ANATOMY OF THE LUNGS. 479 



pigment, upon the surface of the lung ; and by inflating a portion 

 of the organ, the cellular structure may be seen. The several 

 lobules are united together by connective tissue without fat ; and 

 each is attached to a terminal branch of the airtube, and receives 

 offsets of the pulmonary vessels. 



The lung is invested by the pulmonary pleura, except at the Serous 

 hilum, where the vessels enter. The serous membrane is thin and covenn s 

 transparent, and is closely attached to the lung-substance by means 

 of a fine layer of subserous areolar tissue, which is continuous with 

 the interlobular tissue. Both the pleura and the subserous tissue and sub- 

 are very elastic, so that in the collapsed state the surface of the 8e 

 lung is still smooth. 



Arrangement of the airtube and pulmonary artery entering the lung. Kelation of 

 It has already been seen that in the root of the lung the pulmonary bronchus, 

 artery lies at first in front of the bronchus ; but before entering the 

 organ the artery crosses over, and gains the posterior surface of the 

 airtube. On the left side the artery passes backwards above the on left side, 

 undivided bronchus ; but on the right side the bronchus gives off and on right, 

 the branch (epiarterial bronchus) to the upper lobe of the lung before 

 it is crossed by the arterial trunk, which therefore runs between the 

 upper and middle divisions of the airtube. From this arrangement 

 it would appear that the lower half of the left bronchus and the 

 two lobes of the left lung are represented on the right side by the 

 continuation of the bronchus below the artery and by the middle 

 and lower lobes of the lung ; and that the upper lobe of the right 

 lung with its division of the airtube have no representatives on the 

 left side. 



Bronchial branches in the lung. If the primary divisions of the Airtubes in 

 bronchi be followed into the lung, they will be found to give off Iung : 

 secondary branches ; and these, together with the smaller offsets of 

 the air-passnges, divide for the most part dichotoniously, that is mode of 

 evenly into two. The branches of the airtube within the lung are br 

 known as the bronchia or bronchial tubes, and differ from the bronchi 

 in being circular in section. Their structure resembles that of the structure ; 

 bronchi ; but the pieces of cartilage are irregular in shape and occur 

 on all sides of the tube, and the muscular tissue is proportionately 

 greater in amount and completely surrounds the canal. The 

 ultimate bronchial tubes are about half a line in diameter; and and ending, 

 each leads to a group of somewhat funnel-shaped dilatations 

 (infundibula), which are beset with air-cells and form -the lobules 

 of the lung. 



VESSELS OF THE LUXG. Two sets of vessels are furnished to the Vessels are 

 lung, viz., the pulmonary, which bring blood to the lung to be 

 aerated, and then return it to the heart and the smaller bronchial, 

 which convey the blood destined for the nutrition of the lung. 



The pulmonary artery divides like the bronchus, and within the Pulmonary 

 lung its branches run usually on the posterior surface of the artery ' 

 bronchial tubes, which they accompany to the lobules. The arterial 

 branches do not anastomose together ; and they end in the capillary 

 network of the air-cells. 



