STUDIES ON THE PERMEABILITY OF THE SKIN TO MERCURY 83 ment still remaining on the skin, the animal was wrapped in a piece of celluloid sheeting formed into a cylinder fitted snugly around the trunk. The edges of the cylinder were cemented to the skin at the shoulders and hips so as to prevent slipping of the shield or possible seepage of the ointment. The pro- cedure is illustrated in Fig. 1. As additional protection, the celluloid Figure l.--For studies on the permeability of the skin to mercury shield was covered with a layer of adhesive tape applied in such a way as to overlap the edges of the shield by about •/4 in. Thus "trussed," the animals were placed in individual cages with access to food and water. Experience with several hundred rats so prepared has shown that the celluloid shield proved to be sufficiently rigid to prevent the animal from getting in oral contact with any part of its trunk, yet did not interfere with walking. Although the animals occasionally struggled, the celluloid- to-skin seals were effective in con- fining the ointment beneath the shield. B. METHODS FOR DETECTING CUTANEOUS PENETRATION OF MERCURY The method for detecting and quantitating the cutaneous pene- tration of mercury depends on an entirely new principle. This method substitutes storage of mercury for excretion of mercury as a measure of how much has penetrated the skin. Of all the organs of the body, the kidney has the greatest capacity for storing mercury. This is shown very clearly in Table 1. Here it can be seen that the concentration of mercury in the kidney, following 24- and 48-hour exposures to calo- mel ointment, is of an entirely dif- ferent order of magnitude than in other tissues. Convenient also for purposes of assay is the fact that the concentration of mercury in the control kidney is so low as to be of no quantitative significance. At any time following exposure to mercury, the amount in the kidney may be assumed to be a balance be- tween three dynamic factors: (1) the rate of entrance of mercury, (2) the rate of excretion of mercury, and (3) the rate of storage of mer- cury. If the rate of excretion is slow (experiments have proved this) and the rate of storage rapid (intra- venously injected mercury finds its way into the kidney in maximum amount with 1'./•. hours), then it becomes possible to use the mercury content of the kidney as a measure of how much has penetrated the skin. Obviously this measure is only a relative one, but is chiefly useful in

84 JOURNAL OF THE SOCIETY O17 COSMETIC CHEMISTS '• comparing the pen. etrating abilities of et•cient types of mercurial oint- ments. This method is so sensitive that if as little as 9 micrograms enter the body of a 250-gm. rat, 25 to 50 per Cent thereof can be detected in the kidneys in 24 hours. The procedure which we have used for assaying mercurial oint- ments may be summed up as fol- lows: A clipped area on the back is inuncted for 3 minutes with a mer- cury ointment, covered with rubber sheeting or celluloid, and contact of the ointment with the skin main- tained for 24:'hours. At the end of this time, the animal is sacrificed by exsanguination under ether an- esthetic, and the kidneys removed for analysis of mercury (5). All results are expressed in micrograms of mercury per gram of wet tissue. Owing to the natural variability in test animals, 6 to 8 are used for an assay in order to be able to recognize significant differences be- tween inunction procedures or oint- ments. Before discussing the findings of chief interest, namely, the effects of various types of ointment bases and mercury compounds on the cutaneous penetration of mercury, a number of factors relating espe- cially to the test a. nimals and the methods of application must be con- sidered briefly. L Effect of Covering the 2rnunction Site According to Rothman (6), cover- ing the skin results in interferences. with the normal escape of moisture.