If, after cutting off the head of a very young animal, we stop the hæmorrhage by means of a bandage and then introduce a bellows into the trachea, so that respiration may be artificially stimulated, we see that the headless animal begins to breathe after the cessation of the perturbing influence of the first shock. With kittens it is sufficient to administer a small dose of strychnine (0·0005 gr.) in order to produce respiratory movements in the trunk after decapitation, which, however, become gradually weaker as they no longer suffice to maintain the excitability of the nerve-centres through the oxidation of the blood.
This simple experiment proves that respiration is accomplished by nerves branching off from the brain and spinal cord. The consciousness of our ego is not necessary; even a decapitated animal responds by modifications of respiration when we pinch or squeeze his paws, because the sensory nerves of the skin transmit the impressions of the external world to the spinal cord. The deep, noisy, and interrupted inspirations of anyone taking a plunge-bath are also involuntary, and when we are overcome by fear it is the same mechanism which produces a deep inspiration.
At every step which physiology makes, we discover fresh complications, other wheels, if I may use the expression, within the wheels of our organism. Until quite recently it was thought that the brain controlled the respiratory centre, accelerating or arresting its movements; but it has now been shown by Christiani that by means of a vivid light which must strike the eyes of the animal, deep and frequent inspirations may be produced even after the brain has been removed. Impressions of sound, which would have frightened the animal, produce the same effect, the oppression being even greater than in normal conditions. This experiment shows us that, independently of cerebral action and psychic operations, the rhythm of breathing alters at every change which takes place in the external world, at every peripheral irritation of the sensory nerves. Thus are explained the oppression and palpitation of the heart which befall us at the banging of a door, at a clap of thunder, and which we cannot suppress, which are produced by a sudden noise on a thousand occasions before we learn the cause, and do not disappear even when we have done so, and recognised the unimportant origin of our perturbation.