intervertebral Substance in Fish and Quadrupeds. 181 
fish. The structure of the spine in the lamprey eel resembles 
that of the sturgeon. 
The intervertebral joint which is common to fish, is not met 
with in any of the whale tribe, whose motion through the water 
is principally effected by means of their horizontal tail; in 
them the substance employed to unite the vertebras together 
is the same as in quadrupeds in general, and from the size of 
the vertebras it is on a larger scale, and rendered more con- 
spicuous. 
The external portion is very firm and compact, is ranged 
in concentric circles with transverse fibres uniting the layers 
together, it becomes softer towards the middle, and in the 
centre there is a pliant soft substance without elasticity, but 
admitting of extension more like a jelly than an organized 
body, corresponding in its use to the incompressible fluid in 
the fish. 
To ascertain whether this structure was generally met with 
in the spines of quadrupeds, Mr. Brodie, at my request, exa- 
mined the intervertebral substance in a great many animals, 
and found, what, undoubtedly, was very little to be expected, 
that in the hog and rabbit, in the central part, there is a cavity 
with a smooth internal surface of the extent of half the dia- 
meter of the vertebra, in which is contained a thick gelatinous 
fluid, so that in some quadrupeds there is an approach towards 
the intervertebral joint in fish; but whether this is to answer 
any essential purpose to these animals, or is only to form an 
intermediate link in the chain of gradation of structures, which 
is so uniformly adhered to in the productions of nature, cannot 
at present be determined. 
In the bullock, sheep, deer, monkey, and man, the struc- 
