Page:Popular Science Monthly Volume 1.djvu/428

414 muscles. The first of these systems, consisting of the brain, the spinal cord, and the nerves of sense, sensation, and motion, is called the cerebro-spinal or voluntary system of nerves; the second, consisting of a series of nervous ganglia with nerves which communicate with the involuntary muscles and with nerves of the voluntary kind, is called, after Harvey the vegetative, after Bichat the organic system: a sketch of this organic system is depicted in the accompanying diagram.

In sleep, the cerebro-spinal system sleeps; the organic system retains its activity. Thus in sleep the voluntary muscles and parts fail to receive their nervous stimulation; but the involuntary receive theirs still, and under it move in steady motion; while the semi-voluntary organs also receive sufficient stimulation to keep them in motion.

Of all the involuntary organs, the heart, which is the citadel of motion, is most protected. To itself belongs a special nervous centre, that which feeds it steadily with stimulus for motion; from the cervical ganglia of the organic nervous system it receives a second or supplementary supply; and from the brain it receives a third supply, which, passive under ordinary circumstances, can under extraordinary circumstances become active and exert a certain controlling power. Then the arteries which supply the heart with blood are the first vessels given off from the great feeding arterial trunk, and the veins of the heart winding independently round it empty their contents direct again into it. Thus is the heart the most perfect of independencies: thus during sleep and during wakefulness it works its own course, and, taking first care of itself in every particular, feeds the rest of the body afterward; thus, even when sleep passes into death, the heart in almost every case continues its action for some time after all the other parts of the organism are in absolute quiescence; thus, in hibernating animals, the heart continues in play during their long somnolence; and, thus, under the insensibility produced by the inhalation of narcotic gases and vapors, the heart sustains its function when every other part is temporarily dead. Next the heart in independent action is the muscle called the midriff or diaphragm; and, as the diaphragm is a muscle of inspiration, the respiratory function plays second to the circulatory, and the two great functions of life are, in sleep, faithfully performed. In sleep of illness bordering on sleep of death, how intently we watch for the merest trace of breath, and augur that, if but a feather be moved by it or a mirror dimmed by it, there is yet life!

In natural sleep, then, sleep perfect and deep, that half of our nature which is volitional is in the condition of inertia. To say, as Blumenbach has said, that in this state all intercourse between mind and body is suspended, is more perhaps than should be said, the precise limits and connections of mind and body being unknown. But certainly the brain and spinal cord, ceasing themselves to receive impressions, cease to communicate to the muscles they supply stimulus for motion, and the muscles under their control, with their nerves, therefore