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Show 340 MR. J. B. SUTTON ON THE INTERVERTEBRAL [June 29, body and two additional centres for the epiphysial plates, as in other regions of the spine. When the segments of the sacrum commence to ankylose, the adjacent epiphysial plates fuse with each other before they join the bodies of the vertebrae to which they belong. This fusion of epiphyses is of a deceptive character, for it does not occur throughout the whole width of these bony menisci, but only around their circumferences. Hence if a section be carried through the sacrum, a piece of cartilage may be detected situated in a central cavity, the boundaries of which are constituted by the epiphysial plates ; this piece of cartilage persists long after the various segments of the sacrum have, from all external evidence, become A section through the human sacrum, showing the epiphysial plates uniting with each other peripherally before fusing with their centra. firmly united. This remarkable arrangement of the epiphysial plates is represented in fig. 3. It is quite possible that this mode of fusion is applicable to the cervical vertebrae of Whales ; for an examination of this region of the column in a young Porpoise shows well-marked indications of peripheral union of the epiphysial plates, whilst they are still separate from the bodies of the vertebrae to which they respectively belong. M y attention was first drawn to this question when examining the sacral vertebrae of a young skeleton of the Great Anteater, Myrmecophaga jubata. After the skeleton had been macerated, the sacrum broke up into its component elements, the epiphysial plates separated from the vertebrae, but the contiguous plates were firmly united in pairs. So far as m y observations on other mammals have extended, this mode of fusion appears to be general. There are other points in the axis which demand some notice. I was unaware, until reading Prof. Cunningham's paper, that any |
