114 ANATOMY AND PHYSIOLOGY. 



air-cells, and are enveloped in fine cellular tissue ; and except this 

 cellular tissue, the lung has no proper parenchyma, its structure 

 being entirely filamento-vascular. The lungs are supplied with 

 blood by the bronchial arteries, derived from the thoracic aorta ; 

 these vessels run along the bronchial tubes, subdivide as they pro- 

 ceed, and form a minute net-work on the attached surface of the 

 bronchial mucous membrane ; the blood they convey to the lungs 

 is returned to the vena azygos, or superior cava. The nerves dis- 

 tributed to the lungs are derived from the eighth pair, and a few 

 filaments from the sympathetic. 



We have now to 'Consider the changes the blood undergoes in its 

 course to and through the lungs. It will be recollected that the left 

 side of the heart sends the blood into the general system ; this is 

 called the systemic circulation. The right side of the heart sends it 

 into the lungs, and this has received the name of the pulmonic circu- 

 lation. But if the blood that goes to the lungs were received in the 

 same state as it is sent, death would be the consequence, for venous 

 blood is poison to the body ; and this is the reason why an animal 

 dies when the air is prevented from getting into its windpipe by hang- 

 ing or drowning. Bichat showed this very decisively. He connected, 

 by a tube, the jugular vein of one dog with the carotid artery (which 

 sends the blood to the brain) of another, and allowed the venous 

 blood to flow into it. The immediate effect of this was, that the 

 dog in whose brain the venous blood was made to circulate, became 

 completely insensible, and would in a short time have died. On 

 allowing the arterial blood, however, again to circulate in its brain, 

 the dog was quickly restored. 



What are the changes, then, that take place in the lungs, and 

 how are these changes effected ? The lungs, then (vulgarly called 

 lights), are principally composed, 1st, of air-tubes (bronchi), of 

 which the windpipe (trachea) is the commencement, and which 

 divide and subdivide until they terminate in very minute bags or 

 air-vesicles ; and, 2dly, of the branches of the pulmonary artery, 

 which branch out upon the sides of these air-tubes. The annexed 

 plate shows the windpipe, with the lungs entire on one side, and 

 with the branches of the air-tubes dissected on the other. These 

 tubes terminate in vesicles, which are said to vary in size from the 

 fiftieth to the one hundredth part of an inch in diameter. 



