When the incision has been made one and a fourth of these proportional parts inward from the outside of the clavicle, you have reached the artery. It lays external to that protuberance of the first rib into which, as already observed, the scalenus primus is inserted, and consequently to the insertion of the muscle itself. The twig of the nerve actually crosses the artery immediately below the point where the latter ought to be tyed. The course of the artery is, in descending, obliquely outward, and the precise space in which it lays resembles an isosceles triangle formed by the clavicle, clavicular portion of the sterno-cleido-mastoideus and the omo hyodeus; its base being the clavicular portion of the sterno-cleido-mastoideus. The nerve is external and anterior to the artery, particularly its branch. Both the twig and trunk of the nerve ought to be seen by the operator before he attempt to take up the artery. These directions are so precise, that, were the operator blind and the artery destitute of pulsation, he need not scruple to take up that which presents itself in the above mentioned situation. By similar measurements, may every great artery in the body, liable to be the subject of surgical operation, be discovered with similar ease and precision, nor does there exist any other method of effecting this purpose. I shall now then state the objections which may be urged against this principle. * * * * * It may be objected to the adoption of this fundamental principle of all surgical operation. _1st_, That the accidental and probable fracture of the bone affording the fixed points, would, by deranging these points, render all such measurement inapplicable. _2d_, That varieties in the distribution of arteries, and those deviations which are often produced by disease, would have the same effect. I know not of any other objection that can be urged against it, and these I shall immediately answer. * * * * * _1st_, To the objection that the accidental and probable fracture of the bone affording the fixed points would, by deranging these points, render all such measurements inapplicable, I answer, that, to chuse as an example the operation I formerly described, it is, in such a case, only necessary to transfer the measurement to the sound clavicle, and to convert the three inner or four outer proportional parts into absolute inches, quantities similar to which may then be measured from the corresponding ends of the fractured one, and thereby indicate the situation of the vessel or nerve with as great precision as if no fracture were involved.