93. The hand is an organ, its function is prehension. The performance of this function in any of its numerous applications is rigorously limited by the structure of the hand--the bones, muscles, nerves, circulating and absorbent vessels, connective tissue, fat, etc. Fatigue the nerve, and the function will be feebly performed; exhaust it, and the function ceases; diminish the action of the heart, tie an artery, or vitiate the structure of the blood, and the function will be correspondingly affected; stiffen the tendons, soften the bones, diminish the synovial fluid, or increase the fat--in short, make _any_ alteration whatever in the structure of the hand, and an alteration is necessarily produced in its function. So rigorously is function dependent upon structure, that the hand of one man will execute actions which are impossible to another. The hand of a baby is said to be the same in structure as the hand of a man; and since the powers (functions) of the two are notoriously different, we might rashly conclude that here function was dissociated from structure. The case is illustrative. In baby and man the structure is similar, not the same; the resemblance is of kind, not of degree; and the function likewise varies with the degree. The penny cannon which delights the child is similar in structure to the ten-pounder which batters down walls; and though, speaking generally, we may say that the function of both is to fire gunpowder for human ends, no one expects the penny cannon to be employed in warfare. In physiology, as in mechanics, the effect varies with the forces involved. There can be no doubt that an exaggerated activity will produce a modification in the active organ, for this is only the familiar case of increased growth with increased exercise, and this is the biological meaning in which Function can be said not, indeed, to _create_, but to _modify_ an existing Organ. Preceding the activity there must be the agent. Every organ although having its special function has also the properties of all the tissues which constitute it. The function is only the synthesis of these properties to which a dominant tissue gives a special character. The eye, for example, though specially characterized by its retinal sensibility to light, is largely endowed with muscles, and its movements are essential to Vision. The intestinal canal, again, though specially characterized by its secretions for the decomposition of food, has muscles which are essential to Digestion. In many animals, especially vegetable-feeders, there is an exaggeration of the muscular activity in certain parts of the intestinal canal which is only possible through a corresponding development of the muscular tissue, so that in some birds, crustaceans, and molluscs we find a gizzard, which is wholly without a mucous membrane to secrete fluids, and which aids Digestion solely by trituration.