314 THE RESPIRATOEY SYSTEM 



THE LOBULE OF THE LUNG 



If carefully examined, the surface of the pulmonary pleura presents 

 minute polygonal areas (10 to 25 mm. in diameter), the bases of the 

 anatomical (or secondary) lobules,, whose borders mark the attachment 

 of fine bands of interlobular connective tissue, outlined by pigmented 

 lines. In microscopical preparations still finer bands may be found, 

 which traverse the pulmonary tissue in the direction of the root of the 1 

 lung, and which partially outline minute conical areas, the true pul- 

 monary (or primary) lobules, or lung units, whose bases are directed 

 toward the pleura, and their apices toward the root of the lung. In many 

 of the lower mammals, e.g., ox, these lobules are more distinctly outlined 

 by interlobular connective tissue than is the case in man. 



At the apex of the secondary lobule a small bronchiole (intralobular 

 bronchiole) enters and divides into its respiratory bronchioles (from 30 

 to 100). This secondary pulmonary lobule consists of a collection of 

 the smaller units or primary pulmonary lobules above described. At 

 the same point a terminal branch of the pulmonary artery enters with 

 the bronchiole and supplies the anastomosing capillary plexus in the 

 alveolar walls. Branches of the bronchial artery do not supply any of 

 the primary intralobular structures, and the pulmonary veins which 

 return the blood from the alveolar capillaries arise at the periphery of 

 the primary lobule and immediately enter the interlobular connective 

 tissue. 



The interlobular connective tissue contains the smaller branches of 

 the pulmonary veins, the lymphatics returning from the pleura, and 

 the non-medullated nerve trunks which are destined for the supply of 

 the pleura and the intralobular pulmonary tissue. 



BLOOD SUPPLY or THE LUNGS 



The blood supply of the lungs is derived from two distinct sources, 

 the pulmonary arteries and the bronchial arteries. The former is des- 

 tined chiefly for aeration in the capillaries of the alveolar walls, the 

 latter for the nutrition of the bronchial walls. 



The pulmonary artery enters at the hilum in company with the 

 vein and the bronchus. It follows the bronchus throughout its course 

 and gives an arterial branch to each of its subdivisions. The large ar- 

 teries nearly equal in size the bronchus in relation to which they lie, 

 but the smaller branches are not more than one-fourth to one-fifth the 



