intervertebral Substance in Fish and Quadrupeds . 179 
those of other animals, in which the parts are not so readily 
distinguished from one another, and consequently their pre- 
cise use has not been accurately ascertained. 
The fluid contained in the cavity being incompressible, pre- 
serves a proper interval between the vertebras to allow of the 
play of the lateral elastic ligaments, and forms a ball round 
which the concave surfaces of the vertebras are moved, and 
readily adapts itself to every change which takes place in the 
form of the cavity. 
The elasticity of the ligaments, by its constant action, ren- 
ders the joint always firm, independent of any other support, 
and keeps the ends of the vertebras opposed to each other, so 
that the whole spine is preserved in a straight line, unless it 
is acted on by muscles or some other power. When a mus- 
cular force is applied to one side of the spine, it stretches the 
elastic ligament on the opposite side of the joint, and as soon 
as that force ceases to act, the joint returns to the former 
state. This is one of the most beautiful instances in nature 
of elasticity being employed as a substitute for muscular 
action. 
The extent of the motion in each particular joint is un- 
doubtedly small, but this is compensated by their number, and 
the elasticity of the vertebras themselves. 
Fish in general have their vertebras formed with similar 
concavities to those of the squalus maximus ; these, when ex- 
amined after death, contain a solid jelly, but in the living 
fish it is found in a fluid state. This fact was ascertained 
in the skate, the smallness of the quantity of fluid in any one 
joint, and the readiness with which it coagulates after death, 
prevented it from being before observed : the fluid in the 
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