ON THE FUNCTIONS OF THE BRAIN. 69 



the separation of a liquid, to the formation of which the blood must 

 more or less give its aid ; it rather expresses a phenomenon generally 

 remarked in all the organs, whatever the nature of their function may- 

 be. The muscular system, which produces nothing but mechanical 

 work, is in this regard like the glands, which act chemically. At the 

 instant of muscular action the blood circulates with greater activity, 

 which relaxes when the organ begins to rest. The peripheral nervous 

 system, the spinal marrow, and the brain, which serve to manifest the 

 phenomena of innervation and intelligence, are equally subject to this 

 law, as we are about to see. 



The relations existing between the phenomena of circulation in the 

 brain and the functional activity of that organ have long remained 

 obscure, owing to mistaken ideas of the conditions of sleep, which is 

 rightly considered the state of rest of the cerebral organ. The ancients 

 suppose! that sleep resulted from compression exerted on the brain by 

 the blood when its circulation declined. They imagined that this press- 

 ure was chiefly exerted at the back part of the head, at the point 

 where the veined folds of the dura mater unite in a common confluent, 

 which is still called the torcular or compress of Herophilus, from the 

 name of the anatomist who first described it. These conjectural ex- 

 planations have been handed down to us ; and it is only of late years 

 that experiment has succeeded in proving their falsity. In fact, it has 

 been shown by direct experiment that, during sleep, the brain, instead 

 of being congested, is on the contrary pale and bloodless ; while in a 

 state of wakefulness the circulation, becoming more active, provokes a 

 flow of blood proportioned to the intensity of cerebral activity. In this 

 respect natural sleep and the anaesthetic sleep of chloroform are alike ; 

 in both cases, the brain, sunk into rest or inactivity, presents the same 

 paleness and relative bloodlessness. 



The experiment is made in this manner : A part of the bony cover- 

 ing of an animal's skull is carefully removed, and the brain laid bare 

 so as to study the circulation at the surface of this organ. Then chloro- 

 form is administered to produce insensibility. In the first exciting stage 

 of the action of the chloroform, the brain is observed to grow con- 

 gested and to lap over at the edges ; but as soon as the stage of anaes- 

 thetic sleep is reached, the substance of the brain sinks in and grows 

 paler, presenting a languid movement of capillary circulation, which 

 lasts as long as the state of sleep or cerebral rest continues. For the 

 study of the brain in natural sleep a circular trepan is made on a dog's 

 head, and the piece of bone removed is replaced by a watch-glass care- 

 fully adjusted to the exact opening, so as to prevent the irritating 

 action of the air. The animals subjected to the operation survive it; 

 and observations on their brain through this sort of window, while 

 awake and when asleep, prove that when the dog is asleep the brain is 

 always paler, and that a fresh afflux of blood is regularly noticed on 

 his awaking, when the functions of the brain resume their activity. 



