No. I.] THE STRUCTURE OF THE LUNG. 1 79 



and the artery on the periphery. This same statement may 

 be applied to the liver. The veins keep on the periphery of 

 the lobule (Figs. 17, 18 and 19), except in a few instances, when 

 they send branches between the air-sacs to collect the blood 

 from the terminal bronchus and the capillaries of the atria. 

 All the larger veins run in the interlobular spaces, and are 

 situated as far from the bronchus and artery as possible. 



Considering the termination of the air passages and the 

 arteries and veins, we have as histological unit of the lung, 

 the air-sac. In all cases it has one artery and one vein which 

 represent the termination of each system. The artery is on 

 the central side of the sac, the vein on the peripheral, and 

 between the two a rich capillary net-work, thus giving an 

 arterial and a venous side to each air-sac. 



Bronchial artery. — The distribution and termination of this 

 vessel has caused much discussion on account of the difficulty 

 in isolating it from the surrounding vessels. The coarser 

 vessels are best separated by injecting them with gelatine in 

 which there is suspended ultramarine blue. The granules do 

 not pass into the capillaries. If, however, the capillaries which 

 arise from this artery are to be injected, it is best to use Prus- 

 sian-blue gelatine in which there are suspended granules of 

 cinnabar. The blue passes through the capillaries and the 

 cinnabar lodges in them, thus marking very definitely the 

 direct path of the vessels. 



The bronchial artery is distributed to the bronchus through- 

 out its entire length, the walls of the blood and lymph vessels, 

 and their connective tissue sheaths. There are two or more 

 bronchial arteries for each bronchus and at no point do they 

 anastomose with the pulmonary artery. Free injections into 

 the bronchial arteries always flow into the pulmonary veins, 

 and at no time into the pulmonary arteries unless the former 

 are clamped when the backward pressure forces the fluid through 

 the capillary into the pulmonary artery. The bronchial artery 

 breaks up into capillaries which communicate quite freely 

 with the pulmonary capillaries, but in general form quite large 

 but short veins, which empty directly into the pulmonary 

 veins. These small bronchial veins extend throughout the 



