146 ORGANS OF RESPIRATION. 



bronchial. The pulmonary artery, coming from the right ven- 

 tricle of the heart, divides under the arch of the aorta into two 

 large branches : one for the right lung, and the other for the left. 

 The right branch is larger than the left. Each of these branches 

 having reached the upper part of the root of its respective lung, 

 begins there to distribute itself in large trunks, which divide and 

 subdivide throughout the substance of the lung. The terminating 

 branches finally become capillary, and ramify in the parietes of 

 the air cells, where the blood which they carry, from being 

 dark-coloured and venous, is so altered as to have the arterial 

 qualities restored to it, and to become of a bright red. From 

 the ultimate branches of the pulmonary artery, arise the 'first 

 branches of the pulmonary veins. These are successively ac- 

 cumulated into two large trunks on each side, which, issuing at 

 the lower part of the root of the lung, go to open into the left 

 auricle of the heart. It has been remarked by Mr. Boyer, that 

 the two pulmonary veins are less capacious than the pulmona- 

 ry artery of the same side in which they manifest a peculiarity 

 of blood vessels, differing from what exists in other parts of 

 the body. The pulmonary artery and veins are distributed in 

 company with the bronchia. From the observations of Pro- 

 fessor Mayer, it appears that valves exist in the pulmonary 

 veins, contrary to the general opinion of anatomists. They 

 are found where smaller trunks join the larger ones, at an acute 

 angle, but there are none when they join at a right angle.* 



The second order of blood vessels, being the bronchial, also 

 consists in arteries and in veins, and are for the nourishment of 

 the lungs. They, too, attend the branches of the bronchia. The 

 arteries pervade the substance of the lung by innumerable fine 

 branches, and anastomose with the pulmonary arteries. The 

 bronchial veins also anastomose with the pulmonary veins, but, 

 finally, come out in small trunks from the root of the lung: the 

 right one empties into the vena azygos, and the left into the 

 4runk of the superior intercostal veins of the left side about the 

 third dorsal vertebra. 



The Lymphatics of the lungs are numerous ; after traversing 

 the black bronchial glands, those of the left side empty into the 



* Am. Mod. Jour. vol. iii. page 186. 



