742 



CRANIUM. 



Fig. 377. 



In infancy the rela- 

 tive proportion of the 

 cranium to the face is 

 much greater than in 

 adult life; and this 

 causes the foramen 

 m agnu m to appear to be 

 situated much further 

 forward, in the infe- 

 rior region of the base, 

 than it is when the 

 face is more 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 have 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 diplb'e 

 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 (fgss. 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 foetus 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 domes, and, 

 as the ossific matter radiates 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 an egg-like cavity ; and that 

 their margins are so arranged as to enable them 

 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. 



Assuming that the skull involved the pro- 

 perties of an arch, its defensive power has by 

 some been attributed to the circumstance of its 

 being of that figure. An arched form, how- 

 ever, would serve it only in the case of force 

 descending from above ; it would not provide 

 resistance to those severe shocks which are 

 communicated from below, as in jumping, 

 nor protect it from blows that might arrive on 

 its sides. 



But the cranium is not an arch, for there are 

 neither piers on which the extremities of that 

 arch could rest, nor abutments to resist their 

 lateral thrust. Supposing a barrel to be sawed 

 lengthwise, and the edges to be connected by 

 a base, if the centre be applied on a column, 

 (the proportion of which to the base is the 

 same as that of the spine to the width of the 

 skull,) it is manifest that, since the extremities 

 of the arch are received on the ends of two 

 long levers which have a common fulcrum, an 

 inconsiderable force would have a tendency to 

 sever them at their junction. On the other 

 hand, if the barrel were entire, force would be 

 transmitted through the parietes to a point 

 exactly opposite to that on which it impinged, 

 if it were not dissipated in its transit. Such 

 a degree of force however might be applied, 

 that its vibrations, distributed at the moment 

 of its application, might pass through the entire 

 walls, and, accumulating at one spot, by their 

 intensity cause the fracture of the part. The 

 natural mode of providing against this occur- 



