DISEASES OF THE NEEVOUS SYSTEM. 



SECTION I. 

 DISEASES OF THE BRAIN. 



CHAPTER I. 



HYPERJ3MIA OF THE BRAIN AND ITS MEMBRANES. 



ETIOLOGY. For a time the fact was ignored that on autopsy the 

 olood-vessels within the skull were sometimes found distended and at 

 others empty, and it was supposed that the amount of blood contained 

 in the closed cranium of an adult could neither increase nor dimmish, but 

 was constant; and that anaemia or hyperaemia was only supposable 

 when the brain-substance was increased or diminished, that is, when 

 there was hypertrophy or atrophy of the brain. This view was based 

 on the following reasoning : The brain is not compressible, at least not 

 by the pressure to which it is subjected from the contents of the 

 blood-vessels ; and it is surrounded by walls which do not expand ; 

 consequently only the same amount of blood can enter the skull as 

 passes out from it, and conversely only as much blood can pass out of 

 the skull as enters it. This reasoning is false, as it starts with the 

 supposition that the contents of the cranium consist only of the mem- 

 branes of the brain, the brain-substance and the blood-vessels with 

 their contents ; it leaves the cerebrospinal fluid out of consideration. 

 This, which is a simple transudation, can rapidly increase or diminish, 

 and can at least partly pass into the spinal canal, which is not entirely 

 enclosed by rigid walls. In almost all autopsies it may be seen that 

 the amount of blood contained in the vessels and the amount of cere- 

 brospinal fluid are in inverse proportion ; that a distention of the ves- 

 sels of the menmges is accompanied by a decrease of arachnoid fluid, 

 and conversely that, when the vessels are less full, the meshes of the 



