A few words are first of all necessary to indicate precisely what anæsthesia is. By definition, an anæsthetic is a substance which, without paralysing the activity of the heart and the respiration, abolishes sensibility. Indeed, whenever general sensibility is abolished, there is, at the same time, abolition of consciousness, of intelligence, and of memory. Another characteristic of an anæsthetic is that its action is of a transient nature. At the end of a certain time, it disappears; and then intelligence, consciousness, and memory return gradually with sensibility. It is well known that the admirable discovery of general anæsthesia, allowing operations to be performed on man without the accompaniment of pain, was due to chance. It was an American dentist, Horace Wells, and his colleague, Morton (and others also perhaps), who discovered by chance that protoxide of nitrogen (commonly called laughing gas) has the power, when inhaled, of annulling all sensibility to pain for a certain length of time--sufficiently long for a surgical operation (1840). Then they discovered the effects of ether (1842). Since then, many other anæsthetics have been introduced, notably chloroform, prepared by Soubeiran in 1832, but the anæsthetic properties of which were only discovered in 1847 by Flourens and Simpson; so that physiologists and surgeons are now quite familiar with the mode of action of anæsthetics. Anæsthetics, in appropriate doses, poison the nervous cells, which are the seat of intelligence and sensibility, but leave unimpaired the functions of the cardiac nervous system and of the nervous system governing the respiration. An individual under chloroform breathes regularly; his heart beats rhythmically, but all intelligence has disappeared; he has no longer any will or memory or reflex actions, and the most painful operations can be performed on him without provoking the smallest phenomenon of sensibility. Further, we have no hesitation in asserting that the anæsthetised animal behaves like the anæsthetised man; that is to say, chloroform given to an animal abolishes all sensibility to pain. Vivisection, therefore, on an anæsthetised animal, does not provoke any pain. Physiologists are so convinced of this that, however humane they may be, they have no scruple in performing lengthy vivisections on an animal which is thoroughly anæsthetised.