1877.) OF THE INTERVERTEBRAL SUBSTANCE. 49 
bree, 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 
Y SSL 
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the ground, the momentum acquired by the head and upper extre- 
mities 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 move- 
ment, 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 vertebree on one another (fig. 3). 
These difficulties are entirely surmounted by the existing mechan- 
ism (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). 
Proc. Zoou. Soc.—1877, No. IV. 4 
