18865. | 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. 
A diagram of the axis yertebra to show the various nuclei. The dotted portions 
represent cartilage. s, the suspensory ligament, marking the former 
position of the notochord; ¢, e, epiphyses. 
In common with the majority of the vertebrae, the axis ossifics 
from three primary centres—one for the centrum, and two lateral 
for the laminze 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 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 
vertebree. The body of each of the segments has its centre for the 
