. Animal and vegetable physiology considered with reference to natural theology. Natural theology; Physiology; Plant physiology; Biology. 490 THE MECHANICAL FUNCTIONS. leaves of the first cranial vertebra, and assimilat- ing it more to a hinge joint. In birds, however, where, as we have just seen, the most extensive lateral motions are required, the plan of the ball and socket joint is again resorted to; and the occi- pital bone is made to turn upon the atlas by a single pivot. So great is the freedom of motion in this joint, that the bird can readily turn its head completely back upon its nec
. Animal and vegetable physiology considered with reference to natural theology. Natural theology; Physiology; Plant physiology; Biology. 490 THE MECHANICAL FUNCTIONS. leaves of the first cranial vertebra, and assimilat- ing it more to a hinge joint. In birds, however, where, as we have just seen, the most extensive lateral motions are required, the plan of the ball and socket joint is again resorted to; and the occi- pital bone is made to turn upon the atlas by a single pivot. So great is the freedom of motion in this joint, that the bird can readily turn its head completely back upon its neck, on either side. As spinous, or transverse processes of any length would have interfered with the flexions of the neck, we find scarcely a trace of these processes in the cervical vertebrae of birds. But another, and a still more important consideration was to be attended to in the construction of this part of the spine. It must be recollected that the spinal mar- row passes down along the canal formed by the arches of the vertebrae, and that any pressure ap- plied to its tender substance would instantly para- lyze the whole body, and speedily put an end to life. Some extraordinary provision was therefore required to be made, in order to guard against the possibility of this accident occurring during the many violent contortions into which the column is liable to be thrown. This is accomplished in the '^25 ms^ simplest and most effec- tual manner by enlarging the diameter of the canal at the upper and lower part of each vertebra, while at the middle it re- mains of the usual size; so that the shape of the cavity, as is well seen in Fig. 225, which shows a. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Roget, Peter Mark, 1779-1869. London : W. Pickering
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