THIS study is undertaken with the purpose of investigating certain normal shadows seen commonly at the apices of the lung on roentgenograms of the healthy chest. Of little significance in themselves, they derive considerable importance from the fact that they may be misinterpreted as representing disease of the lungs or pleura, particularly apical pulmonary tuberculosis. It is believed that accurate evaluation of apical shadows requires not only precise anatomic knowledge of the pleural domes and pulmonary apices, but also familiarity with certain simple concepts of the mechanism of shadow production by which normal structures may cast visible shadows on the roentgenogram. Apical parenchymal disease is briefly discussed, mainly for the purpose of contrasting normal with abnormal apical linear shadows. Special reference is made to apical subpleural scars. Anatomy The pleural dome by strict anatomic definition is that portion of the parietal pleura covering the summit of the lung and situated above the superior orifice of each thoracic cage. The plane of this orifice corresponds to the upper surface of the first rib and forms an angle of 45 degrees with the horizontal. In exposing the pleural dome, the various structures encountered may be considered in the nature of a series of enveloping sheaths, as follows: (1) skin and subcutaneous tissues; (2) an outer muscular layer; (3) an inner muscular layer; (4) a fibrous sheath; (5) a vasculo-nervous sheath; (6) the parietal pleura. Roentgenographically, the “apical region” is ordinarily understood to designate that portion of the chest situated above the level of the clavicle, as seen on the flat postero-anterior film. This region includes, in addition to the above named structures, the posterior and lateral portions of the upper three or four ribs, the associated intercostal soft tissues, and the upper mediastinum. Skin and Subcutaneous Tissues.—The thickness and tonus of the subcutaneous tissues is a direct expression of the subject's age, habitus, and state of nutrition and is of importance in determining the relative radiolucency of the apices on the film. The thin broad platysma muscle, located anteriorly, is included as an essentially cutaneous structure. External Muscle Group.—The sternomastoid muscle arises by a sternal head from the anterior surface of the manubrium and by a clavicular head from the sternal third of the clavicle. It covers the anterior aspect of the apex more or less completely. Postero-lateral to the apices are the levator scapulæ muscles (above) and the serratus anterior muscles (below), while directly posterior, associated with the upper thoracic spine, are the heavy erector spinæ muscles (splenius, semispinalis, etc.), as well as the rhomboids and trapezii. Internal Muscular Group.—Posterior and lateral to the apices are the three scalene muscles forming a box-like compartment about the pleural dome.