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 ; 



