it of a weight held in the hand. Their united cross-section is about 16 square centimetres: (16 × 10) / 10 = 16. One cannot hold out in the hand, the elbow being pressed against the side, so that these muscles alone are acting, a greater weight than 16 kilogrammes (34 pounds), although the muscles are exerting a traction ten times as great as this. The strength of muscle when pulling straight is well illustrated by the thick white mass in the centre of an oyster. It keeps the shell closed until a force equal to 1,300 times the animal’s weight has been applied. This muscle also affords a good illustration of the part played by reflex contraction in opposing stretching—the reaction by which tone is maintained. Anyone who inserts an instrument, such as the end of a screwdriver, between the slightly open valves of an oyster lying under water will find that he needs to give it an exceedingly smart twist if he would catch the muscle asleep. Stretching it causes a reaction proportional to the stretching force. [Illustration: FIG. 17.—BICEPS MUSCLE IN ACTION.] The fact that the output of energy by muscle is proportional, within certain limits, to the work to be done, is brought out even in laboratory experiments. A nerve-muscle preparation teaches that the amount of work is not a function of the stimulus. Within certain limits a stronger stimulus evokes a higher and stronger lift; but the stimulus remaining the same, the work done by muscle (_i.e._, the product of weight multiplied by height) is, up to a certain optimum, increased by increasing the weight. Often a very light load is not lifted as high by a nerve-muscle preparation as a slightly heavier one. No satisfactory theory of this reaction to load has yet been formulated. Explanations have been put forward, but they merely substitute one unknown for another, a not uncommon drawback to explanations. Muscles are strongest when at their full physiological length. As he dips an oar into the water a man exerts the greatest force of which he is capable, provided that he is not guilty of “missing the beginning.” Hands over the stretcher, body between the knees, ankle, knee, hip, fully flexed, arms straight—all his strongest muscles are at their greatest physiological length. Rowing is an exercise which has no rival. Every muscle in the body, from little toe to little finger, comes into play under the conditions which suit it best. And not less admirable is the effect upon the abdominal muscles during recovery at the end of the stroke; and the rhythmic movement which encourages deep and measured respiration.