MUTUAL CONNEXIONS OF VERTEBRE. 473 
instead of being supported upon four fixed points, are buoyed 
up in every part by a liquid of nearly the same density with 
ves. The extreme flexibility of the spine of Fishes, 
enables them to propel their bodies by the movements of the 
hinder portion and tail from side to side ; their members, or 
pectoral and ventral fins (fig. 243), being but little used 
except for influencing the direction of their motion. And 
thus we see that in the lowest Vertebrata, as in the lower 
Articulata (such as the Leech and Earth-worm), the propul- 
sion of the body being accomplished by the movements of 
the trunk itself, its skeleton (internal in the one case, external 
in the other) is left in the soft condition which it has in all 
at an early period: whilst in the higher classes of both series, 
—Birds and Insects for example,—the extremities being so 
developed, and being furnished with muscles so powerful 
that the function of locomotion is entirely committed to them, 
the skeleton of the body undergoes great consolidation, its 
Various pieces being so knit together as to make the trunk 
almost immovable. 
630. This knitting-together is partly accomplished by 
means of projections or processes from the several vertebra, 
which are united to one another by muscles and ligaments. 
Of these processes there are seven in Man from each vertebra. 
One of these, termed the spinous process (6, fig. 225), projects 
directly backwards ; and thus is formed the prominent ridge 
on the back, in which the ends of these projections can be 
istinguished. The spinous processes serve in Man to give 
_ attachment to the muscles, by which the trunk and head are 
kept erect ; in Animals whose spine is horizontal, they are 
_ generally much longer, in order to give firm attachment to the 
muscles and ligaments which support the head (fig. 229, ve, xd). 
And in Fishes they are greatly prolonged (fig. 243), so as to in- 
crease the surface by the stroke of which from side to side the 
body is propelled through the water. On each side of the 
vertebra is a process (c, fig. 225) which is called transverse; 
this serves for the attachment of the ribs to the vertebra. 
_ And lastly, from the upper and under side of each vertebra, 
two articulating processes project, which lock against each 
_ other in such a manner as to prevent the movements of the 
vertebre: from being carried to an injurious extent. These 
processes are peculiarly long in Birds, where they almost 
