DESCRIPTION. 85 



blood, and the lungs are nourished by the bronchial arteries, which, 

 running alongside the bronchi, form an arterial tree corresponding, 

 in some measure, with their ramifications; the blood of the bron- 

 chial arteries being brought back out of the lungs into the circula- 

 tion by the bronchial veins, which again form an inverse system of 

 twigs, branches, and trunks, answering to that of the bronchial ar- 

 teries. The bronchial vessels are of small calibre. 



Besides these there are the pulmonary artery and veins, the for- 

 mer a very large vessel, which, coming direct by a single trunk from 

 the venous side of the heart, accompanies the bronchia, and splitting 

 into finer and finer ramifications, forms at last a " wonderful net- 

 work" of blood-vessels around the air cells, the blood in which is se- 

 parated from the air only by a membrane of extreme thinness. From 

 the air cells this network reunites from twigs into branches, and 

 from branches into the four trunks of the pulmonary veins, which 

 pour the arterialized blood direct into the left side of the heart. 

 Thus, the lungs are like forests of blood trees, the air cells being 

 open spaces between, whereby the atmosphere is admitted to nourish 

 and ventilate them; one set of trees, dull and venous, representing 

 the blood before the ventilation, the other set blooming and arterial, 

 representing the beauty and flower which succeeds where the vernal 

 air has blown. This turn from autumn to organic spring is mo- 

 mentaneous in the lungs, which may not inaptly be compared to trees, 

 inasmuch as leaves are the lungs of plants, and the vegetable king- 

 dom, transmuting the earth and the atmosphere, belongs to the lung- 

 department of material nature. 



The lungs, like the other important organs, have a plentiful supply 

 of nerves, which coming from both the cerebral and sympathetic 

 systems, pursue the bronchia to the air cells. 



The parts enumerated make up the active constituents of the 

 lungs. In recapitulation, they are the tree of the air tubes, four 

 other arterial and venous trees, and a nervous tree, terminating 

 around and within the air tubes. All these are compacted by a 

 system of membranes or skins, which make of the lungs not a five- 

 fold or sixfold system of trunks, boughs, branches and twigs, but 

 one solid though distinctly lobulated organ. 



These membranes may be generalized under the name of the 

 8 



