The bones, however, as is seen in nearly all of the skiagraphs illustrating this paper, cast well-defined shadows. This is at once an advantage and a hindrance. To illustrate the latter first, even one thickness of bone is difficult to penetrate, so that the attempt to skiagraph the opening which had been made in a skull of a living person by a trephine entirely failed, since the bone upon the opposite side of the skull formed so dense an obstacle that not the slightest indication of the trephine opening appeared. To take, therefore, a skiagraph of a brain through two thicknesses of skull, with our present methods, is an impossibility. Even should the difficulty be overcome, it is very doubtful whether there would be any possibility of discovering diseases of the brain, since diseased tissues, such as cancer, sarcoma, etc., are probably as permeable to the X rays as the normal tissues. Thus Reid ("British Medical Journal," February 15, 1896) states that a cancerous liver showed no difference in permeability to the rays through its cancerous and its normal portions. Foreign bodies, such as bullets, etc., in the brain may be discovered when our processes have become perfected. Figure 7 shows two buck-shot skiagraphed inside of a baby's skull, and therefore through two thicknesses of bone. It must be remembered, however, that not only are the bones of a baby's skull much less thick than those of an adult's skull, but they are much less densely ossified, and so throw far less of a shadow. The dense shadows cast by bone are, at least at present, an insuperable obstacle to skiagraphing the soft translucent organs of the body which are enclosed within a more or less complete bony case, as the rays will be intercepted by the bones. Efforts, therefore, to skiagraph the heart, the lungs, the liver, and stomach, and all the pelvic organs, probably will be fruitless to a greater or less extent until our methods are improved. While a stone in a bladder outside the body would undoubtedly be perceptible, in the body the bones of the pelvis prevent any successful picture being taken. [Illustration: FIGURE 6.--SKIAGRAPH OF A DEAD HAND AND WRIST, SHOWING TWO BUCK-SHOT AND A NEEDLE EMBEDDED IN THE FLESH. ("American Journal of the Medical Sciences," March, 1896.)]