[Illustration: DIAGRAM 27.—EFFECT OF TEMPERATURE.] [Illustration: DIAGRAM 28.—VERATRIA CURVE.] Finally, there are the electrical changes in muscle. These, again, may be passed over briefly, since they are not easily understood or described. To put the facts in a nutshell, the part of a muscle which is in activity is negative to all other parts. Thus, if a muscle be dissected out and cut across, the activity at the seat of the injury, while it lasts, causes a current to pass through a galvanometer from uninjured parts to the wounded. (See Diagram 29.) Again, if a muscle be dissected out without injury, connected at two points with a galvanometer, and then stimulated at one end, as the wave of contraction passes along it, first one, then the other, contact becomes negative. (See Diagram 30.) S, Stimulating electrodes; N, contraction which marks the wave of excitation passing along the muscle; G, galvanometer which shows that the seat of activity (N) is negative to the rest of the muscle. [Illustration: DIAGRAM 29.—INJURY CURRENT: CROSS-SECTION OF MUSCLE NEGATIVE TO REST.] [Illustration: DIAGRAM 30.—ACTION CURRENT.] In passing, it may be mentioned that, as the heart is a muscle slung obliquely across the body, and waves of contraction are continually passing down its long axis, the whole body is affected by continual electrical changes. By very delicate instruments it can be demonstrated that with each beat the two hands alternately become electrically positive and negative to each other. Whilst dealing with the electrical phenomena of muscle, it may be as well to state that nerve fibres, which are studied with very much the same apparatus, show the same electrical changes, the point of injury or of the greatest activity being negative to all the rest. Single cells are less easily investigated, but in glands it is possible to show that the same rule holds. Undoubtedly the most curious fact about the generation of electricity by protoplasm is that, by a modification of muscle and nerve, which causes them to lose their ordinary properties, they are converted into a special organ for giving electric shocks. Armed with powerful batteries of this description, an otherwise rather helpless class of fish are enabled to defend themselves from their enemies, and deal unexpected death to their more agile prey. * * * * * Having now run over a few of the physical properties of protoplasm, we may pass on to a brief investigation of the movements we find in the body of man. III. In describing the movements of the body, we shall have to treat them as several and distinct, as indeed they are; but the fact should not be lost sight of that they cannot really be isolated: one idea embraces the whole. Two kinds of movement may, however, be distinguished in the vital functions: movement of the actual cells, such as muscles; and movement of non-protoplasmic elements acted upon by the cells—_e.g._, lymph.