414 THE POPULAR SCIENCE MONTHLY. 



tary 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, consist- 

 ing 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 tbe organic system : a sketch 

 of this organic system is depicted in the accompanying diagram. 



In sleep, the cerebro-spinal system sleeps ; the organic system re- 

 tains 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 cervi- 

 cal ganglia of the organic nervous system it receives a second or sup- 

 plementary 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 ves- 

 sels 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 ani- 

 mals, 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 cir- 

 culatory, and the two great functions of life are, in sleep, faithfully per- 

 formed. 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 na- 

 ture which is volitional is in the condition of inertia. To say, as Blu- 

 menbach 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 cer- 

 tainly the brain and spinal cord, ceasing themselves to receive impres- 

 sions, cease to communicate to the muscles they supply stimulus for 

 motion, and the muscles under their control, with their nerves, therefore 



