The forehead of these animals is frequently marked by transverse furrows, and especially, as Darwin justly observes, when they raise their eyebrows. The same great observer considers that the countenances of anthropoids are, in comparison with those of men, generally inexpressive, and indeed, chiefly in consequence of the fact that they do not wrinkle the forehead when they are excited. The wrinkling of the forehead, which is one of the most significant forms of expression in man, is due to the action of the corrugatores supercilii, by which the eyebrows are drawn down and closer to each other, so as to form vertical folds on the forehead. It has been asserted that the orang and chimpanzee possess these muscles, but they seem to be rarely exercised--at any rate, to any remarkable extent.[67] When Darwin brought a chimpanzee out of his dark chamber into bright sunshine, he only once observed a slight wrinkling of the forehead. When the same observer tickled the nose of a chimpanzee with a straw, its face was slightly wrinkled, and faint vertical furrows appeared between the eyebrows.[68] Darwin never observed any wrinkling of the forehead in an orang. I myself have observed a contraction of that region of the brows which is covered with bristly hairs, and a wrinkling of the skin which covers the bridge of the nose in the gorilla and the chimpanzee, and have illustrated this expression by a drawing. Darwin goes on to say that when a young chimpanzee is tickled, to which, as in the case of children, their armpits are peculiarly sensitive, he generally utters a chuckling or laughing sound, although sometimes the laugh is silent. The corners of the mouth are then drawn back, and this sometimes causes the eyelids to be slightly wrinkled. This wrinkling, which is so characteristic of the human laugh, is still more apparent in some of the other apes. In the chimpanzee the teeth of the upper jaw are not exposed when he utters this laughing sound, and in this respect he differs from man. Darwin further observes that when the tickled young orang ceases to laugh, an expression passes over his face, which, according to Wallace, may be called a smile. Darwin has observed something similar in the chimpanzee.[69] My own observation confirms what has been said of the chuckling of a tickled chimpanzee. When Dr. Hermes, the director of the Berlin Aquarium, played with the chimpanzee which was kept in that establishment, a contortion of the corner of the mouth, resembling a somewhat sardonic smile, at once appeared. No specimen displayed this smile with so much effect as the lively Augustus, who delighted visitors by his inexhaustible humour in 1879. The gorilla, of which an illustration is given in Fig. 3, also drew down the corner of his mouth when he was pleased, by means of the muscular system which we have just described.