Head and Neck. 403 



tration of blood that follows, produces the raised edge. 

 When the cephalhsematoma is beneath the pericranium, 

 i.e., between it and the bone, it is limited to that bone, be- 

 cause of the union of the pericranium to the sutural mem- 

 brane, around the margins of the bone. When, however, 

 the effused blood is beneath the aponeurosis of the mus- 

 cle, it may spread over the whole head, as explained above ; 

 whereas, if it be confined to the superficial fascia, it may 

 be situated anywhere, but is localized. The danger of 

 erysipelas and other septic affections of the scalp will be 

 dealt with when discussing the venous circulation of the 

 bones of the skull and brain. Tumors of the scalp. Nsevi 

 affecting the scalp may be either the capillary form, the 

 cavernous or the mixed. These vascular tumors have 

 special predelection for the tempero-frontal region prob- 

 ably, as Virchow states, arising from the vascular tissue 

 of the branchial arches. Plexiform angeiomata, or ag- 

 gregations of dilated and tortuous arteries, generally 

 affect the superficial temporal and the occipital arteries. 

 Sebaceous cysts or wens frequently occur in the scalp, and 

 in their extirpation, it is necessary to remove every vestige 

 of the cyst, since, otherwise, recurrence may take place. 



The Skull. The skull is composed of two tables 

 with an intervening cancellous tissue termed the diploe. 

 The skull averages in thickness one-fifth of an inch, being 

 thinnest over the temporal fossa and thickest in the neigh- 

 borhood of the external occipital protuberance or inion. 

 The outer or compact table of the skull affords strength and 

 elasticity, while the diploe, being cancellous, acts as a 

 damper to vibrations that may have been started in the 

 outer table by a blow received on the vault, so that, on 

 reaching the inner table these vibrations would be much 

 diminished in force. The diploe is normally absent in the 

 thinnest parts of the skull, viz., in the squamous portion of 



