156 



THE HUMAN MECHANISM 



circulation in this organ is seriously interfered with, imperfect 

 mental action or even unconsciousness is a result. Thus 

 when all the arterioles of the body dilate, or the heart beat 

 is slowed down, in consequence of some sudden " shock," so 



that pressure in the arterial 

 reservoir falls too far, the 

 driving force for the flow 

 of blood through the brain, 

 as well as through other or- 

 gans, is diminished, and. the 

 person loses consciousness, 

 or faints. Most cases of 

 fainting are traceable to 

 one or the other of these 

 causes. 



The most familiar and 

 most common example of 

 unconsciousness, however, 

 is that of sleep, which in 

 so many respects resembles 

 fainting as to suggest that 

 the unconsciousness in both 

 cases is due to the same 

 cause, namely a lessened 

 blood supply to the brain. 

 Unquestionably, the amount 

 of blood flowing through 

 the brain is greatly lessened 

 during sleep. The evidence 

 for this statement cannot 

 be given here in full, but it 

 is known that where accident has destroyed a part of the 

 rigid bone of the skull, and the wound has been covered 

 over by connective tissue and skin, the scar sinks in dur- 

 ing sleep indicating less blood in the brain and returns 



FIG. 75. Showing the relation between 



general arterial tone and the supply of 



blood to the brain 



In A the arterioles of the organs m, n, s 

 are constricted, raising general arterial 

 pressure, which forces a large amount of 

 blood through the brain b. In B the ar- 

 terioles of m, n, and s are dilated, general 

 arterial pressure is low, and less blood is 

 forced through the brain. //, heart 



