To determine whether the renal arterial system has a fractal structure, the fractal dimension of renal angiograms from 52 necropsy cases was measured using an implementation of the box-counting method on an image analysis system. The method was validated using objects with known fractal dimensions. The method was accurate with errors of less than 1.5 per cent and reproducible with initial values within 1.2 per cent of the mean of ten sets of measurements (reliability coefficient 0.968, 95 per cent confidence limits 0.911-0.984). In the 36 satisfactory angiograms the mean fractal dimension was 1.61 (SD 0.06), which was significantly greater than the topological dimension of 1 (P < 0.0001), indicating that the renal arterial tree has a fractal structure. There was no significant relationship between age (P = 0.494), sex (P = 0.136), or systolic (P = 0.069) or diastolic (P = 0.990) blood pressure, but two congenitally abnormal kidneys (hypoplastic dysplasia and renal artery stenosis) had fractal dimensions at the lower end of the normal range (third percentile). Since the renal arterial tree has a fractal structure, Euclidean geometric measurements, such as area and boundary length, are invalid outside precisely defined conditions of magnification and resolution.

Marking resection margins in surgical biopsy specimens We were interested to read the recent papers and correspondence in the Journal concerning the marking of resection margins in surgical biopsy specimens, using artists pigments suspended in acetone,' Tipp-Ex fluid,2 organically coloured gelatins' and alcian blue.' The traditional method using India ink is not particularly satisfactory due to the prolonged period of drying required and the tendency for the ink to spread. We have overcome this problem by applying Bouin's solution to the specimen immediately after painting on the ink, either by immersion or using a syringe. The ink seems to become instantaneously "fixed" to the specimen, and the cutting up of the specimen can proceed without delay. The ink remains firmly adherent to formalin fixed or fresh tissue throughout processing and is clearly seen in histological sections (figure). The ink also firmly adheres to any tissue retained in formalin for further examination.

No abstract

, but negatively for iron (Figure 2). At this point, the ibopamine drops were discontinued. Comment Bacterial organisms can produce condition that promotes adrenochrome autoxidation. An acidic environment is created by the formation of lactic acid as the major metabolic end-product of carbohydrate fermentation by S. viridans. In our patient, the concurrent infection with S. viridans may have facilitated the oxidation of ibopamine into its degradation products, resulting in the rapid pigment deposition in the cornea. It is also possible that the deposition would have occurred even without the favorable environment created by the infectious keratitis. Animal models have shown that for pigment deposition to occur, oxidized adrenochrome and a susceptible corneal surface must be present, 6 a condition satisfied by our patient's persistent epithelial defect. Thus, based on our observations, patients using topical ibopamine eye drops should be carefully monitored for pigment deposition in the cornea, particularly in the presence of a compromised epithelial surface.

No abstract