a. The scalp, turned down.
b. The cut edge of the bones of the skull.
c. The external strong membrane of the brain (Dura Mater)
suspended by a hook.
d. The left hemisphere of the brain, showing its convolutions.
e. The superior edge of the right hemisphere.
f. The fissure between the two hemispheres.
1. Hemispheres of the brain proper, or cerebrum;
2. hemispheres of the smaller brain, or cerebellum; 3. spinal
cord continuous with the brain, and the spinal nerves proceeding
from it on each side.
84. The spinal cord itself, continuous with the substance of the brain, passes also out of the cranium through the foramen magnum into the spinal canal (fig. XLIX. 3), enveloped in the delicate membranes that cover it, and surrounded by the aqueous fluid contained between those membranes. The size of the spinal canal, accurately adapted to that of the spinal cord, which it lodges and protects, is of considerable size, and of a triangular shape in its cervical portion (fig. XLIX. 3), smaller and rounded in its dorsal portion (fig. XLIX. 3), and again large and triangular in its lumbar portion (fig. XLIX. 3).
85. The spinal column performs several different, and apparently incompatible, offices.
First, it affords a support and buttress to other bones. It sustains the head (fig. XXXIV. 1); it is a buttress to the ribs (fig. XLVI. 7); through the sternum and ribs it is also a buttress to the superior, and through the pelvis, to the lower, extremities (fig. XXXIV. 2, 3, 4).
Secondly, it affords a support to powerful muscles, partly to those that maintain the trunk of the body in the erect posture against the force of gravitation, and partly to those that act upon the superior and inferior extremities in the varied, energetic, and sometimes long-continued movements they execute.
Thirdly, it forms one of the boundaries of the great cavities that contain the chief organs of the organic life. To the support and protection of those organs it is specially adapted; hence the surface in immediate contact with them is even and smooth; hence its different curvatures, convexities, and concavities, have all reference to their accommodation; hence in the neck it is convex (fig. XLV. 2), in order to afford a firm support to the esophagus, the wind-pipe, the aorta, and the great trunks of the venous system (fig. LX. 3, 4); in the back it is concave, in order to enlarge the space for the dilatation of the lung in the act of inspiration (figs. XLV. 3, and LX. 5); in the loins it is convex, in order to sustain and fix the loose and floating viscera of the abdomen (figs. XLV. 4, and LX. 6, 7, 8, 9); in the pelvis it is concave, in order to enlarge the space for lodging the numerous delicate and highly-important organs contained in that cavity (fig. XLV. 5).