Within the last few years considerable interest has been aroused concerning certain anatomic and pathologic characteristics of the petrous bone. Among those who have been active in this work should be mentioned: Druss, Friesner, Kopetzky, Almour, Eagleton, Frenkner, Profant, Glick, McMahon, Boemer, Lillie, Lawson and others. Restudy of the anatomy of the petrous bone has thrown much light on the pathologic possibilities. ANATOMY In the present investigation twenty-five temporal bones were studied from microscopic sections and twenty-five were examined from gross sections. The bones were studied to determine the variations in the distribution of the bone marrow and the pneumatic spaces. Of the twenty-five bones of which gross sections were made, seven, or 28 per cent, showed pneumatic spaces in the petrous apex. In four of the seven these were paratubal cells. Of the twenty-five bones of which microscopic sections were made, ten, or 40 per cent, had pneumatic spaces