Analogous to the bony tissues are the so-called ossified (really, calcified) arteries. In the dead body, arteries filled with substances opaque to the X rays, such as plaster of Paris or cinnabar mixtures, have already been skiagraphed successfully. It is not at all improbable that calcified arteries in the living subject may be equally well shown. So, too, when we are able to skiagraph through thick tissues, we may be able to show such deposits in the internal organs of the body. Stones in various organs, such as the kidney, will be accessible to examination so soon as our methods have improved sufficiently for us to skiagraph through the thicker parts of the trunk. The presence of such stones in the kidney is very often inferential, and it will be a great boon, both to the surgeon and the patient, if we shall be able to demonstrate positively their presence by skiagraphy. For the reason already given (the pelvic bones which surround the bladder), it is doubtful whether we can make use of it in stone in the bladder. Gall stones, being made not of lime and other similar salts, as are stones in the kidney and bladder, but of cholesterine, are, unfortunately, permeable to these rays; and it is, therefore, doubtful whether the X rays will be of any service to us in determining their presence. The chief use of the method up to the present time, besides determining the diseases, injuries, and abnormities of bone, has been in determining with absolute accuracy the presence of foreign bodies, especially of needles, bullets, or shot and glass. It is often extremely difficult to decide whether a needle is actually present or not. There may be a little prick of the skin, and no further positive evidence, as the needle is often imperceptible to touch. The patient, when cross-questioned, is frequently doubtful whether the needle has not dropped on the floor; and it might be, in some cases, a serious question whether an exploratory operation to find a possible needle might not do more harm than the needle. Moreover, though certainly present, to locate it exactly is often very difficult; and even after an incision has been made, though it may be embedded in a hand or foot, it is no easy task to find it. [Illustration: FIGURE 9.--SKIAGRAPH OF A HUMAN FOOT, SHOWING THE DEFORMITY IN THE LAST TWO TOES CAUSED BY TIGHT BOOTS. (Skiagraphed by Mr. Sydney Rowland, and published in the "British Medical Journal.")]