encing and promoting its production without danger or detriment to general health. The few facts we have so far may be stated as follows: In healthful sleep the blood circulates more slowly; hence kidney action and perspiration diminish. The breathing is slower, and the exhalations of moisture and carbon dioxide from the lungs are less. Hence, these are doubtless produced in smaller amount, as they should be, from the diminution of muscular work and of combustion of animal fiber. And it is highly probable, though not yet fully tested by experiment, that certain gaseous, vaporous, or other products of the transformation of the muscles, nerves, and other tissues, partially stored up in the blood during working hours, are eliminated during sleep by the lungs, skin, and kidneys. The digestion is stated to be more active during normal sleep, and the temperature in the vital organs at least is stated by some authorities to remain at its normal point, though in the limbs it probably falls. This may be due to a tendency of the blood to the internal digestive organs. It is known, as was proved independently by Hammond and Durham, that the volume of the brain diminishes during natural sleep.
Returning to the old edition of the Encyclopædia Britannica (1779) before quoted, a few sentences possessing interest may be further taken from the article on Sleep. Referring back to the article on Dreams, it is here added: "Sleep we have shown to arise immediately from the communication between our sentient principle and external objects being cut off, in consequence of which memory is also lost, and the person becomes insensible of existence. This state may be induced either by such causes as affect the brain, the nervous system, or the blood, though it probably depends in most cases on the state of the vital fluid." This "vital fluid" or "vital spirit," as it is elsewhere denominated, is apparently not the blood itself, but an agent or influence then unknown, believed to reside therein. If we were now—with our present chemical knowledge—to substitute for "vital fluid" oxygen of oxyhæmoglobin of the arterial blood, this part of the citation would become rational. Oxygen, however, had then been discovered but four years previously, and its functions must have been yet but vaguely understood. The Italics have been introduced into the above excerpt to emphasize certain notions about sleep whose fallacy has been previously pointed out.
Dr. Hammond suggests that in the most profound natural slumber the spinal cord, with its nervous ganglia, remains awake, though not quite so much so possibly as when the brain itself is wholly conscious. Of course, the involuntary muscles, as those which operate the heart, the respiratory organs, etc., governed by ganglia of the spinal cord, never slumber. Their slumber is the last sleep of all. But Dr. Hammond cites proof independent of