28 SURGICAL APPLIED A A ATOMY. [Chap. in. 



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

 be torn against the many projections in the base of 

 the skull. As it is, the only part that usually suffers 

 is the under surface of the frontal lobe (a compara- 

 tively unimportant segment), which, being brought 

 violently in contact with the irregular orbital plate, is 

 readily contused. The subarachnoid space communi- 

 cates with the ventricles through the foramen of 

 Magendie. 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 

 from thence to the lateral ventricles by the foramen of 

 Monro. 



The cerebro-spinal fluid prevents the ill effects that 

 irregularities in the blood circulation might have upon 

 the brain, situate as it is within an unyielding cavity. 

 Jf the great nerve-centres in the lateral ventricles 

 are swollen by congestion, they are not met by an un- 

 yielding wall, but merely displace some of the cerebro- 

 spinal fluid through the foramen of Magendie, until 

 such time as their circulation is normal again. 



Hilton has shown that closure of this foramen 

 may lead to that excessive accumulation of fluid 

 within the ventricles known as hydrocephalus. If the 

 brain, too, becomes enlarged by congestion, it is not 

 met by unyielding bone, but rather by an adjustable 

 water-bed, and during its period of enlargement it 

 merely displaces into the spinal part of the sub- 

 arachnoid space some of the fluid that surrounds it. 

 This mutual effect is well illustrated in a case reported 

 by Hilton of a man with a fracture of the base, from 

 whose ear cerebro-spinal fluid was escaping. The 

 discharge of this fluid was at once greatly in- 

 creased by expiratory efforts when the nose and 

 mouth were held closed, and the veins compressed 

 in the neck. 



