contract. There can be no doubt that in such cases the contraction of one part produces some kind of disturbance, it may be electrical, which is propagated to adjoining parts, and acts upon these as a stimulus. Thus a kind of wave of contraction passes through the structure. Something of the same kind has been observed in muscle; but to this we shall return when we come to the consideration of the electrical phenomena in muscle.
We have now studied muscle as a producer of what we may call mechanical energy. At this stage, I shall leave the order in the syllabus and take up a subject mentioned in connection with the fifth lecture, namely, the production of heat by a muscle. We detect heat usually by a thermometer; but the heat we must look for in a muscle is so small in quantity as to oblige us to use a more delicate method. It is well known that minute quantities of heat may be detected by the use of what are called thermal piles. To understand the principle involved in the working of a thermal pile, look at this simple experiment. I have here a number of strips of the metals
