THE LUNGS. 



KJU 



Ki-. -': 



i The minute polygonal cells lining the air or pulmonary vesicles 

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

 l-2800th to l-3800th of an 

 inch in thickness. Between the 

 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 walk 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.) AIR-CELLS OF LUNG, WITH IXTEUVEXIXI; n 



-KLS. The lung is a a> Epithelium; 6, Elastic trabecuhe; c, Membranous 



very vascular organ. The nu- wall, with fine elastic fibres. 



merous ramifications it receives 



divide into two orders the functional and the nutritive vessels. 



Finu-floiuil vessels of the lung. We know that the blood returns from all 

 j p;uts 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 

 rated by mediate contact with 

 the air. It is the pulmonary artery 

 which conveys this fluid into the 

 parenchyma of the organ, and by 

 liniiiiary veins it is can-it <1 

 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 fight principal 

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



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



AltltAV.I Ml M "1 Illl M-ll I \K1I> VI 

 Illl MIX I 1 \.-. 



