RESPIRATORY SYSTEIVJ. 233 



of the lungs, in the substance of which it branches forth, and 

 takes the course of the bronchial tubes. These tubes they supply- 

 as well as the coats of the pulmonary vessels, and the parenchyma 

 of the lungs, with blood : in fact, they may be regarded as the 

 nutrient vessels of these organs. It has been, however, and still 

 remains, a subject of dispute, whether these vessels do exclusiveli/ 

 nourish the substance of the lungs or not; some say that they 

 do ; while others assert that they are assisted in this function by 

 the pulmonary artery, with some of the branches of which they 

 anastomose. The latter opinion certainly does not appear to be 

 supported by facts of much weight; on the contrary, the blood 

 which the pulmonary arteries contain is dark-coloured, and unfit 

 for the nutriment of any organ; and as for anastomosis, we have 

 no demonstrative proof of its existence. The bronchial veins end 

 in one trunk, which returns the blood into the vena azygos. 



The nerves of the lungs are derived principally from a large 

 plexus within the chest, constituted of the par vagum and sym- 

 pathetic. They enter the pulmonary structure in company with 

 the bronchial tubes and bloodvessels, and continue their course 

 with them, to be dispersed upon the bronchial membrane and pa^ 

 rietes of the air-cells. 



The absorbents of the lungs are large and numerous, particularly 

 the deep-seated : and of the superficial, we may often succeed in 

 injecting considerable numbers, by introducing a quicksilver-pioe 

 under the pleura pulmonalis. They all pass through the absorbent 

 glands situated around the roots of the bronchial tubes. 



Farenchynia. — The connecting medium of the various consti- 

 tuent paits of these organs, or, as it is termed, their parenchyma, 

 appears to consist of little else than cellular tissue, without any 

 intermixture of adipose matter : it admits of the free diffusion of 

 any fluid that may be extravasated into it — of air that may have 

 escaped from the air-cells, or of serous fluid poured out when the 

 lungs become anasarcous ; but, as was observed before, there is no 

 intercommunication between it and the cells or vessels, as long as 

 the organs preseive their integrity of structure. 



Specific Gravity. — The lungs, when healthy, are exceeding 

 light in comparison to their volume; so that if they be immersed 

 in water, unlike most other parts, they will float upon the surface, — 

 a fact familiar to every one who has seen the liver and lights of an 

 animal thrown into a pail of water to be washed : indeed, the name 

 of lights itself seems to have been given to them from this very 

 property. If the foetal lungs, however, be so treated, they will in- 

 stantly sink to the bottom of the vessel: and this experimental 

 result at once shews why those of an animal that has once 

 breathed should swim; for, in the one instance they contain air, 

 H h 



