I have now before me the skull of a Gangetic Crocodile, in which, although the animal was an adult when killed, the bones of the long lower jaw are so loose that unless they were tied together the jaw would fall to pieces.
This analogy between Art and Nature is thus described by Professor Owen in the work which has just been mentioned:—
“The purpose of this subdivision of the lower jaw-bone has been well explained by Conybeare and Buckland, by the analogy of its structure to that adopted in binding together several parallel plates of elastic wood or steel to make a crossbow, and also in setting together thin plates of steel in the carriage spring.”
Dr. Buckland also adds: “Those who have witnessed the shock given to the head of a Crocodile by the act of snapping together its thin, long jaws, must have seen how liable to fracture the lower jaw would be were it composed of one bone only.... The splicing and bracing together of thin flat bones of unequal length and of varying thickness afford compensation for the weakness and risk of fracture that would otherwise have attended the elongation of the parts.”
A good example of the value of this structure of bone may be found in young children. Before they are old enough to take care of themselves they are perpetually falling down, and never hurting themselves. I have seen a little girl of five years old roll from top to bottom of a lofty staircase. It looked as if the child must be killed, but she was only giddy with her many revolutions, and a little bruised about the elbows. The reason of this curious immunity from injury is, that the bones, especially those of the skull, are not completely united, and so act on the principle of the compound spring.
The Spiral Spring.
This subject is so large, and there are so many examples, both in Art and Nature, that it is not very easy to make selections which will sufficiently answer the purpose.
The upper left-hand figure of the illustration represents the ordinary Spiral Spring made of wire, and used for its power of resuming its shape when compressed. In early childhood most boys have had practical experience of this spring in the toy guns and cannons with which they are supplied. The spring is compressed by the ramrod, and held in its place by a catch. If a pellet be placed in the gun, and the catch released by pulling the trigger, the spring flies back to its former shape, and drives the pellet.
An exactly similar spring is used in the well-known “Needle-gun,” the spring driving a needle through the explosive mixture, and so igniting the charge.