476 DISEASES OF THE BRAIN. 



is compressible or not, whether, that is, it be or be not 

 reducible by pressure into a smaller compass, it is clearly 

 capable of having different degrees of pressure applied to 

 it, and of being pressed out of its ordinary form. We shall 

 see hereafter, that by pressure exercised from within by the 

 distension of what are called the ventricles of the brain, the 

 convolutions on its surface are sometimes flattened, and the 

 natural furrows between them nearly effaced. Pressure 

 there certainly is, in what I shall have to describe to you 

 as hypertrophy of the brain. There must be considerable 

 pressure on the nervous pulp when blood is poured out 

 within it from a ruptured artery in cerebral haemorrhages 

 But the phenomena noticeable when a portion of the skull 

 has been removed by the trephine, show very clearly that 

 the encephalon sustains pressure from varying states of 

 the circulation during perfect health. The surface of the 

 brain, seen through the circular opening in the bone, is 

 observed to pulsate, and to pulsate with a twofold motion. 

 With every systole of the heart the surface protrudes a little, 

 and it again subsides with the succeeding diastole. This 

 shows that the tension of the arteries produced by every 

 contraction of the ventricles of the heart, exerts a degree of 

 pressure upon the contents of the cranium. But the brain 

 has also an alternate movement corresponding with the 

 movements of the thorax in breathing, rising with every act 

 of expiration, and sinking with every act of inspiration. 

 Now, during expiration, the blood escapes less freely from 

 the head through the veins, arid thus again vascular fulness 

 is found connected with evidence of pressure on the parts 

 within the head." 



The lower animals suffer severely at different times from 

 modifications in the amount of blood in the cranium, and 

 greater or less pressure on the brain substance. In the horse 



