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THE RESPIRATORY SYSTEM 



The interlobar fissure (fig. 998) begins about 6 cm. below the apex of the lung at the level 

 of the head of the third rib. With the arm hanging at the side, a line drawn across the back 

 from the third thoracic spine to the root of the scapular spine would indicate the course of 

 the upper part of this fissure. (Merkel.) Thence it passes downward and around the chest to 

 the end of the sixth bony rib in the mammillary line. Merkel points out the use of the root 

 of the scapular spine as a landmark for finding the limits of the lobes posteriorly: with the 

 arm hanging at the side all above this spot is superior lobe; all below it the inferior. The short 

 fissure of the right lung begins at the main interlobar fissure in the axillary line, about the 

 level of the fourth rib or fourth interspace, and passes nearly horizontally to the anterior margin 

 of the lung at the level of the fourth costal arch. 



The roots of the hmgs are placed opposite the fifth, sixth, and seventh thoracic vertebrEe. 

 The right root lies behind the inferior vena cava and under the arch of the azygos vein; the left 

 root is beneath the aortic arch and in front of the thoracic aorta. The phrenic nerve passes 

 in front of each root, the vagus behind. On the front and back are the pulmonary plexuses, 

 anterior and posterior. The ligament of the pleura goes from the lower edge of the root. 



Vessels and nerves of the lungs. — The bronchial arteries (see p. 588), belonging to the 

 systemic system, carry blood for the nourishment of the lungs. They arise from the aorta or 

 from an intercostal artery, two for the left lung and one for the right, and, entering at the hilus, 



Fig. 998.— Position op the Lungs from Behind. (Merkel.) 

 The pleura is represented as in Fig. 997. 



reach the hinder wall of the main bronchus. The bronchial arteries accompany the bronchi, 

 whose walls they supply, as far as the distal ends of the alveolar ilucts, beycuid which they do 

 not go. The.se vessels also supply the lyinjjh glands of the hilus, the walls of the large pulmonary 

 ve8.sels, and the connective-ti.ssue septa of the lung. Bronchial veins (see p. G64), anterior 

 and posterior, arise from the walls of the first two or three divisions of the bronchi and end in 

 the innominate and the azygos or in one of the intercostal veins; those arising from the walls 

 of the smaller tubes, inchujing tlie alveolar ducts, join the ))ulinonary veins. The pulmonary 

 artery (see p. 52S), entering t\\v liihis in a jjlane anterior to the l)r()iu^hus, turns to the posterior 

 aspect of the main-stem, following its brandies and tlieir subdivisions to the lobules. Entering 

 the lobule, the last branch of the vessel gives off as many twigs as there are atria (fig. 992), 

 and these twigs end in d(!nse cajiillary nets in the walls of the alveoli. Here the venous blood 

 brought by the pulmonary artery, separated from the air in tlie alveolus only by a thin septum, 

 is changed to arterial blood in the respiratory process. According to Miller, anastomosis 

 between the branches of the i)uhnonary artery are exceptional. Anastomosis between the 

 bronchial and pulmonary arteries has been (;humed, but the connection aj)parently existing 

 between these vessels is throngli the radicles of the bronchial veins which join the pulmonary 

 veins. The pulmonary venous radicles begin ;i,t the cai)iilary networks and drain the arterial 

 blood into the pulmonary veins, which run between adjacent lobules and which receive also 



