CONDITION OF THE BRAIN, ETC., DURING SLEEP. 747 



tional activity and during its state of repose or sleep was most remarkable." There 

 can be hardly any doubt, from these experiments, that the circulation in the cerebral 

 substance is more active when we are awake than during sleep ; but the question has 

 been raised by Dr. Cappie, in a very interesting little work upon the causation of sleep, 

 whether, during a state of diminished activity of the capillary circulation in the brain- 

 substance, the veins be not congested, and sleep be immediately due to pressure from 

 these distended vessels on the gray matter. This point is one very difficult to decide, 

 and it has not been made the subject of experimental inquiry. Dr. Cappie accepts, in 

 the main, the experiments of Durham as accurate, but he regards his observations as ap- 

 plying only to the circulation in the arteries and capillaries. His view is that, when the 

 capillary circulation in the brain-substance is diminished in sleep, the nervous matter is 

 more or less collapsed, and that the veins are necessarily congested. At present, how- 

 ever, we can only accept the experimental results of Durham, that the circulation in the 

 brain is notably diminished in sleep. . . 



The influence of diminished supply of blood to the brain has been Illustrated by com- 

 pression of both carotid arteries. In an experiment performed upon his own person, Dr. 

 Fleming produced immediate and profound sleep in this way, and this result invariably 

 followed in subsequent trials upon himself and others. We have, however, the observa- 

 tions of Waller, who produced anesthesia in patients by pressure upon both pneumogas- 

 tric nerves ; but the nerves are so near the carotid arteries that they could hardly be 

 compressed, in the human subject, without interfering with the current of blood, and 

 such experiments do not positively show whether the loss of sensibility be due to 

 pressure upon the nerves or upon the vessels. In some rare cases, in which both carotid 

 arteries have been ligatured in the human subject, it has been stated that there is an 

 unusual drowsiness following the necessary diminution in the activity of the cerebral 

 circulation ; but this result is by no means constant, and the morbid conditions in- 

 volving so serious an operation are usually such as to interfere with their value as facts 

 bearing upon the question under consideration. As far as the human subject is concerned, 

 the most important facts are the results of compression of both carotids in healthy per- 

 sons. These, as well as experiments on animals, all go to show that the supply of blood 

 to the brain is very much diminished during natural sleep, and that sleep may be induced 

 by retarding the cerebral circulation by compressing the vessels of supply. When the 

 circulation is interfered with by compressing the veins, congestion is the result, and we 

 have stupor or coma. 



If diminished flow of blood through the cerebral vessels be the cause of natural sleep, 

 it becomes important to inquire how this condition of physiological anasmia is brought 

 about. It must be that, when the system requires sleep, the vessels of the brain contract 

 in obedience to a stimulus received through the sympathetic system of nerves, diminish- 

 ing the supply of blood, here, as in other parts under varied physiological conditions. 

 The vessels of the brain are provided with vaso-motor nerves, and it is sufficient to have 

 noted that the arteries are contracted during sleep, the mechanism of this action being 

 well established by observations upon other parts of the circulatory system. Contraction 

 of the vessels of the pia mater has been observed, although there is some discussion with 

 regard to its exciting cause. 



It must be acknowledged that we know but little of the intimate nature of the pro- 

 cssses of nutrition of the brain during its functional activity and in repose; but there 

 can be no doubt of the fact that there is more or less cerebral action at all times when 

 we are awake. Although the mental processes are much less active during sleep, even at 

 this time, the operations of the brain are not always suspended. It is equally well estab- 

 lished, that exercise of the brain is attended with physiological waste of nervous sub- 

 stance, and, like other parts of the organism, its tissue requires periodic repose to allow 

 of the regeneration of the substance consumed. Analogies to this are to be found in 

 parts that are more easily subjected to direct observation. The muscles require repose 



