chap, in.] THE CRANIAL CONTENTS. 25 



grooves for the meningeal vessels. It is important to 

 remember in trephining that the inner table is not 

 always parallel with the outer. 



CHAPTER III. 



THE CRANIAL CONTENTS. 



THE membranes of the brain. The dura 

 mater, from its toughness, forms an excellent protec- 

 tion to the brain. It is very intimately adherent to 

 the bone over the whole of the base of the skull, and. 

 consequently in this situation extravasations between 

 the membrane and the bone are scarcely possible. Over 

 the vault its attachments are comparatively loose, 

 although it is more closely adherent along the lines of 

 the sutures. This lax attachment allows large hsemorr- 

 hagic and purulent extravasations to collect between 

 the dura mater and the bone. Such extravasations 

 usually lead to compression of the brain, and it may be 

 noted that in the great majority of all cases of com- 

 pression the compressing force is outside the dura 

 mater. Thus, in uncomplicated cases, when symptoms 

 of compression come on at the time of an accident, the 

 cause is probably depressed bone ; when they appear 

 after a short interval, the cause is probably extrava- 

 sated blood between the membrane and the bone ; and 

 when a long interval (days or weeks) has elapsed after 

 the accident, the cause is probably a collection of pus 

 in the same situation. 



When blood is poured out between the dura mater 

 and the bone in cases of fracture, the vessel that, as a 

 rule, gives way, is the middle meningeal artery. In 

 thirty-one cases of such haemorrhage, this vessel was 



