ONE FIGUREThe specimen exhibiting the anomaly recorded here is a newborn female orang-utan (Pongo pygmaeus, U. S. N. 31. no. 153825), examined through the courtesy of Blr. Gerrit S. Miller, Jr.In this specimen much of the ventricular part of the heart is herniated to the left through an opening in the pericardium.The oval pericardial opening ( fig. 1B) measures approximately 15x13 mm., and has a thick, smooth, clearly defined circumference. This opening is on a plane dorsal to the phrenic nerve and below and ventral to the root of the left lung.A very thin, transparent membrane, apparently continuous with the serous layer of the parietal pleura, separates the left lung and the herniated portion of the heart. However, from nhat is known of serous surfaces, it may be confidently asserted that this thin membrane contains also a layer of mesothelium representing the parietal pericardium, since if at any time such a layer m r e absent and the visceral pericardium were in contact with coiiiiective tissue, either the parietal mesothelium would promptly regenerate o r else adhesions would form. I n the case a t hand no adhesions whatsoever are present. Rather both cardiac and pulmonary faces of the membrane have the smooth, glistening appearance characteristic of normally functioning serous surfaces. Thus, the opening spoken of as pericardiaI must involve only the connective tissue of the pericardinm. Also from this it follows that, 69
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