32 SUXGTCAL APPLIED ANATOMY. [Chap. m. 



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 contre-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 clanger 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, and were it not for the 

 " water-bed," the under parts of the cerebrum would 

 be torn against the many projections in the base of 

 the skull. . These projections, which are so conspicuous 

 in the dry skull, are moulded off in the living by the 

 dura mater, and also by the blood sinuses which occupy 

 the hollows about these projections, and also certain 

 margins of bone. As it is, the only part that usually 

 suffers is the under surface of the frontal lobe (a com- 

 paratively unimportant segment), which, being brought 

 violently in contact with the irregular orbital plate, is 

 readily contused. The subarachnoid space commu- 

 nicates with the ventricles through the foramen of 

 Majendie. This foramen leads into the fourth ven- 

 tricle, and is an aperture in the pia mater that closes 

 in that ventricle. From the fourth ventricle fluid 

 can pass along the iter into the third ventricle, and 



