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Show 1886.] DISK O F T H E A X I S IN M A N . 339 On becoming acquainted with these observations of Prof. Cunningham, I lost no time in testing the statements by independent observation, and am able to confirm them in every particular. As in so many other instances, it is not remarkable that this piece of cartilage should exist, but that it has remained so long undetected. M y intention in bringing the subject under the notice of the Society is not merely to confirm Cunningham's statement, but to show that the presence of this piece of cartilage, in the midst of the axis, merely harmonizes with the condition of things found in situations where other vertebrae normally fuse together, as for example in the sacrum. Before discussing the question it will be desirable to briefly review the chief facts connected with the development of the axis. A diagram of the axis vertebra to sbow the various nuclei. The dotted portions represent cartilage, s, the suspensory ligament, marking the former position of tbe notochord; e, e, epiphyses. In common with the majority of the vertebrae, the axis ossifies from three primary centres-one for the centrum, and two lateral for the laminae and processes. Subsequently two nuclei appear for the odontoid process, arranged side by side as represented in fig. 2. After birth these centres fuse, and an additional nucleus, detected by Prof. Humphry, is deposited for the tip of the odontoid process, usually visible about the second year. Finally two epiphysial plates appear to complete the ossification of the true centrum of the axis, as shown in the figure. As growth proceeds the various nuclei fuse, the cartilage becomes replaced by bone, except the piece referred to at the commencement of the paper, and the growth of the bone is complete. The band of fibrous tissue passing from the summit of the odontoid process represents the thickened sheath of the notochord, and is known as the suspensory ligament. Turning our attention to the sacrum, we shall find in that bone an explanation of the persistence of the piece of cartilage which remains unossified in the axis. The sacrum in man is composed of five fused vertebrae. The body of each of the segments has its centre for the |
