The first question that meets us in the use of the method in medicine is what normal constituents of the body are permeable or impermeable to the X rays. It may be stated, in a general way, that all of the fleshy parts of the body are partially permeable to the rays in a relatively short time; and if the exposure is long enough, they become entirely permeable, so that no shadow is cast. Even the bones, on _prolonged_ exposure, do not present a sufficient obstacle to the passage of the rays, and the shadow originally cast becomes obliterated. Hence, skiagraphs of the same object exposed to the rays for varying times may be of value in showing the different tissues. The most permeable of the normal tissues are cartilage or gristle, and fat. A kidney (out of the body) is stated by Dr. Reid of Dundee to show the difference between the rind, or secreting portion, which is more transparent, and the central portion, consisting chiefly of conducting tubes, which is less transparent. On the contrary, in the brain the gray cortex, or rind, is less transparent than the white nerve tubules in the centre. The denser fibrous tissues, such as the ligaments of joints and the tendons or sinews of muscles, cast very perceptible shadows, so that when we come to a thick tendon like the tendo Achillis, the shadow approaches even the density of the shadow cast by bone. I presume that it is for the same reason (the dense fibrous envelope, or sclerotic coat) that the eye-ball is not translucent to the rays, as is seen in Figure 5, of a bullock's eye. [Illustration: FIGURE 5.--SKIAGRAPH OF A BULLOCK'S EYE. (From the "American Journal of the Medical Sciences," March 1896.)] Mr. Arthur H. Lea has ingeniously suggested that the translucency of the soft parts of the living and of those of the dead body might show a difference, and that, if such were the case, it might be used as a definite test of death. Unfortunately Figure 6, of a dead hand, when contrasted with Figure 11, of a living hand, shows virtually no difference, and the method cannot be used as a positive proof of death. That we are not able at present to skiagraph the soft parts of the body, does not imply that we shall not be able to do it hereafter; and should this be possible, especially with our increasing ability to penetrate thick masses of tissue, it is evident, without entering into details, that the use of the X rays may be of immense importance in obstetrics.