The igferior fontanelles are found, the anterior (a, fig. 375) between the spinous angle of the parietal, and the great wing of the sphenoid bone; the posterior (b, jig. 375) between the mastoid angle of the first-named bone and the mastoid process of the temporal. These two fontanelles are, therefore, situated at the extremities of the squamous suture.
In infancy the rela tive proportion of the cranium to the face is much greater than in adult life; and this causes the foramen magnum to appear to be situated much further forward, in the infe rior region of the base, than it is when the face is niore expanded.
The lower part of the occiput is flattened, the superior is very projecting, and, altogether, the cranium has a character of rotundity which is speedily exchanged for the oval form which prevails in the adolescent age.
When the sutures ha.ve become conjoined, and the cranium is constituted a defensive in vestment of the brain in virtue of its mechan ism, the internal table (the tabula vitrea) is secreted in greater abundance, and the diphie between it and the outer table is rendered more manifest. The spongy tissue of the sphenoid bone is absorbed and the sinuses formed ; but it is not until a period nearly coeval with puberty, that those of the frontal bone are developed.
It is not until the diploe is fully formed that we can demonstrate those venous canals with which that structure has been shown to abound by the researches of Chaussier, Dupuytren, and Breschet (figs.187, 188, p. 436).
:Mechanical adaptation of' the cranium.—It will now be noticed that the properties of the cranium, those on which its defensive qualities are founded, differ in the several periods of life ; but that, nevertheless, there is in each as perfect an adaptation of it to these purposes as seems consistent with the schemes of Provi dence in the creation of a finite being.
The pressure which the brain has to sustain during the process of parturition, is directed solely to that part which is not essential to life; the condition of the bones of the calvaria ad mits of the volume of the hemispheres being diminished at the time the fcetus is ushered into the world. Not so the base ; the parts which it is destined to protect require to be maintained in all their integrity, and the ex tent to which it has acquired solidity is such as to forbid the encroachment of the parietes on parts which are essential to the continuance of life, and which are highly intolerant of pres sure.
In infantile life, also, protection is afforded on the same principle. The bones of the calvaria are notoriously capable of sustaining indentations, and afterwards, by their resili ency, of regaining their normal form. The preponderance, too, of the organic over the inorganic texture, blunts the force which may be applied, and resists its transmission to the parts below. But there is an addition even to these provisions, a mechanical disposition of the bones highly favourable to resistance. At the back, on the sides, and in front—opposed in every direction from which force may pro ceed—are the summits of ovoidal dome,s, and, as the ossific matter mdiates from these summits to the circumference, the force will be received on one extremity of a bundle of diverging lines, and that which would sever the structure if it fell on any other point, here falls compa ratively innoxious. Hence it is that the cen ters of ossification are so much more projecting during infancy than in after life ; for, although the mechanical contrivance abides through the whole term of existence, it is not, when asso ciated with other means, of that predominating character which we observe in youth.
The manner in which the cranium (when fully formed) defends the brain, differs widely from the preceding. In proportion as its several parts become consolidated, and the relation between its animal and earthy consti tuents is reversed, so its power of deadening, or neutralizing the vibrations which pass through it, is diminished. It is here on its general shape and the disposition of its parts that its protective properties depend.
It has been already stated that the bones of the cranium are so fashioned as to concur in the production of art egg-like cavity; and that their margins are so arranged as to enable thein to bind and be bound by each other, in such a manner that if one bone be taken away the whole will have a tendency to separate. This ovoid form ensures (much better than any other which has no fixed basis or point of resistance beyond itself) the transmission of the vibrations which are distributed from any spot on which force may be applied.