STRUCTURE AND FUNCTIONS OF THE CEREBRUM 191 



are very dependent upon their blood supply. A slight diminution 

 in the rate of blood flow through the brain may depress the cor- 

 tical cells to such an extent that consciousness is lost. The prob- 

 lem of retaining consciousness is, then, the problem of keeping 

 the cerebral circulation up to the proper level. How this is ac- 

 complished during our waking hours, and how its falling off af- 

 fords opportunity' for needed intervals of sleep will be discussed 

 in connection with the circulation of the blood (Chap. XXII). 



