After we had assured ourselves that these alterations of the pulse were much less marked after other noises which did not resemble the loading of a gun, we wished to convince ourselves whether the agitation was caused by a fear of weapons. The next day the dog was again brought into the laboratory, and while he lay on the table, and the action of the heart was being marked, someone walked past him with a gun on his shoulder. The dog recognised it again, became restless, tried to rise, his heart beat violently, he began to wag his tail, and followed the hunter with satisfied glances. V 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. [Illustration: _F_ _G_ _H_ _I_ 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.