Spencer and Darwin gave no proof of their statements, so that it devolves upon us physiologists to discover experimentally whether the intuition of these great philosophers is correct. Darwin was, as usual, very cautious, and in Chapter III., in which he treats of the general principles of expression, after having mentioned the above theory, says: 'Our present subject is very obscure, but, from its importance, must be discussed at some little length; and it is always advisable to perceive clearly our ignorance.’ III I made a few experiments in order to see whether in reality, as Spencer assumes, there exists a difference of conductibility amongst the various nervous filaments which move the muscles of the face, and which, as we know, are united into one bundle called the facial nerve. I irritated this nerve at its point of departure from the skull, and near the ear, where it is most easily isolated. There is no need to describe the process of irritating the nerves by electric currents. Galvani began in the last century to produce contractions of the muscles by means of electric currents originated by the contact of two metals; and we all know that the contractions of the legs of some skinned frogs, observed by Galvani in Bologna while he was hanging them on the iron railings in his garden, were the beginning of one of the greatest conquests of science, and one of the discoveries which have exercised the greatest influence on civilisation. The apparatus made use of to irritate the nerves is an invention of Professor Du Bois-Reymond. Many who are not physicians will yet know the apparatus, which is often used in the cure of diseases by electricity; its greatest advantage consists in the facility with which the intensity of the electrical stimulus may be increased or diminished. After having produced such a profound sleep in a dog by means of chloral that he was insensible, I tried to irritate the nerve which moves the muscles of the face by a very weak electric current. At first the current was so weak that no effect was visible, but as it was increased, a slight contraction of the cutaneous muscle of the neck appeared, and a little movement at the corner of the mouth. It might perhaps be of use to explain here how, in the muscles situated under the skin of the neck, other facial muscles originate, amongst others the so-called risorial muscle; but in order not to interrupt the relation of this experiment, I shall reserve these remarks till the beginning of the next section. The intensity of the electric current when the slight movement of the mouth appeared was equal to 400 units.