594 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
to increase rapidly on awaking. Brush and Fayerweather observed a 
lowering of the pressure during the first five hours of sleep, followed 
by an increase up to the time of waking. 
Externally you find at the approach of sleep, effects of a slackening 
of the circulation; sometimes the surface of the body becomes con- 
gested, indicating the need of a loosening of the clothing. Mosso, then 
Fran¢ois Franck, measured this expansion of the blood vessels near 
the surface, and Howell, then Lehmann, observed that it increases 
until the second hour of sleep, then diminishes until awaking. It 
has been possible to note the conditions of the circulation in the 
brain in subjects who have lost part of the skull, or on very young 
infants whose skull bones have not yet united. The first to make 
this observation was Blumenbach in 1795. He had occasion to exam- 
ine a young man 18 years old, who before the age of 5 had fallen 
upon his forehead, sustaining a fracture of the skull, causing loss of part 
of the bone. When Blumenbach saw it the wound was healed, but 
one could feel an opening underneath; this was depressed during 
sleep, diminishing when awake, and replaced by a bump during a 
stram. Blumenbach concluded from this that there is less blood in 
the brain during sleep. Durham made these observations on some 
animals that were trepanned and chloroformed, and he noticed that 
the arteries as well as the veins were less swollen during sleep. Ham- 
mond confirmed these conclusions, and even before Durham, he had 
watched a man whose brain was laid bare by a railroad accident, 
and he saw the pressure of the brain diminish during sleep and rise 
up when the man awoke. He found the same condition in young 
infants. But the accuracy of these observations was doubted and 
was disputed by various writers, and it was only the registering 
experiments made by Mosso, Francois Franck, and Salathé that 
settled the question. Salathé held that the fontanels of young infants 
beat more strongly during sleep, indicating a diminution of pressure 
within the skull. Fran¢ois Franck, in the case of an invalid stricken 
with necrosis of the right parietal bone, observed the same move- 
ments of the brain and the same diminution of pressure. Mosso at 
last registered in several cases where the brain was accessible the 
movements which take place during sleep; the pulse, regular and 
uniform, was not so high as during wakefulness. Furthermore, 
Brodmann has added to these observations that the act af going to 
sleep is characterized by a sudden increase in the volume of the 
brain, and waking up by a diminution in its size. 
These statements concerning anemia of the brain and expansion 
of the blood vessels of the extremities during sleep has led certain 
writers to seek here an explanation for sleep. Five hundred years 
before Christ, Aleméon of Crotone said: ‘‘Sleep comes from a flowing 
back of the blood into the veins, and waking up is caused by its 
