408 ON THE INTERVERTEBRAL SUBSTANCE. 
64. ON THE MECHANISM OF THE INTERVERTEBRAL 
SUBSTANCE, AND ON SOME EFFECTS OF THE 
ERECT POSITION OF MAN.* 
Page 49. In all works on human anatomy the structure of the disks of fibro- 
elastic tissue which intervene between the bodies of the vertebre is 
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. 
J a — => [XN 
wT ——} 
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 vertebra 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 vertebra. 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 
* “ Proceedings of the Zoological Society,” 1877, pp. 48-50. Read, Feb. 6, 
1877. 
