650 NERVOUS SYSTEM. 



of the pia mater were seen moderately distended and the circulation was 

 active ; but during perfectly natural sleep, the brain retracted and became 

 pale. " The contrast between the appearance of the brain during its period 

 of functional activity and during its state of repose or sleep was most re- 

 markable." There can be hardly a.ny doubt, after these experiments, that 

 the cerebral circulation is considerably diminished in activity during sleep. 



The influence of diminished supply of blood to the brain has been illus- 

 trated by compression of both carotid arteries. In an experiment performed 

 upon his own person, Fleming produced immediate and profound sleep in 

 this way, and this result invariably followed in subsequent trials upon him- 

 self and others. Waller produced anaesthesia in patients by pressure upon 

 both piieumogastric nerves ; but the nerves are so near the carotid arteries 

 that they could hardly be compressed, in the human subject, without inter- 

 fering 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 instances in which both carotid arteries have 

 been tied 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 condi- 

 tions involving 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 persons. These, as well as experi- 

 ments on animals, all go to show that the supply of blood to the brain is 

 diminished during natural sleep, and that sleep may be induced by retarding 

 the cerebral circulation by compressing the vessels of supply. When the cir- 

 culation is interfered with by compressing the veins, congestion is the result, 

 and there is 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 physio- 

 logical anaemia 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, diminishing 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 circu- 

 latory system. 



Little is known of the intimate nature of the processes of nutrition of 

 the brain during its 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 one is 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 established that exercise of the brain is attended with physiological 

 wear of nervous tissue, and like other parts of the organism, its tissue re- 

 quires periodic repose for regeneration of the substance consumed. Analo- 



