72. From above downwards, that is, from the first bone of the neck to the first bone of the sacrum, the separate bones forming the column progressively increase in size; for this column is the chief support of the weight of the head and trunk, and this weight is progressively augmenting to this point (fig. XLV. 2, 4). From the sacrum to the coccyx, the bones successively diminish in size, until, at the extremity of the coccyx, they come to a point (fig. XXXV.). The spinal column may therefore be said to consist of two pyramids united at their base (fig. XLV. 4, 5). The superior pyramid is equal in length to about one third of the height of the body, and it is this portion of the column only that is moveable. 73. The two surfaces of the spinal column, the anterior and the posterior, present a striking contrast (figs. XXXIV. and XXXV.). The anterior surface, which in its whole extent is rounded and smooth, is broad in the region of the neck, narrow in the region of the back, and again broad in the region of the loins (fig. XLV. 2, 4.). It presents three curvatures (fig. XLV. 2, 4); the convexity of that of the neck being forwards, that of the back backwards, and that of the loins again forwards (fig. XLV. 2, 4). 74. From the posterior surface of the column, which is every where irregular and rough, spring, along the median line, in regular series, strong, sharp, and pointed projections of bone (fig. XXXV.), which from being sharp and pointed, like elongated spines, are called spinous processes, and have given name to the whole chain of bones. These processes afford fixed points for the action of powerful muscles. Extending the whole length of the column, from the base of the skull to the sacrum, on each side of the spinous processes, are deep excavations, which are filled up with the powerful muscles that maintain the trunk of the body erect. 75. From the lateral surfaces of the column likewise spring short but strong projections of bone, termed transverse processes, which also give attachment to powerful muscles (fig. XLVI.). [Illustration: Fig. XLV. Bones of the trunk. 1. Spinal column; 2. the seven cervical vertebræ; 3. the twelve dorsal vertebræ; 4. the five lumbar vertebræ; 5. the sacrum; 6. the sternum; 7. the true ribs; 8. the false ribs; 9. the clavicle; 10. the scapula; 11. the ilium; 12. the ischium; 13. the pubes; 14. the acetabulum; 15. the brim of the pelvis.]