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Show 1877.] OF THE INTERVERTEBRAL SUBSTANCE. 49 brae, are described as being composed of a central elastic cushion with a laminated fibrous investment, the individual fibres of which, instead of running straight from the lower edge of one vertebra to the upper edge of the one below it, are arranged obliquely, those of one layer crossing those of the next at a considerable angle. That this is an accurate statement of the condition which exists no one will doubt. Of its mechanical advantages, however, I have nowhere found any explanation. If the fibres, instead of crossing had run parallel, and at right angles to the surfaces which they joined (fig. 1), it is evident that the median elastic pad would have efficiently retained the vertebrae at a distance from one another under ordinary circumstances. But in the act of jumping, for instance, when the feet have just reached the ground, the momentum acquired by the head and upper extremities would compress the elastic pad, and diminish the distance between each two vertebrae. At this moment, if the upper part of the body had the least tendency to obliquity in its downward movement, the relaxed outer fibres of the intervertebral substance would allow the body of the upper vertebra to slide upon the one below it, (fig. 2), and so diminish the capacity of the spinal canal, as well as the general stability of the column. A forcible attempt to rotate the body upon the spine would, under similar conditions, be also attended by compression of the elastic pad, and considerable rotatory gliding of the vertebrae on one another (fig. 3). These difficulties are entirely surmounted by the existing mechanism (fig. 4), as may be most satisfactorily demonstrated by the employment of a model composed of two circular disks of wood bound together, with an interval between them, by tapes of similar lengths arranged obliquely and crossing one another, attached to opposite points on the margins of the disks. So connected, no gliding of any kind of the disks upon one another can be produced, and the only movements possible are their approximation either at all points, or at any part where compression is employed (figs. 5 and 6). P R O C ZOOL. Soc-1877, No. IV. 4 |