DISEASES OF THE BRAIN. 



fecting the space in the skull, have shown me that, in the above rea 

 soning, I overlooked an important fact and hence came to a false conclu 

 sion. Indeed, in spite of the brain being incompressible, and being 

 surrounded by an unyielding capsule, partial pressure on it may occur } 

 this is so, because the cranial cavity is divided into three chambers by 

 two tensely-stretched membranes, because the falx and tentorium pro- 

 tect, to some extent at least, parts of the brain lying on one side of 

 them from pressure acting on the other side. 



The three chambers of the skull communicate with one another, it 

 is true, and, if the cerebral substance were liquid, pressure on any part 

 of it would affect the whole organ equally, in spite of the tense septa 

 traversing it. But the consistence and tenacity of the cerebral sub- 

 stance, which only permit a slight protrusion of a section of brain 

 pressed from an opposite chamber of the skull toward the excavation 

 of the tentorium or the lower border of the falx, cause portions of 

 the brain lying in one chamber to be protected to some extent from 

 pressure on those lying in one of the other chambers, in spite of the 

 openings in the septa, particularly the large opening in the falx. The 

 protection afforded by the tentorium is greater than that given by the 

 falx, and the posterior lobes of the cerebral hemispheres are far better 

 protected against a pressure acting on the opposite hemisphere than 

 the frontal lobes are, because the falx is much broader posteriorly, and 

 hangs much farther down than it does anteriorly. In subsequent chap- 

 ters we shall frequently refer to these circumstances also, whose great 

 significance in the explanation of the symptoms of disease encroaching 

 on the cranial cavity I was, to the best of my knowledge, the first to 

 point out. 



ANATOMICAL APPEARANCES. Partial anaemia of the brain cannot 

 by any means always be definitely made out in the dead body. The 

 distribution of blood after death and during life is not the same; 

 places which, during life, were distinguished by their vascularity from 

 other less vascular parts, often become just as bloodless as the latter 

 after death. In the cutis, where we have an opportunity for compari- 

 son, we may see this daily ; in the brain the state of affairs is just the 

 same. It is remarkable that emboli are almost always found in the left 

 arteria fossae Sylvii. Perhaps this is partly because the left carotid ar- 

 tery leaves the arch of the aorta almost in the direction of the current of 

 blood, while the innominate forms a considerable angle with it (Rtthle). 

 The necrosis resulting from obstruction of the vessels and insuffi 

 cient development of collateral circulation induces relaxation and soft- 

 ening of the brain-substance; hence necrosis caused by anasmia is 

 designated as a peculiar form of softening of the brain, as " simple " or 

 " yellow " softening. Corresponding to the most frequent and obsti- 



