If the current is increased, the movement of the lips becomes more apparent, and when the electric irritation of the _facialis_ reaches the intensity of 700, a contraction of the orbicular muscle of the eyelids appears, which closes the eyes. At an intensity of 750 the muscles which elevate the upper lip contract. At 820 the nostrils are dilated and elevated. At 950 the contraction of the lips becomes so pronounced that the dog shows his teeth, and his face assumes an aggressive expression. At 1,250 there is a depression of the corners of the mouth, as though produced by pain and disgust. At 1,500 this expression becomes more intense, and the eye is forcibly shut. If the stimulus is still further intensified, the face assumes the fierce expression of an animal about to attack. I obtained the same results with the animal soon after death. These experiments show that Herbert Spencer’s hypothesis is correct; but we shall presently see that the matter is exceedingly complicated, and that we must take into account other factors no less important in the expression of the face. IV The muscles of the face have certainly not the office, as Duchenne de Boulogne thought, of expressing the passions of the soul. To speak frankly and without sentimentality, pedantry, or conventionality, we must recognise that the most important feature of the face is the mouth, and that the mouth is a funnel of flesh attached to the alimentary canal. Sometimes it only serves to seize the prey and receive the food before sending it to the stomach, as is the case in fishes, reptiles, and birds, in which the face is reduced to a minimum. As the apparatus for mastication becomes more complicated with the appearance of teeth for cutting and crushing the food, and of lips for sucking, drinking, and closing the mouth, the more complicated also does the structure of the face become. One of the most curious things discovered by anatomists is, that many muscles of the face, of great importance in the expression of the emotions, were originally, that is, in the inferior animals, muscles serving a very different purpose. I shall develop this fact briefly, so that the reader may see how difficulties continually increase the nearer science approaches to the origin of things. We all know that the hedgehog rolls itself up on the approach of danger, its body having the appearance of a ball covered with bristles. This movement is executed by means of a muscle under the skin which covers nearly the whole body, and which contracts in a similar manner to a bag drawn together by a string. Many other animals are similarly furnished with a fine muscular layer which covers the body. In the mole, for instance, this muscular system is well developed. Amongst domestic animals we may mention the dog, the cat, and the horse, in which, although these layers of cutaneous muscles are less compact, they are still sufficiently developed to be noticeable when they enter into action.