The requisites for skill in performing this operation are very obvious. It is plain that the man who knows the most about the relations of the parts, will best know how to adjust those parts when they get out of place; just as one who understands most thoroughly a machine is the best fitted to repair it when it is out of order. And there is no such thing as an innate instinctive knowledge of a machine made of bones, muscles, and ligaments, any more than there is such a knowledge of a machine made of wood and iron. In both cases the knowledge is _acquired_ knowledge—acquired by observation and study. In order that the knowledge which one has, even of the most common machine, shall be accurate and complete, he must be familiar with the parts of it when separated, and then with their connection as a whole. For the same reason, in order that the surgeon may understand so compound a machine as a human joint, he should become familiar with the several bones and muscles, and ligaments, and tendons, of that joint separately, and then with their connection as they make up the whole machine. And by familiarity with the machine as a whole, I do not mean merely that the surgeon should be familiar with the parts of the joint in their connection, as they are seen when the skin is removed with the fat and the cellular membrane. In order to perfect his knowledge of the joint as a whole, he must be familiar with it as it appears covered with the skin, and observe it in all its various attitudes and motions. This is quite as important a part of the knowledge of the joints, as that which is revealed by the dissecting knife. For the surgeon in his practice has to do with them in this covered state; and if he be familiar with them only in their uncovered condition, he will often find himself much puzzled, and may commit some unfortunate errors. A musician, who had always played upon a piano with the keys open to the eye, would hardly venture to play a tune with the keys covered for the first time at a public concert; but I apprehend that there is many a surgeon who makes his first real study of a joint as a whole in its covered state, when he is called upon to determine whether some part of that joint is out of place. This external geography of the joints, as it may be termed, is not sufficiently attended to, nor is the importance of studying it properly urged upon the students of medicine.