64. The vault of the cranium forms a cavity which contains the brain (fig. XLIII. and XLVIII.) The size of this cavity is invariably proportioned to that of the organ it lodges and protects. The form and magnitude of the cavity, and consequently the shape and size of the cranium, depend upon the brain, and not of the brain upon the cranium. The soft parts model and adapt to themselves the hard, and not the hard the soft. The formation of the brain in the fœtus is anterior to that of the case which ultimately contains it; and the hard bone is moulded upon the soft pulp, not the pulp upon the bone. At every period of life, on the inner surface of the cranium there are visible impressions made by the convolutions of the brain, and the ramifications of the arteries (figs. XXXIX. 1, 2, and XL. 2), and on its external surface are depressions occasioned by the action of the external muscles. Nor does the modifying power of the brain over the bones of the cranium terminate at birth. The formation of bone, always a slow process, is never completed until the child has attained its third or fourth year, and often not until a much later period. At this tender age, the bones, which in advanced life are hard and rigid, are comparatively soft and yielding, and consequently more readily receive and retain the impression of the convolutions and of the other projecting parts of the brain, by which they are sometimes so deeply marked, that an attentive examination of the inner surface of the cranium is of itself sufficient to determine not only that some part, but to indicate the very part of the brain which has been preternaturally active. At this tender age, pressure, internal or external, general or partial, may readily change the form of the cranium. If, by a particular posture, the head of a child be unequally balanced on the spine, the brain will press more on that side of the cranium than on the other; the organ will expand in the direction to which it inclines; that portion of it will become preternaturally developed, and consequently the balance of its functions will be disturbed. An awkward way of standing or sitting, perhaps contracted inadvertently and kept up by habit; a wry neck; any cause that keeps the head constantly inclined to one side, may produce this result, examples of which and of its consequences will be given hereafter.