THE LUNGS 



(The minute polygonal cells lining the air or pulmonary vesicles 

 measure from 1-1 600th to l-2250th of an inch in diameter, and from 

 l-2800th to l-3800th of an 



inch in thickness. Between the Fig. 239. 



vesicles is a trabecular tissue, 

 mainly composed of yellow elas- 

 tic with a few muscular fibres, 

 some of which are united with 

 the lining membrane to strength- 

 en it, especially around the 

 apertures of communication be- 

 tween the adjoining air-cells.) 



3. Capillary vessels ramify 

 in the walls of the vesicles, and 

 even project on their inner 

 face. 



(The capillary plexuses are 

 so arranged between the two 

 layers forming the walls of two 

 adjacent cells, as to expose one 

 of their surfaces to each, in order 

 to secure the influence of the 

 air upon them. These networks 

 are so close, that the diameter 

 of the meshes is scarcely so 

 great as that of the capillaries 

 which inclose them.) 



VESSELS. The lung is a 

 very vascular organ. The nu- 

 merous ramifications it receives 

 divide- into two orders the functional and the nutritive vessels. 



Functional vessels of the lung. We know that the blood returns from all 

 parts of the body by the veins, after 

 losing, along with its bright red 

 colour, the properties which render 

 it fit to maintain the vitality of the 

 tissues. It thus arrives at the right 

 side of the heart, whence it is pro- 

 pelled into the lung, there to be 

 regenerated by mediate contact with 

 the air. It is the pulmonary artery 

 which conveys this fluid into the 

 parenchyma of the organ, and by 

 the pulmonary veins it is carried 

 back to the heart. The artery is 

 at first divided into two branches, 

 which ramify and finally terminate 

 in dense capillary plexuses upon the 

 walls of the air-cells. The veins, 

 innumerable and attenuated at their 

 origin, like the arterial capillaries, terminate in from four to eight principal 

 trunks, which open into the left auricle of the heart. 



These two orders of vessels, which necessarily participate in the physiolo- 



AIR-CELLS OF LUNG, WITH INTERVENING TISSUES. 



a, Epithelium ; 6, Elastic trabeculae ; c, Membranous 

 wall, with fine elastic fibres. 



Fig. 240. 



ARRANGEMENT OF THE CAPILLARIES AROUND 

 THE AIR-CELLS. 



