The accessory sinuses are demonstrable by the Roentgen rays becau!'E" they contain air and are surrounded by bony walls. They are, therefore, more transparent than the surrounding tissues. By means of the Roentger: rays one can demonstrate their size, outline, position and the condition of the walls and the septa. The sinuses 011 the two sides of the head can be projected in such manner that tht')' can be used for direct comparison as to their transparency or density. In this connection it must

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... be borne in mind that if the sinus on olle side is smaller than that of the opposite side, it is normally less transparent and must, therefore, not be interpreted as being diseased. Disease of the accessory sinuses may consi.,t of exudate within the cavities, of tumor formation \·,ithin the cavities, or of any of the bone diseases involving the bony walls. Exudate within the accessory sinuses causes a decrease in transparency. One cannot determine the nature (If the exudate oy the Roentgen rays, and we must expect muct1". pus or blood to cast similar shadows. If this exudate involves only thẽ inuses on one side, there will be shown a striking contrast when compared with the opposite side. If both sides are involved, as for example in the case of the max:lIary sinuses, one would compare the transparency or opacity with the sphenoids and the frontals above. If all of the sinuses ar.e diseased and filled with exudate, one must consider the opacity of-the sinus areas as compared with the detaH of structure in the remainder of the skull. In this connection, care must .