16 
POPULAR SCIENCE REVIEW. 
tion, &c. But these three kinds are so intimately connected in 
the animal economy that one of them may, in certain cases, 
react upon the others, so that all three are capable of giving 
rise to pain ; the first directly, and the others indirectly, by 
acting through the first. But happily for us, the study of 
nature has placed in our possession several means of allaying 
pain ; it has procured us a series of substances which act upon 
the nervous system so as to deprive it of its faculty of produc- 
ing pain, sometimes partially, and sometimes most completely, 
without interfering in any great degree with the other func- 
tions of the body. 
Some of these means of allaying pain consist in producing 
sleep, either perfect or partial. The physiology of sleep is 
little understood, and, indeed, next to life and death, sleep is 
perhaps the most inexplicable phenomenon inherent to our 
nature. At certain intervals, we feel an irresistible desire to 
repose, and sleep comes on gradually. Its influence is felt by 
certain portions of the nervous system ; other portions never 
sleep. The nerves of organic life continue their functions dur- 
ing sleep, the heart beats, circulation goes on, respiration and 
digestion continue, but the nerves of motion and of sensation have 
momentarily lost their power ; consciousness is gone, voluntary 
motion disappears, hunger and thirst are absent, and so like- 
wise is pain. Such is complete or perfect sleep. But it happens 
sometimes that sleep is partial or imperfect, and then some 
curious phenomena are observed. Intelligence may be more 
or less active, and sensation may be possible to a greater or 
less degree, but never to so great an extent as hi the waking 
state. Generally, in imperfect sleep, consciousness is only 
partial, volition incomplete, and sensation dull. This state of 
incomplete sleep gives rise to dreams and to somnambulism. The 
former are odd manifestations of the intelligence no longer 
completely conscious. The latter may be either natural or 
artificial (mesmerism) . 
In somnambulism the intelligence is more active than in an 
ordinary dream ; the nervous system, only partially subdued 
by sleep, is capable of bringing into action the senses and 
motion, but the faculty of feeling pain is almost, if not entirely 
absent, and moreover the somnambulist has no consciousness of 
danger. Hence natural somnambulism, which varies in intensity 
from simple sleep-talking to the most remarkable physical and 
intellectual feats, is a dangerous condition, inasmuch as we are 
no longer able to take care of ourselves. However, somnam- 
bulists have frequently avoided dangers in the most astonishing 
manner. 
Artificial somnambulism, or mesmerism, as it is sometimes 
