66 
POPULAE SCIENCE EEVIEW . 
have the representation of the same thing in the restlessness of 
the muscles, the half-conscious wakings, the dreams. 
Suppose this idea of the change of nervous matter to he 
true, is there any clue to the nature of the change itself? I 
think there is. The change is one very closely resembling 
that which occurs in the solidification of water surcharged with 
a saline substance, or in water holding a hydrated colloid, like 
dialysed silica, in trembling suspension. What is, indeed, the 
brain and nervous matter ? It is a mass of water made suffici- 
ently solid to he reduced into shape and form, by rather less 
than twenty per cent, of solid matter, consisting of albuminous 
substance, saline substance, fatty substance. The mechanism 
for the supply of blood is most delicate, membranous ; the 
mechanism for dialysis or separation of crystalloidal from col- 
loidal substance is perfect, and the conversion of the compound 
substance of brain from one condition of matter to another is, 
if we may judge from some changes of water charged with 
colloidal or fatty substances, extremely simple. I do not now 
venture on details respecting this peculiarly interesting ques- 
tion, but I venture so far as to express what I feel will one 
day be the accepted fact, that the matter of the wakeful brain 
is, on going to sleep, changed, temporarily, into a state of 
greater solidity ; that its molecular parts cease to be moved 
by external ordinary influences, by chemical influences ; that 
they, in turn, cease to communicate impressions, or, in other 
words, to stimulate the voluntary muscles; and that then there 
is sleep which lasts until there is re-solution of structure, 
whereupon there is wakefulness from renewed motion in brain 
matter and renewed stimulation of voluntary muscle, through 
nerve. 
The change of structure of the brain which I assume to be 
the proximate cause of sleep is possibly the same change as 
occurs in a more extreme degree when the brain and its subor- 
dinate parts actually die. The effects of a concussion of the 
brain from a blow, the effects of a simple puncture of nervous 
matter in centres essential to life — as the point in the medulla 
oblongata which Fluorens has designated the vital point — have 
never been explained, and admit, I imagine, of no explanation 
except the change of structure I have now ventured to suggest. 
Here, for the moment, my task must end. My object has 
been to make the scientific reader conversant with what has 
been said by philosophers upon the subject of sleep and its 
proximate cause, and to indicate briefly a new line of scientific 
enquiry. I shall hope on some future occasion to be able to 
announce further and more fruitful labour. 
