsal cartilages of one side.