614 TEXT-BOOK OF PHYSIOLOGY 



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. 

 After this period the volume remains practically the same for several hours, 

 diminishing as the intensity of sleep diminishes and the waking state is 

 approached. Just previous to the return of consciousness there is a rapid 

 diminution in the volume of the arm. If it be accepted that the enlargement 

 of the cutaneous vessels is followed by a diminution in size of the cerebral 

 vessels, it follows that the former condition stands to the latter in the relation 

 of cause and effect, whereby a portion of the blood is diverted from the brain 

 to the skin. It also naturally follows that the withdrawal of the blood from 

 the brain to the skin and possibly other regions as well, is the fundamental 

 condition for brain repose. 



The Intensity of Sleep. Observations of individuals during sleep 

 show that the intensity or the depth of sleep varies from hour to hour. 

 Attempts have been made to estimate the intensity by measuring the 

 loudness of a sound caused in several ways that is necessary to awaken the 

 sleeper. Accepting this criterion it may be stated from the results of many 

 experiments, that sleep increases in. intensity or depth and reaches its maxi- 

 mum between the first and second hours, after which it rapidly decreases un- 

 til the end of the third hour, when consciousness is so nearly restored, that 

 but a very slight stimulus is required to awaken the sleeper. It is during 

 the latter period when the brain is reviving that dreams arise, the elements 

 of which are formed of previous sensations. 



The Causes of Sleep. Different theories have been proposed to account 

 for the causes of sleep, none of which have been wholly satisfactory. From 



