446 
require some organs modified to render them 
capable of reptation. For this purpose we 
find the spinal column is composed of a greater 
number of vertebre than is to be found in any 
other class of animals. The number of verte- 
bre varies in different species of Ophidia. In 
a large Python three vertebrae in their normal 
itions measured one inch, which in an ani- 
mal of six feet in length would give 216 verte- 
bre of equal dimensions ; towards the tail, how- 
ever, their bulk diminishes, by which the flexi- 
bility of that part is augmented ; the vertebre 
are most numerous in some of the smaller spe- 
cies of Ophidian Reptiles. The bodies of the 
vertebre are short, and are articulated together 
by a ball and socket joint which is situated at 
the inferior border of the body of the vertebra ; 
but as this kind of articulation permits of a ro- 
tatory motion in every direction, it would ren- 
der the whole spine exceedingly weak if the 
motion of the vertebre was not restricted in 
some other part; to attain this object, and to 
give steadiness and precision in their move- 
ments, the articulating processes are elongated 
and furnished with double articular surfaces; of 
these the inferior is horizontal, and the superior 
oblique. The horizontal articular surface of 
one vertebra projects backwards as far as the 
extreme convex head of the ball, whilst that of 
the next vertebra projects forward as far as the 
edge of the socket ; by this arrangement the ho- 
rizontal articular surfaces are in contact to the 
Fig. 
The Octopus vulgaris represented in the act of creeping on the shore, its back being turned towards t 
spectator, towards whom it is supposed to be advancing. ‘ 
MOTION. 
extent of the depth of the socket. The superior 
oblique articular surfaces of correspondin 
tions of the two vertebre are also locked into 
each other when the vertebral column is ex- 
tended ; all the articulating processes being 
cuneiform and fitting into cavities or upon cor- 
responding surfaces effectually prevents any 
twisting of the body around it. The areas of 
the planes of the articular surfaces are suffi- 
ciently extensive to enable the animal to rotate 
each vertebra laterally 15° without causing” 
them to slide from each other, consequently 
successive vertebre allow of a sufficient range 
of motion to render the animals capable of 
turning certain portions of its body atright angle 
to each other. Owing to the position of the ar 
throdial joint, a small amount of flexion on the 
abdominal aspect in the mesial plane is suffi- 
cient to produce a considerable space between 
the spinous processes, so that 
e motion of 
the spine in this direction is rather restricted, 
and from the same cause the spine cannot he 
but very slightly flexed on the dorsal 
in the mesial plane without dislocation. ‘ 
movements of the spine for the pu of 
locomotion are, therefore, chiefly lateral. The 
ribs which extend from the atlas to the anus 
are articulated to the short transverse : 
cesses of the vertebra, and in consequence of the 
absence of the sternum, scapula and pelvis, 
are endowed with great om of motion ; 
they act in pairs on the transverse a’ i 
240. 
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