Chap, i.] THE SCALP. 5 



pericranium differs somewhat in its functions from the 

 periosteum that covers other bones. If the periosteum 

 be removed to any extent from a bone, the part from 

 whence it is removed will very probably perish, and 

 necrosis from deficient blood-supply result. But the 

 pericranium may be stripped off a considerable part of 

 the skull vault without any necrosis, save perhaps a 

 little superficial exfoliation, following in consequence. 

 This is explained by the fact that the cranial bones 

 derive their blood supply mainly from the dura mater, 

 and are therefore to a considerable extent independent 

 of the pericranium. A like independence cannot be 

 claimed for the periosteum covering other bones, since 

 that membrane brings to the part it covers a very 

 copious and essential contribution to its blood supply. 

 This disposition of the pericranium is also well illus- 

 trated by its action in cases of necrosis of the cranial 

 bones. In necrosis of a long bone, the separation of 

 the sequestrum is attended with a vigorous periosteal 

 growth of new bone, which repairs the gap left after 

 the removal of such sequestra. In necrosis of the 

 vault of the skull, however, no new bone is, as a 

 rule, formed, and the gap remains unrepaired. The 

 general indisposition of the pericranium to form 

 new bone in other circumstances is frequently illus- 

 trated. 



Abscess in the scalp region may be situated 

 (1) above the aponeurosis, (2) between the aponeurosis 

 and the pericranium, and (3) beneath the pericranium. 

 Abscesses in the first situation must always be small 

 and comparatively insignificant, since the density of 

 the scalp tissue here is such that suppuration can 

 only extend with the greatest difficulty. Suppuration, 

 however, in the second situation (in the loose tissue 

 beneath the aponeurosis) may prove very serious. 

 The laxity of this tissue offers every inducement to 

 the abscess to extend when once pus has found its 



