4fi6 RESPIRATION. 



connective tissue, and the pleura, to terminate in the 

 gean and asygos veins. 



The pulmonary artery, which conveys venous blood to be 

 aerated in the lungs, originates from the base of the right ven- 

 tricle, and curving upwards and backwards, bifurcates over the 

 left auricle of the heart, sending a branch to each lung. It sub- 

 sequently divides in the same manner as the bronchial tubes, 

 and sends an ultimate twig to each lobulette, to be distri- 

 buted on the walls of the air sacs and pulmonary cells. 

 The resulting capillary plexus is consequently exposed on 

 both sides to the action of the air, except in those parts 

 where the walls of one lobulette are applied against those of 

 another; in such a case they are necessarily related on one 

 side to the interlobular connecting tissue. The capillaries 

 of the network vary in size from 3 * O th to 25 1 00 th of an 

 inch. WATERS. 



The pulmonary veins are those vessels which convey the 

 arterialized blood from the lungs to the left side of the heart. 

 They originate by numerous small radicles, passing from the 

 periphery of the lobulettes, and joining to form veins in the 

 interlobular spaces. These veins continue to traverse the 

 interlobular spaces, and generally follow the most direct course 

 towards the root of the lung. Other veins are found run- 

 ning along the bronchial tubes, and returning the blood from 

 their mucous membrane. On reaching the root of the lung, 

 they unite into several large trunks (4 to 8 in number), by 

 which the blood is discharged into the left auricle. 



The whole of the blood from the pulmonary lobulettes, 

 and the mucous membrane of the bronchial tubes, seems to 

 be returned by the pulmonary veins, while that from the 

 outer coats of the larger pulmonary vessels and bronchial 

 tubes, the bronchial glands, and, according to Heale, the pleura 

 and interlobular tissue, is taken up by the bronchial veins. 



