In fever our teeth chatter while the temperature of the body is over 40°, and human economy, far from seeking to heat the blood by trembling, seems rather to stand in need of some mechanism which would cool it, so as to preserve life. After severe exertion or protracted work with the arms, our hands tremble, even when we are panting with heat. When exhausted after the forced marches which I had to make during my studies of fatigue, I have noticed that in the evening, on my return from the peak of Monte Viso or from the highest glaciers of Monte Rosa, my legs trembled, although the temperature of my body was one or two degrees above the normal. Tea, alcohol, coffee, and many stimulating medicines cause a very visible tremor. In convulsive laughter, pleasure, intoxication, voluptuous enjoyment, anger, when the necessity for heating the blood is certainly not apparent, one trembles also, the voice vibrates and the legs shake. All this makes it probable that Darwin is right, and more decidedly do I incline to his side when I think of the disastrous effects which trembling produces during fear. Seals and many animals, of which I shall speak more in detail in the chapter on Fright, tremble so violently that they cannot make their escape, and allow themselves to be overcome and miserably killed. How can we, amongst the sublime perfections which we admire in organisms, admit the contradiction that an animal, in order to warm itself, does not flee from danger but trembles till it is killed, whereas in running away, it could warm itself much better and save its life as well? But the question must not be judged in this way. The divergent opinions which arise in the interpretation of facts are most difficult to resolve in science, because to one of the adversaries there always remains a certain territory in which he may intrench himself. III In order to learn the actual nature of trembling we must first see how the muscles are made and how they act. If we look at a muscular filament as fine as a hair under the microscope, we see that it consists of nearly a hundred extremely fine fibrillæ lying close to each other in the form of bundles. It is these which together form those minute threads seen with the naked eye in fibrous meat. Each fibril, seen under a powerful microscope magnifying three or four hundred times, appears formed of a series of muscular elements, or, let us say, of so many little boxes, piled one on the top of the other like a battery and about two-thousandths of a millimetre in thickness. Each little box is prismatic in form and terminates in two flat ends. It was an English physiologist who described these little boxes for the first time; they are therefore known in science as Bowman’s muscular elements. The resemblance of each fibril to a voltaic pile is so great that some have tried, but in vain, to establish an analogy between their functions.