THE VASOMOTOR MECHANISM 379 



therefore practically constant, and the circulation through the 

 brain can only be altered by changing the rate at which the blood 

 flows through it. In such an arrangement as this, where local 

 vasodilation cannot occur, the only way in which the rate of 

 blood-flow can be altered is by changes in the pressure at which 

 the blood is forced into the region. The arteries feeding the brain 

 spring directly from the aorta; it follows, therefore, that variations 

 in aortic pressure, in other words, in general blood-pressure, are 

 reflected exactly in the rate of cerebral blood-flow. 



General blood-pressure, as we have seen, is maintained by vaso- 

 motor tone, the state of moderate constriction of arterioles gener- 

 ally. Variations in the tone of restricted areas, such as occur in 

 connection with the functioning of individual organs, do not or- 

 dinarily affect general blood-pressure enough to alter the circula- 

 tion through the brain to any extent. 



There is good evidence that the degree of activity of the cells of 

 the cerebral cortex is directly and immediately dependent upon 

 the rate of blood-flow through the organ. A rapid circulation 

 means alertness and efficiency of mental processes; as the flow 

 becomes slower and slower the cells work less and less actively; 

 when a certain point of sluggishness is reached consciousness dis- 

 appears, the ce/ls, if not altogether quiescent, working too freely 

 to arouse that state. 



The phenomenon of fainting, which has already been men- 

 tioned, is the result usually of a sudden inhibition of the vaso- 

 constrictor center whereby over a large area, the whole splanchnic 

 region, for instance, there is general vasodilation and a resulting 

 fall in blood-pressure. The rate of cerebral blood-flow falls to a 

 point below that required for the maintenance of consciousness 

 and the individual falls in a faint. 



Sleep. This periodic loss of consciousness, so important for the 

 proper restoration of the fatigued organs and tissues of the Body, 

 has been the subject of considerable attention and investigation. 

 Its explanation is not simple, involving as it does a number of 

 questions, as, for instance, why fatigue, which ordinarily induces 

 sleep, may, if extreme, prevent it; and what it is that causes one 

 to awake after the proper number of hours of sleep. 



Objectively sleep is marked by its well-known signs, which are 

 not very instructive as to its cause, and also by certain vaso- 



