THE BRAIN. 527 
nent than usual; and it is well known that means calculated to 
diminish the circulation in the brain, as cold and pressure, favor 
sleep. It is also well established by general experience that 
withdrawal of the usual afferent impulses through the various 
senses favors sleep. A remarkable case is on record of a youth 
whose avenues for sensory impressions were limited to one eye 
and a single ear, and who could be sent to sleep by closing 
these against the outer world. Yet this subject after a long 
sleep would awake of his own accord, showing that, while affer- 
ent impulses have undoubtedly much to do with maintaining 
the activity of the cerebral centers, yet their automaticity (in- 
dependence) must also be recognized. 
It is a matter of common experience that weariness, or the 
exhaustion following on pain, mental anxiety, etc., is favor- 
able to sleep. 
A good deal of light is thrown on this subject by hiberna- 
tion, particularly in mammals. 
From special study of the subject we have ourselves learned 
that, however temperature and certain other conditions may 
influence this state, it will appear at definite periods in de- 
fiance, to a large extent, of the conditions prevailing. Hiber- 
nation, we are convinced, is marked by a general slowing of all 
of the vital processes in which the nervous system takes a 
prominent part. Sleep and hibernation are closely related. In 
both there is a diminution of the rate of the vital processes, as 
shown by the income and output, measured by chemical stand- 
ards, with of course obvious physical signs, as slowed respira- 
tion, circulation, etc. While sleep, then, is primarily the re- 
sult of a rhythmical retardation of the vital processes, especially 
within the nervous system, it is like hibernation in some de- 
gree (in the lowest creatures, without a nervous system) the 
outcome of that rhythm impressed on every cell of the organ- 
ism and the influence of which is felt in a thousand ways, that 
no doubt we are quite unable to recognize. 
Dreaming is a partial activity of the mind, corresponding 
doubtless to functional wakefulness or relatively increased ac- 
tion of some limited part or parts of the brain. It is now all but 
certain that these parts are more vascular—i. e., we must reckon 
with a localized vascularity and functional activity. If this be 
recognized, almost all the peculiarities of the dreaming state 
may be understood. Dreams usually lack some elements that 
give the completeness and consistency of waking thought—a 
matter readily understood, as well as the unrest of a dreamy 
