Thus does anatomy, by what I term precise or real proportional measurements between fixed points, afford an obvious and simple, though hitherto unobserved, general means of rendering the operations of surgery perfectly precise and easy. Indeed, such wonderful precision and ease do these real proportional measurements confer, that, by means of them, the finest instrument may be passed from the surface of the body through any great nerve or vessel that is liable to be the subject of surgical operation. Having stated this principle, I may merely observe, that, proceeding with all the ease and security, such accuracy must infallibly confer, it is, in all operations the first duty of the operator to ascertain by feeling, as nearly as possible, the thickness of cellular substance situate immediately under the skin of the patient, and then, with a bold and steady hand, to make an incision, deep in proportion to the thickness of that substance, and long in proportion to its depth, as well as to the additional depth and extent of the part to be operated upon. These general principles excepted, other subordinate rules are chiefly applicable to particular operations. * * * * * In order to illustrate this general principle, I cannot adduce a fairer or a better example than that which, according to it, is necessary to direct the taking up of the subclavian artery above the clavicle. The patient must, in this operation, rest on the fundament, his body slightly inclining backward, so as to form an obtuse angle with the thighs, and his neck and head inclined backward so as to form with the trunk an angle similar to that formed by it with the thighs; the inferior of these angles being concave, the superior convex. This last may indeed be more properly called an irregular segment of a circle, formed in consequence of the vertebrae of the neck bending in that direction. In this position, therefore, the neck is, in general, in the same parallel with the thighs. The head of the patient is to be turned to the opposite side from that in which the operation is to be performed, where also the operator must stand. The light must fall directly downward from the head of the patient. In this case then, the bringing of the shoulder forward and downward, which I deem indispensible, however necessary to the ease of the operator, is not very material to the correctness of the measurement. It is however of some consequence; for, as the subsequent measurement is dependent on the clavicle, and as that bone describes a portion of a circle in having its lateral end brought forward, every single point of it would, in that position, have a different relation to the vessels beneath from that which each would have had, if kept in its usual position.