In the normal waking states, the vagotonic nerves of the autonomic system which upbuild the body and insure the continuance of the race should dominate the organism, being checked in emergencies only by the sympathetic nerves which constitute the human safety system.
The vagotonic nerves contract the pupil, make saliva and gastric juice flow, slow down the heart beats, decrease the blood pressure, promote sexual activities, etc.
The sympathetic nerves on the contrary, dilate the pupil, dry the mouth, stop the gastric activities, increase the heart beats, raise the blood pressure, decrease or arrest the sexual activities, etc.
In peaceful sleep, we observe that the vagotonic functions hold full sway. In sleep, our pupils are contracted. Even when they have been dilated by atropine, they become contracted again in sleep.
In sleep, the digestive organs continue to perform their specific work, all the popular beliefs to the contrary notwithstanding. Infants and animals generally go to sleep as soon as they finish feeding. Animals digest infinitely better if allowed to sleep after being fed, than if compelled to stay awake, walk or run.
The activity of the sexual organs is as great in sleep as in waking life; in certain cases, it is even greater.
At certain times, during sleep, the pressure of the blood in the brain is greatly reduced, and certain authors have concluded that sleep was characterized by brain anaemia, which some of them consider as the cause of sleep.
Indeed, unconsciousness can be induced by producing a temporary brain anaemia, for instance by compressing the carotid arteries of the neck for a minute or so. Sleepiness almost always appears then and lasts as long as the pressure is exerted.
Special manometers show that the fall in the blood pressure invariably precedes the appearance of sleep. In dogs whose skulls have been trephined for purposes of observation, the brain can be seen to turn pale as soon as the animals fall asleep.
But we have here simply one of the vagotonic activities mentioned previously. In the normal organism, the blood pressure should be low, rising only in emergencies, when the organism is facing some danger and must be prepared for fight or flight.