Chap. II.] BONY VAULT OF THE CRANIUM. 19 



The following anatomical conditions tend to 

 minimise the effects of violence as applied to the 

 skull : The density of the scalp and its great mo- 

 bility; the dome-like arrangement of the vault; the 

 number of the bones that compose the head, and 

 the tendency of the violence to be broken up amongst 

 the many segments ; the sutures which interrupt the 

 continuity of any given force, and the sutural mem- 

 brane, which acts as a kind of linear buffer ; the 

 mobility of the head upon the spine ; and the elas- 

 ticity of the cranial bones themselves. 



In children the membranous layer between the 

 sutures is of considerable thickness, but, as age 

 advances, this membrane disappears, and the bones 

 tend to fuse together (synostosis). The sutures 

 begin to be obliterated about the age of forty, the 

 change commencing on the inner aspect of the 

 suture, and appearing first in the sagittal suture, 

 and last in the squamous. The synostosis may 

 be complete by the age of eighty (Tillaux), and 

 its onset is said to be coincident with the cessation 

 of increase in the weight of the brain. This latter 

 assertion is supported by the fact that it appears 

 earlier in the lower races of mankind. As age 

 advances, moreover, the skull bones become less 

 porous, and lose much of their elasticity. They are, 

 therefore, more readily fractured in the aged than in 

 the young. 



As a rule, in fracture the entire thickness of the 

 bone is involved ; but the external table alone may be 

 broken, and may even be alone depressed, being 

 driven into the diploe, or, in the case of the lower 

 frontal region, into the frontal sinus. The internal 

 table may be broken without a corresponding fracture 

 in the outer plate ; and in nearly all cases of complete 

 fracture, especially in such as are attended with 

 depression, the internal table shows more extensive 



