THE CEREBRUM. 563 



tion of functional activity, and the establishment of the condition of fatigue. 

 The irritability of the sense organs, especially of the eyes and ears, in all 

 probability declines in a similar manner. These structures pass into the 

 condition of fatigue and become less responsive to external stimuli. The 

 results of all these conditions is a less active stimulation of the brain cells, 

 which in connection with other factors predisposes to 



Brain Repose or Sleep. Brain repose or sleep is characterized by a 

 greater or less degree of unconsciousness, the non-development of sensations, 

 ideas, feelings and volitional acts, and is the result of a diminution in the 

 physiologic activities of the body at large and more especially of the brain, 

 sense organs, and spinal cord. Coincident with the cessation of brain activity 

 and the onset of sleep, there is a diminution in the rate and force of the heart- 

 beat, and in the frequency and depth of the respiratory movements, and a 

 relaxation of the skeletal muscles, especially those employed in voluntary 

 movements. 



The sense organs are in part protected from the action of external stimuli. 

 The eyeball is so turned that its anterior pole is directed far upward under 

 the eyelid, while the pupil is markedly diminished in size, and in consequence 

 the entrance of light largely prevented. The ear is protected against the 

 reception of sounds of ordinary pitch by an increased tension of the tympanic 

 membrane. The nose and mouth are less responsive to various stimuli 

 because of the dryness of their mucous membranes from diminished secre- 

 tion. The skin appears to be less sensitive to mechanic pressure and other 

 forms of stimulation. 



In addition to the foregoing phenomena, experimental investigations 

 have shown that there is a shunting of a portion of the blood stream from 

 the brain to other regions of the body, especially to the skin and perhaps to 

 the abdominal viscera as well, whereby it becomes incapable of functionating 

 physiologically. The fact that the brain receives a lessened quantity of 

 blood during sleep has been shown by trephining the skull and inserting in 

 the orifice a glass plate through which the circulatory conditions of the brain 

 can be observed. In the waking condition the blood-vessels on the surface 

 of the brain are prominent, and turgid with blood and the whole organ 

 completely fills the cranial cavity, indicating that the blood-vessels in the 

 interior of the brain are in a similar condition. With the onset of sleep the 

 larger blood-vessels begin to diminish in size, the smaller vessels disappear 

 from view, the brain tissues become pale and the volume of the brain shrinks. 

 During the continuance of deep sleep, this anemic condition persists. As 

 the period of sleep approaches its termination, the smaller blood-vessels 

 again fill with blood, the surface of the brain flushes, and in a very short 

 time the former circulatory conditions return, the volume of the brain 

 increases and the waking state is reestablished. 



The fact that the skin receives an increased volume of blood during sleep, 

 has been shown by inserting an arm or leg in a plethysmograph by which 

 means a record of any change in volume can be obtained. Howell thus 

 succeeded in obtaining graphic records in the variations of the volume of the 

 arm during sleep. These records disclosed the fact that with the onset of 

 sleep the volume of the arm gradually increased in size until it attained a 

 maximum which was from one to two hours after the beginning of sleep. 



