3 1 4 THE GROWTH OF THE BRAIN*. 



some reflexes are still easily aroused even in this state, 

 while others, like the knee-kick, are lost. 



The waste products of activity appear as the prime 

 cause of sleep, and on them is probably dependent the 

 distribution of the blood which has been observed 

 during this condition. The effective blood supply of 

 the nerve centres is increased in the first stages of 

 activity, and diminishes in fatigue (Mosso). It is 

 diminished during sleep also. Any stimulus acting on 

 the sensory nerves during sleep tends to cause a with- 

 drawal of blood from the limbs, and its return to the 

 head as shown in the observations of Bardeen and 

 Nichols previously quoted. 



Sleep is more easily induced, the fewer the stimuli 

 that act upon us. Not only, then, the condition of the 

 afferent nerves, but also the responsiveness of the 

 central cells toward the impulses that fall upon them, 

 are to be taken into account. In common practice we 

 reduce stimuli to a minimum as a preliminary to sleep. 

 Striimpell * has reported an interesting case of a lad of 

 low intelligence, who by disease had been reduced to 

 one ear and one eye as the sole avenues by which he 

 received external stimuli. When the ear was plugged, 

 and the eye bandaged, he fell asleep with the regularity 

 of a machine. 



The parts played by the sensory and that by the 

 central cells vary somewhat at different times of life. 

 In childhood the amount of stored material .is small, 

 large at maturity, and small again in old age. Hence 

 the cells would, by reason of this fact, have the greatest 

 capability for work in the middle period. Between child- 

 hood and old age there is, however, this difference, that 

 while in the former the non-available substances in the 

 cell are developing, not yet having matured, those in 

 1 Striimpell, Deutsch. Arch. f. Klin. Med., 1864. 



