CIRCULATION IN THE CRANIUM. 333 



sition to such a view are so numerous and well established, 

 that the question does not demand extended discussion. It 

 is well known, that in certain cases the vessels of the brain 

 and its membranes are found engorged with blood, and in 

 others containing a comparatively small quantity ; but it is 

 nevertheless true that there are anatomical peculiarities in 

 these parts, the effects of which on the circulation present 

 important and interesting points for study. 



In the brain, the venous passages which correspond to the 

 great veins of other parts, are sinuses between the folds of 

 the dura mater, and are but slightly dilatable. In the per- 

 fectly consolidated adult head, the blood is not subjected to 

 atmospheric pressure as in other parts, and the semi-solids 

 and liquids which compose the encephalic mass cannot in- 

 crease in size in congestion, and diminish in anemia. Not- 

 withstanding these conditions, the undoubted fact remains 

 that examinations of the vessels of the brain after death show 

 great differences in the quantity of blood which they contain. 

 The question then arises as to what is displaced to make 

 room for the blood in congestion, and what supplies the 

 place of the blood in anemia. 



An anatomical peculiarity, which has not yet been con- 

 sidered, offers an explanation of these phenomena. Magen- 

 die has shown by observations on living animals, confirmed 

 by dissections of the-human body, that between the pia mater 

 and the arachnoid of the brain and spinal cord there exists a 



necessary to enumerate. These experiments were fully reviewed by Dr. George 

 Burrows, who shows by his quotations from Dr. Kellie that they proved nothing 

 of the kind. Dr. B. repeated the experiments on rabbits, and demonstrated that 

 great variations exist in the quantity of blood in the brain, when the animals are 

 killed in different ways. He showed that the blood-vessels are engorged when the 

 head is left dependent for a number of hours, and that they contain but little 

 blood when it is elevated. Certain of Kellie's experiments, cited by Dr. Burrows, 

 show that the difference is in the conclusions, and not in the experimental 

 facts. For a full discussion of this subject, the reader is referred to the work 

 of Dr. Burrows on Disorders of the Cerebral Circulation, d'c. (American reprint), 

 Philadelphia, 1848. 



