1886.J DISK OF THE AXIS IN MAN. 339 



On becoming acquainted with these observations of Prof. Cun- 

 ningham, 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. 



My 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 situa- 

 tions 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. 



Fig. 2. 



-— — '''^-—.e . 



A diagram of tbe axis vertebra to show the various nuclei. The dotted portions 

 represent cartilage, s, the suspensory ligament, marking the former 

 position of the notochord ; e, e, epiphyses. 



In common with the majority of the vertebra, 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 ossi- 

 fication of the true centrum of the axis, as shown in the figure. As 

 growth proceeds the various nuclei fuse, the cartilage becomes re- 

 placed 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 sheaih 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 

 vertebrse. The body of each of the segments has its centre for the 



