The greatest output of work is obtained when muscles contract against a progressively diminishing load. Towards the end of the lift the load must be small, if contraction is to be carried to its extreme limit. The provision for this is well seen in the case of the muscles of the arm when lifting a weight up to a position above the head. A portmanteau is held in the hand. Its handle is gripped by flexing the fingers. And here it may be noted that, since the range of movement of a muscle varies as its length, the thumb and fingers are not worked only by muscles contained in the palm of the hand. Fingers are bent and wrist flexed by muscles of which the origin is carried up even to the lower end of the humerus. As the portmanteau hangs by the side, biceps and brachialis are at their fullest length. Suppose it to be necessary to place it on a cab. These muscles begin the work under the best conditions. They could not, however, lift the portmanteau far did not the muscles of the shoulder displace the elbow from the side, so that at the end of their pull, the forearm being almost vertical, the muscles of the arm have little more to do than to move the hand inwards towards the head, in preparation for the extensor thrust. The secret of getting the greatest amount of work out of any particular muscle lies in securing for it the due co-operation of other muscles. ELECTRIC ORGANS. Muscle disperses energy in the forms of mechanical work, heat and electricity. Its structure, as already pointed out, is peculiarly favourable for the display of electromotive force. In certain fishes muscle is so modified as to give an electric discharge without developing mechanical work. The production of an electric change is a by-phenomenon of muscular activity. It becomes the sole function of an electric organ. If the skin be removed from the tail of a skate, a cylindrical column of brawny tissue about the size of a finger will be found embedded amongst the muscles near its root on either side. These are electric organs, although so weak that it is barely possible to feel the shock which they give in a live fish. The nearly allied Torpedo of the Mediterranean has far more powerful batteries. They are situate near its gills, occupying the whole thickness of the fish from skin to skin. When the back of a torpedo is pressed, it discharges a current of 30 volts, or even more. Still more violent are the shocks given by an eel—Gymnotus—which haunts the tributaries of the Amazon, a terror to all who have to cross their fords on foot; or the African fish, Malapterurus. The current which these animals develop attains an intensity of 200 volts. With the exception of those of Malapterurus, all electric organs are modified muscle, and closely similar in structure. The organs of Malapterurus appear to be modified glands. The skate’s electric organ may be taken as typical of the rest. When sliced with a knife, it is seen to be divided by firm connective tissue into minute chambers.