PARTIAL ANEMIA OF THE BRAIN. 203 



the death of the tissue is due to abstraction of the supply of nutritive 

 material ; but the necrosed parts within the skull, not being exposed 

 to the action of the atmosphere, are rarely decomposed. This only 

 happens when the embolus, that has stopped the vessel, comes from a 

 suppurating spot, and transfers with it a tendency to suppuration. 

 Closure of a vessel induces necrosis more readily the later and more 

 incompletely a collateral circulation is established. If the degenera- 

 tion of the walls of the vessels that has induced a thrombosis of the 

 cerebral arteries be extensive, the collateral branches, the elasticity 

 of whose walls is diminished, cannot dilate sufficiently to supply the 

 place of any large arteries that may be closed, hence the partial anae- 

 mia is only partly removed, and the anaemic part softens. Whether 

 or not closure of a cerebral artery by an embolus shall induce necrotic 

 softening depends chiefly on the seat of the obstruction, since in such 

 cases the walls of the vessels are usually healthy and distensible. If, 

 as rarely happens, there be anaemia in the parts chiefly supplied by a 

 vessel which has been obstructed by an embolus before reaching the 

 circle of Willis, it will generally pass off soon, and no necrosis results ; 

 if, on the other hand, a vessel be closed by an embolus beyond the 

 circle of Willis, necrosis is the usual termination of the partial ana> 

 mia. 



The second form of partial anaemia of the brain, due to develop- 

 ment of collateral oedema around an apoplexy, or a point of inflam- 

 mation or softening, or a tumor, etc., has already been mentioned as 

 the not unfrequent termination of excessive fluxionary hyperaemia. 

 When speaking of the different diseases of the brain, we shall fre- 

 quently recur to this form. 



The third division that we have made of partial hyperaemia of the 

 brain, resulting from compression of the capillaries in certain sections 

 of the brain by extravasations of blood, tumors, and other diseases 

 contracting the space in the skull, has hitherto been too little appre- 

 ciated. Along with the best authorities, I myself denied the occur- 

 rence of " partial cerebral pressure." From the fact that the brain is 

 incompressible and is enclosed by an inelastic capsule, I reasoned 

 that an increased pressure, acting on any part of the brain, would 

 spread evenly over the entire organ. In support of this view, I ad- 

 vanced the popular illustration that, if a cork be driven forcibly into a 

 bottle, the latter is not by any means always broken at the neck, but 

 just as often at some distance from the point where the force was ap- 

 plied, perhaps at some particularly weak part. However, a series of 

 observations where there was no doubt that those portions of the 

 brain where the disease was located were far more bloodless than the 

 rest of the brain, and a careful consideration of all circumstances af- 



