. Human embryology and morphology. Embryology, Human; Morphology. THE RESPIRATORY SYSTEM. 255 (4) The Azygos Lobe.—On the inner side of the right lung of man the azygos lobe is frequently present, sometimes as a mere pulmonary projection or trace, sometimes as a lobule. It projects into and fills a slight recess between the pericardium and diaphragm, behind the intra-thoracic part of the inferior vena cava. The lobe is always well developed in quadrupedal mammals. In them the pericardium is separated from the diaphragm by a diverticulum of the right pleura—the sinus sub- pericardiacus (Fig. 20
. Human embryology and morphology. Embryology, Human; Morphology. THE RESPIRATORY SYSTEM. 255 (4) The Azygos Lobe.—On the inner side of the right lung of man the azygos lobe is frequently present, sometimes as a mere pulmonary projection or trace, sometimes as a lobule. It projects into and fills a slight recess between the pericardium and diaphragm, behind the intra-thoracic part of the inferior vena cava. The lobe is always well developed in quadrupedal mammals. In them the pericardium is separated from the diaphragm by a diverticulum of the right pleura—the sinus sub- pericardiacus (Fig. 209). With the assumption of the upright posture (in man and anthropoids) the heart sinks until its rests on the diaphragm, the sub-pericardiac sinus and azygos lobe being thus obliterated. The reappearance of the azygos lobe in man is an atavism—that is to say, a recurrence of an ancestral feature. sup-uen. cau pericardium sin. sub-pericard. of rt. pleura. ensiform inf. uen. cau. Fig. 209.—The Relationship of the Heart to the Diaphragm in Quadrupedal Mammals. Blood Supply of the Lung.—The pulmonary aorta is formed, with the ascending part of the aortic arch, out of the bulbus or conus arteriosus (see page 230). The right and left pulmonary arteries spring as branches from the right and left 5 th aortic arches (Fig. 29, p. 37). They enter the lung buds, and are carried backwards with them. The pulmonary veins grow out from the pulmonary buds and enter the left auricle through the venous mesocardium about the 3rd month (see page 246). Changes at Birth.—When the child begins to breathe at birth the expansion of the lungs opens up the pulmonary circulation;. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Keith, Arthur, Sir, 1866-1955. London : E. Arnold
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