472 STRUCTURE AND CONNEXIONS OF VERTEBRiE. 
628. It has been already noticed (§71) that an ordinary 
character of the vertebras consists in their being perforated by 
an aperture (fig. 225), which, when several vertebrae are united 
together, forms a continuous tube or canal for 
the lodgment of the spinal cord. This cha¬ 
racter is usually lost, however, in the coccy¬ 
geal vertebrae; which are so much contracted 
and simplified as to contain no aperture. 
The purpose of the division of the spinal 
column into so large a number of separate 
hones, is obviously to allow of considerable 
freedom of motion by a slight shifting amongst 
the individual parts ; whilst any such sudden bend as would 
be injurious to the spinal cord, is avoided. Each vertebra con¬ 
sists of a solid “ body ” a, which is situated in front of the 
spinal canal in Man, but below it in animals whose back has 
a horizontal position, and which serves to give solidity to the 
structure,—and of “ processes” or projections, b and c, that 
serve to form the spinal canal, and to unite the vertebrae to 
each other. In Man and other warm-blooded animals, the 
two surfaces of the “body” are nearly flat and are parallel to 
each other; and they are united to the corresponding surfaces 
of the neighbouring vertebrae by a disk of fibro-cartilage (§ 47), 
which extends through the whole space that intervenes be¬ 
tween them, and which, being firmly adherent to both, prevents 
them from being far separated from each other. 
629. But in Beptiles and Fishes, a different plan is adopted. 
In the animals of the former class, particularly in Serpents, 
we find one surface of each vertebra convex or projecting, and 
the other concave or hollowed-out; and the convex surface of 
each vertebra fits into the concave surface of the next, in such 
a manner that the whole spinal column becomes a series of 
ball-and-socket joints, and is thus endowed with that flexibi¬ 
lity which is essential to the peculiar movements of these 
animals. In Fishes both surfaces are concave, and between 
each vertebra there is interposed a bag containing fluid, and 
having two convex surfaces, over which those of the vertebrae 
can freely play. Extreme facility of movement is thus given 
to the spinal column ; but its strength is proportionally dimi¬ 
nished. It is to be remembered, however, that strength is 
not required in the bony framework of animals, whose bodies, 
