CHAP, vi.] THE CRANIUM. 139 



the violence applied directly to the vertex is spent upon the base of 

 the skull , and causes a fracture there. 



The compound structure of the cranial bones is an important 

 element in the architecture of the skull as a protecting case to the 

 brain. Most of these bones are formed of two tables : the outer 

 one is tough, strong, and fibrous ; the internal table is dense and 

 brittle, and hence called tabula vitrea ; and there is interposed 

 between them a spongy texture, the diploe, in which blood-vessels 

 are freely distributed (fig. 20, p.107). The varying density of these 

 three layers evidently diminishes their power of conducting vibra- 

 tions to the parts within, whilst it does not oppose the propagation 

 of those vibrations in the direction of the layers themselves. 



The manner in which the bones of the skull are united together 

 has an evident reference to the physical properties of their inner and 

 outer tables. The sutures are formed by the dovetailing of the 

 outer table ; the inner being cut straight, and simply placed in appo- 

 sition (a layer of cartilage intervening), forming a sort of harmonia. 

 The inner table, which is the brittle one, is not dovetailed, because 

 its teeth would break readily ; but the toughness and elasticity of 

 the outer table fit it well for such a mechanism. On the same 

 principle, Sir C. Bell remarks, a carpenter joins wood, which is 

 tough and elastic, by tenon and mortise or by dovetailing ; but, if 

 pieces of glass or marble are to be joined, cement is employed for 

 that purpose. 



The principal part of the vault of the cranium is formed by the 

 parietal bones, which rest upon the wings of the sphenoid, and upon 

 the temporal bones : these overlap the lower borders of the parietal 

 bones in such a way as to prevent them from starting outwards. 

 They act on the principle of the tiebeam in the roofs of houses. 



At certain exposed situations the bones experience a thickening 

 of their structure, causing tuberosities, which are familiar to descrip- 

 tive anatomists. These contribute to the strength of the roof of 

 the skull : in front, in the frontal bone on each side of the middle 

 Hue ; laterally, in the parietal bones; and, behind, in the centre of the 

 occipital bone. At this last situation two ribs, analogous to groinings 

 in architecture, intersect each other : one extends from the centre 

 of the frontal bone to the most projecting part of the occipital fora- 

 men ; the other passes horizontally across the occipital bone, and 

 terminates immediately behind the wedge-like processes which are 

 formed by the petrous portions of the temporal bones. The occipi- 

 tal protuberance, which is the point of intersection of these groin- 

 ings, is the " thickest and strongest part of the skull ; and it is the 



