MAURY ON SLEEP AND DREAMS. 415 



peculiar distribution of the arterial and venous system 

 in the medullary and cineritious substance, in the 

 membranes and sinuses of this organ, have embarrassed 

 hitherto every question on the subject. It has been 

 the most general opinion of physiologists that a certain 

 amount of pressure on the brain, chiefly from conges- 

 tion of venous blood, was necessary for the state of 

 sleep. More recently, this opinion has been modified, 

 if not contradicted, by the experiments of Mr. Durham, 

 Dr. Hammond, and others ; furnishing evidence that 

 sleep depends on a lessened quantity and force of blood 

 in the brain, and especially in the arterial part of the 

 cerebral circulation. Though this inference is fortified 

 by various known facts, such as the sleep produced by 

 exposure to intense cold, by loss of blood, by pain, and 

 other causes of vital exhaustion, it still leaves the phy- 

 sical theory an ambiguous one ; embarrassed by our 

 ignorance of the relative proportions of arterial and 

 venous blood during sleep by questions as to the 

 mode of action of the vascular portion of the brain 

 upon the medullary and other cerebral tissues and by 

 a further question, of higher interest but harder of 

 solution, viz., the nature of those changes in the cere- 

 bral substance itself through which dreams, and other 

 concomitant phenomena of sleep, have their origin. 



The latter question involves difficulties which, with 

 all just regard to the prowess and high attainments of 

 modern science, we must yet believe to be insuperable. 

 It is in truth the selfsame problem as that put before 

 us by the normal and waking state of our sensorial 

 existence. The dream of the night is connected with 



