chap, in.] THE CRANIAL CONTENTS. 27 



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 subaraclmoicl space is between the arach- 

 noid and the pia rnater, 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 

 of fluid as upon a water-bed, to use a comparison of 

 Mr. Hilton's. The only part of the base of the brain 

 that rests directly upon bone is that part in contact 

 with the orbital plates and lesser wings of the sphe- 

 noid. The posterior two-thirds of the brain rests 

 upon the " water-bed," and is thus admirably pro- 

 tected. 



It is well known that the brain may be damaged 

 by centre-coup. That is to say, if the head be struck 

 at one particular part, the brain may be found unin- 

 jured at the spot struck, but damaged at a correspond- 

 ing place on the opposite side of the skull. The 

 cerebro-spinal fluid is much concerned in modifying 

 the effects of contre-coup. Thus, if the vertex be 

 struck, the important structures at the base of the 

 brain would be in the greatest danger of being 

 severely injured, did they lie in actual contact with 

 the bone. Moreover, when a blow falls upon the 

 occiput, there may be no damage to the brain beneath 

 the spot struck, but the yielding mass is thrown 

 forward within the skull, arid were it not for the 



