VOLUME 2 3, NO. 6, JUNE 1951 919 in a patient with myelogenous leukemia before, during, and after administration over a period of 25 days (Figure 4). During this whole period the serum arsenic levels, although showing a tendency to increase after arsenic administration, ranged from 3.5 to 7.0 micrograms % which is within the normal range. The 24-hour urinary arsenic increased from 5.6 micrograms before medication to 500 during medication, and was still elevated (30 micrograms) 10 days after the last administration of arsenic.

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The factors controlling heat flow into and from the head of an exhaust valve are discussed for the purpose of indicating the means by which temperature, the overriding important factor in exhaust valve failure, can be reduced. This analysis suggests the lines along which the designer and the metallurgist must co-operate. The causes of exhaust valve failure are reviewed, and the conclusion is reached that the majority of these are the result of corrosion fatigue. Even for resistance to burning the two properties inherent in resistance to corrosion fatigue, namely resistance to corrosion and good hot strength, are essential. Laboratory tests for determining resistance to high temperature corrosion fatigue and hot hardness are described, and some results given for exhaust valve materials. Corrosion fatigue resistance is considered to be so significant a factor in exhaust valve failure that it is suggested that laboratory tests to determine this property should form the basis for the development of improved exhaust valve alloys.

The results obtained by computed tomography (CT) in a few cases of difficult diagnoses are compared with diagnoses and preoperative information obtained by conventional diagnostic methods including sonography. Clinical pictures are discussed of the intradiaphragmatic cyst, pelvic spleen, hypostatic abscess in psoas muscle, bronchogenic cyst, and hemangioma of the liver. By clearly outlining the boundary surface and the reaction of surrounding tissue, CT can give valuable information, which may be essential for correct preoperative diagnosis and therapeutic plan. Even so, the diagnosis itself can usually be made without CT. Zusammenfassung. Anhand einiger schwieriger Differentialdiagnosen werden Diagnose und pr~ioperativeInformation mit herk6mmlichen Mitteln incl. der Sonographie mit den Ergebnissen der CT verglichen. Die Krankheitsbilder umfassen Zwerchfellcyste, Beckenmilz, Psoas-Senkungsabszeg, bronchogene Cyste und Leberh~imangiom. Durch Umgebungsreaktion und Abgrenzung erbringt die CT wertvolle Information, die fiir pr~toperative Diagnostik und Therapieplanung entscheidend sein kann. Die Diagnose selbst kann mit grol3er Wahrscheinlichkeit oft ohne CT gestellt werden. Die CT wird oft iibersch~itzt.Summary. C-cell tumours are rare in children. They must be classified as hereditary types of C-cell carcinoma. C-' cell hyperplasias are considered to be a precancerous condition of this tumour. The present article reports two cases of C-cell hyperplasia discovered in the framework of a family-screening study. The surgical procedure should include at least total thyroidectomy and the examination of the specimen should include immunohistochemical methods for the detection of calcitonin. Attention is drawn to the importance of screening for the diagnosis of early tumour stages, which results in improved prognosis.

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