When strong emotions such as fear are concerned, one must have recourse to other methods of writing the pulse, as the animal is very uneasy and tries to escape. As this is a question as yet little considered in physiology, I shall communicate a few experiments which I made bearing upon it. Fig. 7 represents the pulse of the carotid artery of a dog. During line F the animal was quiet; the pulse is somewhat irregular, which, in the dog, is a physiological fact. In line G five normal pulsations may be seen, while at A a shot was fired two steps from the dog. The report caused such a vibration of the air that the pen trembled, as may be seen from the irregular outline of the first two pulsations. The effect of fear on the heart is immediate. The frequency of the beats becomes at once three times greater than before.
We waited till the dog was quiet again. Fifteen minutes afterwards the curve H, which represents the normal pulse-line, was written, then came six pulsations of the line I, while at B a second shot was fired, and immediately the pulse was accelerated.
Fig. 7.—Acceleration of the Cardiac Pulsations through Fear (in A and B)
But why does the heart beat more rapidly and frequently in fear? In order to explain the cause of this phenomenon I must remind the reader of the observations which I made during my studies of the pulse, and of the circulation of the blood in the human brain during sleep. In a sleeping person, at the slightest noise or touch, the pulse becomes more frequent without the sleep being interrupted. This change is indispensable in order to accelerate the circulation and to utilise to the utmost the strength of the organism in preparing it for defence. Our machine is so made that it changes automatically as required, without the interference of the will being necessary. The palpitation of the heart from fear is the exaggeration of a fact which we always notice, whenever the organism must develop its maximum of energy and increase the circulation in the nerve-centres; the heart does not work for itself, but for the brain and muscles, which are the instruments of combat, attack, defence, and flight.
The greater or lesser frequency and force of the pulse in emotion depends upon the greater or lesser excitability of the nerve-centres. Women and children, who are more sensitive, experience this palpitation in greater intensity. When we say that women have more tender hearts, we refer to the fact that their hearts respond to stimuli to which the hearts of men remain impassive. We say of anyone who blushes and grows pale easily, and is soon moved to tears or laughter, that he has a good heart and a sincere character. But even cold, sceptical, egotistic, impassive men, when they are suffering from some illness, or when the excitability of their nervous system is intensified by some cause or other, may be deeply moved, and betray their feelings like children.
One must be a physician in order to see how the most courageous men become faint-hearted at a trifling loss of blood, and timid people, in consequence of a more abundant flow of blood to the brain, perform miracles of bravery. Weakness quickens the heart-beat even when we are not moved by anything. We all know that we avoid giving certain news to the convalescent which at other times would have produced little effect upon them.
One of my colleagues had been ill eight days from a quinsy. When he recovered and came to the laboratory, I hastened to visit him, and found him sitting in an arm-chair, pale and exhausted. I asked him how he was, to which he answered 'Very well,’ but that while scolding his servant on account of some trifling matter, such a feeling of oppression had seized him that he had to desist, as he was scarcely able to draw breath. I felt his pulse, and found that it was above a hundred. He laughed, and said, 'I never dreamt that my strong body was such a paltry piece of machinery as to run down during the few days I have not eaten as usual.’