254 THE CIRCULATION OF THE BLOOD 



they are not compressible by any force which the fullness of the arteries 

 might exercise through the substance of the brain. Nor do they admit of 

 distention when the flow of venous blood from the brain is obstructed. 



The mechanical conditions in the brain and skull formerly appeared 

 enough to justify the opinion that the quantity of blood in the brain must 



FIG. 208. Showing the Origin and Course of the Vaso -constrictor Nerves for the 

 Head. M, medulla; C 8 , eighth cervical spinal nerve; V, vagus, S.c.g., superior cervical 

 ganglion. Modified from Moret. 



be at all times the same. But it was found that in animals bled to death 

 without any aperture being made in the cranium, the brain became pale and 

 anemic like other parts. And in death from strangling or drowning, there 

 was congestion of the cerebral vessels; while in death by prussic acid, the 

 quantity of blood in the cavity of the cranium was determined by the position 

 in which the animal was placed after death, the cerebral vessels being con- 

 gested when the animal was suspended with its head downward, and com- 

 paratively empty when the animal was kept suspended by the ears. Thus, 

 although the total volume of the contents of the cranium is probably nearly 

 always the same, yet the quantity of blood in it is liable to variation, its in- 

 crease or diminution being accompained by a simultaneous diminution or 

 increase in the quantity of the cerebro-spinal fluid. The cerebro-spinal 

 fluid being readily removed from one part of the brain and spinal cord to 



