What keeps this centre active? It has been already stated that all the conditions of the body which cause an increased tissue change, use up a greater amount of oxygen and give off more carbonic acid, therefore are accompanied by more active movements of the respiratory muscles. From this it would appear that there exists some relation between the activity of the respiratory centre and the condition of the blood, a deficiency of oxygen or an excess of carbonic acid gas calling forth increased action. One has only to hold one's breath as long as possible, and note the series of rapid and deep respirations that follow such a temporary impediment to the proper oxygenation of the blood, in order to see that this involuntary respiratory centre is profoundly influenced by a deficiency of oxygen. Experimentally, it can be shown that the centre is excited, in a great measure, at least, by the poorly oxygenated blood flowing through the medulla, and possibly also by the action of the venous blood circulating through distant organs, and reflexly affecting the centre. It has also been shown that the temperature of the blood circulating through the medulla affects the activity of the centre, for, if the blood in the carotids be warmed, the respiratory movements become more rapid.
The respiratory centre is a good example of what is called an "automatic nerve centre," not depending upon the arrival of nerve impulses from afar for its excitation, nor merely reflecting the influences of other centres, but acquiring its energy from its nutritive income and the thermal condition of the warm blood which flows through it.
So long as the amount of oxygen flowing through the centre is kept up to a certain standard, the normal excitability of the centre continues, and we have natural quiet breathing, called Eupncea. When the oxygen falls below the normal standard, the respiratory centre becomes more excitable, and more active movements or "difficulty of breathing" called Dyspnoea is produced.
If the theory that a deficiency of oxygen is the normal stimulus to action of the respiratory centre be correct, a superabundant quantity should diminish the activity of the centre, and a condition the opposite of dyspnoea would be produced. This is difficult to show in natural breathing, though every one knows the efficiency of taking a few deep breaths before a dive into water or an attempt to hold the breath. With artificial breathing, if the movements be carried on very energetically for some time, and then be stopped, the animal will not at first attempt to breathe, but after a short time, somewhat less than a minute, gentle and slow respiratory movements commence. This cessation of breathing, called apnoea, depends upon the blood being so charged with oxygen that it no longer acts as a stimulus to the centre.
It is probable that dyspnoea is produced by a deficiency, in oxygen rather than by ah excess of carbonic acid gas. This is proved by the fact that it occurs when the carbon dioxide is removed from the blood by breathing air which is free from C02, and is only deficient in oxygen, and, secondly, because an excess of C02 gas in the air causes a drowsy condition rather than an active dyspnoea.