278 THE LUNGS, 



Pulmonary vessels. — The capillary network of the pulmonary 

 vessels (tigs. 18G and 187) is spread beneath the epithelium of the air- 

 cells, and is found wherever the finest air-tubes have lost their cylin- 

 drical character, and become beset with alveoli. Around the exterior 

 of each alveolus there is an arterial circle, which communicates freely 

 with neighbouring circles, as may be seen near the surface of the lung. 

 From these circular vessels, which vary in diameter from y^^^th to 

 -g^:-uth of an inch, the capillary network arises, and covers jthe 

 bottom of each alveolus, passing also into the interalveolar septa 

 between the walls of adjacent air-cells, and surrounding the mouths 

 of these. As was pointed out by Rainey, the capillary network, in the 

 partitions between contiguous alveoli, is single in the lungs of man and 

 mammalia, although it forms a double layer in the lungs of reptiles. 



The capillaries are very fine, measuring, in injected specimens, 

 from ^-5^4 0^11 to "soVo^^^ of ^^^ ^^^ch ; the network is so close that 

 the meshes are scarcely wider than the vessels themselves. Those 

 vessels which lie nearest to the mouths of the alveoli are observed arching 

 and coiled over and amongst the elastic fibres found in the interalveolar 

 septa. The capillaries are very superficial, being covered only by the 

 thin layer of tesselated epithelium above mentioned. 



The branches of the pulmonary artery accompany the bronchial tubes, 

 but they subdivide more frequently, and are much smaller, especially 

 in their remote ramifications. They ramify without anastomoses, and 

 at length terminate upon the walls of the air-cells and on those of the 

 bronchia in the fine and dense capillary network, from which the radicles 

 of the pulmonary veins arise. The smaller branches of these veins, 

 especially near the surface of the lung, frequently do not accompany the 

 bronchia and arterial branches, but are found to run alone for a slK)rt 

 distance through the substance of the organ, finally joining some 

 deeper vein which passes by the side of a bronchial tube, and also 

 forming, according to Itossignol, frequent lateral communications. 

 The veins coalesce into large branches, which at length accom- 

 pany the arteries, and thus proceed to the root of the lung. In their 

 course through the lung, the artery is usually found above and in front 

 of a bronchial tube, and the vein below. 



The pulmonary vessels differ from the systemic in regard to their 

 contents, inasmuch as the arteries convey dark blood, whilst the veins 

 carry red blood. The pulmonary veins, unlike the other veins of the 

 body, are not more capacious than their corresponding arteries ; indeed, 

 according to Winslow, Santorini, Haller, and others, they are somewhat 

 less BO. These veins have no valves. Lastly, it may be remarked that, 

 whilst the arteries of different lobules are independent (except where a 

 branch of artery supplies two or three lobules) their veins freely 

 anastomose. 



Tlie bronchial vessels. — The bronchial arteries and veins, which are 

 much smaller than the pulmonary vessels, carry blood for the nutrition of 

 the lung. The bronchial arteries, from one to three in number for each 

 lung, arise from the aorta, or from an intercostal artery, and. follow the 

 divisions of the air-tubes through the lung. They are ultimately dis- 

 tributed in three ways : (1) many of their branches ramify in the 

 bronchial lymphatic glands, the coats of the large blood-vessels, and 

 in the fibrous and muscular walls of the large and small air-tubes, and 

 give supply to a copious capillary plexus in the bronchial mucous mem 



