IT is not an easy task to give the Christmas Lectures at the Royal Institution, especially on a subject like physiology, in which experiments are much more difficult than in some others, at least when required to work without fail, to be intelligible and not to cause offence. The difficulty is increased by the fact that one's audience does not consist simply, in the words of the Royal Institution, “of juveniles between the ages of 10 and 16”; one finds oneself addressing distinguished adults in many walks of life, and not only these, but also through the publicity given to these lectures in the press, a considerable proportion of the people of Great Britain. One must reflect, however, in preparing them that they are intended for ‘juveniles,’ and that if others dare to come they do so at their own risk and will be assumed to be ‘juveniles’ for the purpose of the lectures. It is true, as a matter of fact, that most lecturers are apt to pitch their lectures at a level much too high for their audience, and that by attempting to make them interesting to a child of thirteen they may well succeed in absorbing the attention even of adults comparatively expert in the matter with which the lecture deals. We all like to have things put to us simply, and no lecturer to a juvenile auditory should pay any attention whatever to people above the age of sixteen. The Christmas lectures at the Royal Institution are intended to include a considerable number of demonstrations and experiments, and without the use of live animals experiments on the functional working of the body are impossible. Two fortunate facts, however, make it practicable to give suitable phsyiological demonstrations at the Royal Institution: (1) that the isolated organs and tissues of dead cold-blooded animals will continue to function for considerable periods after removal from the animal; and (2) that in recent years a great variety of perfectly good physiological experiments, often of precise quantitative as well as qualitative significance, have become possible upon men; and if upon men, then also upon children. The fact that isolated tissues of animals will function for a long time after removal, although a commonplace of phsyiology, is not generally realised, and in itself is a matter of entrancing interest to those who come upon it for the first time: it awakens all kinds of questions, scientific and philosophical, which not only a ‘juvenile’ but also an adult urgently desires to see answered. The beating of an isolated heart, which will continue for hours or days after removal from its previous owner, provided that it be properly treated, is an extraordinary eye-opener to most people previously ignorant of phsyiology. There are few physiologists who do not remember the astonishment and interest which was aroused in them by their first sight of this phenomenon, or of the reflex movements of a frog whose head has been cut off, or of the contraction of a muscle preserved, sometimes for days, in salt solution, or of the electric current produced by an isolated nerve, or of the many other strange things that happen to little bits of tissue removed from the ordinary environment which their previous owner supplied. It is such things that bring physiology first into the region where exact experiments can be made, and allow the factors at work in living tissues to be examined carefully and at leisure in the laboratory. The astonishment which we, as physiologists, have felt at intervals at such things, and which, if we have not grown too old, we still remember, is one thing we can and should communicate to our listeners, not only perhaps at the Royal Institution, but also at our classes in universities. The experience of the lecturer himself, at such a course of lectures as these, is valuable as showing him the kind of thing that appeals to intelligent, keen, unsophisticated, and natural people. There was no doubt from their applause that the audience approved of experiments in which the functions of organs were revealed by such experiments on isolated tissues. Even more, however, than experiments on surviving portions of dead animals did the audience appreciate the fact, which I am sure many of them had never realised before, that experiments of all kinds on the mechanism of the body can be performed on man, or in this case on children, without discomfort or danger. How many are the things which, when we were young, we wanted to know about the machinery inside which we live-and nobody told us. Gradually, perhaps, we thought that these things were not to be known, or were unknowable; gradually, perhaps, we reached the state, in which I fear a good many adults exist, of regarding the machinery of the body as something unwholesome or ‘nasty.’ If my audience had been simply one of adults, I might perhaps have been afraid of shocking them in taking a life-like model of a man to bits and trying to fit it together again, after describing its various parts. In the case of children no such fear is necessary. As I have already said, there are very few small boys and not many grown men who do not want to take machinery to bits to see how it works; and children are quite ready to realise that they live inside a mass of complex and beautiful machinery, and are only too glad to be given the chance of taking it to bits, at any rate scientifically speaking, and examining its parts.