Vertebræ.
The vertebral column in the Ichthyosaurus was composed
of more than one hundred joints; and although united to a
head nearly resembling that of a Lizard, assumed, in the
leading principles of its construction, the character of the
vertebræ of fishes. As this animal was constructed for rapid
motion through the sea, the mechanism of hollow vertebræ,
which gives facility of movement in water to fishes,
was better calculated for its functions than the solid vertebra
of Lizards and Crocodiles[1] (See Plate 12, A. and B.)
This hollow conical form would be inapplicable to the vertebrae
of land quadrupeds, whose back, being nearly at right
angles to the legs, requires a succession of broad and nearly
iiat qrfaces, which press with considerable weight against
each other. It is quite certain, therefore, that such large
and bulky creatures as the Ichthyosauri, having their vertebræ
tion of weight or bulk; a similar structure may be noticed in the overlapping bones of the heads of fish, and in a less degree, in those of Turtles.—Geol. Trans. Lond. Vol. V. p. 565, and Vol. I. N. S. p. 112.
- ↑ The sections of the vertebrae of a fish (A c. c.) present two hollow cones, united at their apex in the centre of each vertebræ, in the form of an hour-glass; but the bale of each cone, (b. b.) instead of terminating in abroad list surface, like the base of the hour-glass, is bounded by a thin edge, like the edge of a wine-glass, and by this alone touches the corresponding edge of the adjacent vertebrae. Between these hollow vertebræ, a soft and flexible intervertebral substance, in the form of a double solid cone (e. e.) is so placed that each hollow cone of bone plays on the cone of elastic substance contained within it, with a motion in every direction; thus forming a kind of universal joint, and giving to the entire column great strength, and power of rapid flexion in the water. But as the inflections in the perpendicular direction are less necessary than in the lateral, they are limited by the overlapping, or contiguity of the spines.
This mode of articulation gives mechanical advantage to animals like fishes, whose chief organ of progressive motion is the tail; and the weight of whose bodies being always suspended in water, creates little or no pressure on the edges, by which alone the vertebra touch each other.
