chap, in.] CRANIAL CONTENTS. 33 



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. 

 If 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 Majendie, 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. 



The relations of the brain to the skull. 

 The lower level of the brain in front corresponds to a 

 line drawn across the forehead just above the eye- 

 brows. At the side of the head it corresponds 

 approximately to a line drawn from the external 

 angular process of the frontal to the upper part of the 

 external auditory meatus. A line drawn from this 

 latter spot to the occipital protuberance corresponds 

 to the lower level of the posterior lobe, while below 

 that line will lie the cerebellum (Fig. 7). The com- 

 mencement of the Sylvian fissure corresponds to the 

 pterion. Its ascending limb is nearly parallel to, and 



