Chap, in.] CRANIAL CONTENTS. 31 



carotid artery and cavernous sinus are so intimate 

 that arterio-venous aneurism has followed injury in- 

 volving these parts. It will be seen also with what 

 ease this sinus could become thrombosed in cases of 

 inflammation within the orbit by the extension of the 

 mischief along its great tributaries, the two ophthalmic 

 veins. 



Sarcomatous growths, springing from the dura 

 mater or other of the meninges, may make their way 

 through the cranial bones, and project as pulsating 

 tumours beneath the scalp. Such growths are included 

 under the title of " fungus of the dura mater." 



Between the dura mater and the arachnoid is the 

 subdural space, formerly known as the " cavity of 

 the arachnoid." According to Prescott Hewett, ex- 

 travasations of blood after injury are more common 

 in this space than in any other part within the skull. 

 The blood so poured out may become more or less 

 discoloured, and present in time the aspect of a thin 

 and peculiar membrane ; or the collection may form 

 itself into a species of cyst (" arachnoid cyst ") that 

 was for some time a source of much confusion to 

 pathologists. 



The subdural space contains a small amount of 

 fluid, and acts, probably, like the pleural and peri- 

 toneal sacs in preventing the effects of friction during 

 the movements of the brain. 



The sufoarachnoid space is between the arach- 

 noid and the pia mater, and it is here that the chief part 

 of the cerebro-spinal fluid is lodged. This space is larger 

 in some places than in others. It is insignificant over 

 the convexity of the brain, but is very extensive at 

 the base of the skull in the parts beneath the cere- 

 bellum, the medulla, the pons, and the interpeduncular 

 space as far forwards as the optic nerves. Thus these 

 very important parts of the brain do not rest upon 

 bone, but rest rather on the subarachnoid collection 



