_2d_, To the objection that varieties in the distribution of arteries, and those deviations which are often produced by disease, would have the same effect, I answer, that variety of distribution does not at all take place in some of those parts in which the most important operations have to be performed; namely, high in the groin, above and below the clavicle, &c. and that, wherever great deviation is caused by disease, it will generally be attended by external signs, according to which the operator may modify his measurements. I also further answer, that wherever deviation, from either of these causes, occurs, he who, by means of these measurements, knows the precise situation, according to the most usual and regular structure, will have the most decided advantage over him who is ignorant of it: the former will possess data, from which he can calculate probable deviations, the latter none; the former, even at the worst, will have to _calculate_, the latter will be unable to do more than _guess_. Thus I have answered the only objections that it is possible to urge against the principle proposed. I have already said, that private objections by persons who dare not publicly make one, and who themselves cannot, by one or two incisions, approach within several inches of any artery, I shall uniformly despise, and shall only ask them to perform the operation with my youngest pupil. I shall now deliver a short history of the method proposed. * * * * * The want of some guide to the situation of internal parts, has long been felt. Fixed points have been adopted, but with no regularity. Measurements guessed by the eye have sometimes been used, but even they have almost always been absolute, _i. s._ measurements from one point; rarely proportional, _i. s._ measurements between two points. The few guessed proportional measurements, however, as for paracentesis abdominis, paracentesis thoracis, &c. which have been mentioned in surgical works, have, it must be confessed, been such as, from the magnitude of the parts with which they were concerned, required little more than guess. Such, however, as they were, they evidently depended upon no principle, but seem as if hit upon accidentally, and, at all events, were never generally applied. Precise or real proportional measurements between fixed points, applicable to minute objects, _i. s._ such measurements as I have proposed and described, never have been, as far as I can discover, proposed and described before. And these precise or real proportional measurements, between fixed points, differ exactly as much from guessed ones, as the accurate cabinet, which was regulated in every part by the compass and square, would differ from the clumsy and unserviceable one, where every thing was constructed by guess. Here the parallel is so close, the difference in the results so striking, that I could not have adduced a better illustration.