RESPIRATORY APPARATUS 



297 



has begun, the ductus arteriosus closes, becoming a fibrous cord, and then 

 the volume of blood going through the pulmonary artery equals that in the 

 aorta. (For further details regarding the development of the pulmonary 

 arteries, see Bremer, Amer. Journ. Anat., 1902, vol. i, pp. 137-144). 



The pulmonary veins are at first represented by a capillary plexus 

 around the lung-bud, which receives its blood in part from the pulmon- 

 ary arteries already described, and in part from 

 branches of the dorsal aorta, some of which 

 persist as the bronchial arteries. The capillary 

 plexus is drained partly by branches of the 

 posterior cardinal or azygos veins, representing 

 the future bronchial veins, and partly by a 

 minute vein which has grown out from the left 

 atrium and is destined to become the great 

 pulmonary veins. At a certain stage these 

 veins, two from each lung, have a common 

 orifice in the left atrium; but in later stages, as 

 the heart enlarges, their short common stem is 

 taken up into the wall of the atrium, so that 

 the four pulmonary veins acquire separate 

 openings. The early stages in the development 

 of the pulmonary veins in the cat have recently 

 been studied by Brown (Anat. Rec., 1913, vol. 

 7, pp. 299-330). 



The small bronchial arteries, one or two on 

 each side, are branches of the upper part of the 

 thoracic aorta (Fig. 294); sometimes one of 

 them proceeds from an intercostal artery. 

 The bronchial arteries enter the hilus of the 

 lung and pass into the fibrous tissue in the walls 

 of the bronchi. The main stems branch with 

 the bronchi. They produce capillary net- 

 works in the bronchial mucous membrane, and send branches to the peri- 

 bronchial connective tissue, supplying it with capillaries and becoming 

 the vasa vasorum of the main branches of the pulmonary artery (Miller, 

 Anat. Anz., 1906, vol. 28, pp. 432-436). In some animals Miller finds 

 that the bronchial arteries pass on into the pleura, as in the horse; in others, 

 like the dog, terminal branches of the pulmonary arteries supply the pleura; 

 and in the human lung the pleura receives both pulmonary and bronchial 

 vessels (Amer. Journ. Anat., 1907, vol. 7, pp. 389-407). 



The bronchial veins are small branches of the azygos vein. They do 

 not receive all the blood from the bronchial arteries, since some capillaries 

 from the latter are drained by the pulmonary veins. 



th.ao. 



FIG. 294. RECONSTRUCTION OF A 

 PART OF A HUMAN EMBRYO OF 

 13.8 MM. (Dr. F. W. Thyng.) 



ao., Aorta; d.a., ductus arteriosus; 

 1., entodermal part of the lung; 

 1. at., left atrium; 1. br., left 

 bronchus; 1. r., left ram us of 

 pulmonary artery, p. a.; r. r., 

 its right ramus; oe., oesopha- 

 gus; p. c., pericardia! cavity; 

 p. v., pulmonary vein; s. t., 

 septum transversum; th. ao., 

 thoracic aorta; tr., trachea. 



