RESPIRATORY TRACT. 



2 35 



in connection with the heart. The pulmonary artery of the adult leaves 

 the heart as a subdivision of the ventral aorta; it divides into right and left 

 rami, apparently simple vessels, but in reality each of them consists of the 

 proximal part of a pulmonary arch together with a branch of that arch. 

 In Fig. 268, there is no indication that the left ramus, l.r. includes a part of 

 the left pulmonary arch. 



The pulmonary veins grow out from the left atrium as a single vein 

 with four main branches. By expansion of the 

 atrium the proximal part of the vein is incor- 

 porated in its wall and the four branches, two 

 from each lung, then open separately. The 

 capillary subdivisions of the veins anastomose 

 with those of the pulmonary artery to form 

 the principal blood supply of the lungs. 



The small bronchial arteries which supply 

 the connective tissue of the lungs are branches 

 of the thoracic aorta, one or two on each side. 

 Their capillaries join those of the bronchial 

 veins derived from the azygos veins. In part 

 they connect with the pulmonary veins. 

 Since the bronchial arteries convey 'arterial 

 blood' whereas the pulmonary arteries contain 

 'venous blood,' the former may be compared 

 physiologically with the hepatic artery in the 

 liver. 



The connective tissue in which the entoder- 

 mal part of the lungs ramifies, occurs as a pair 

 of lateral swellings of the mediastinum. The 

 mediastinum is the connect^e tissue surround- 

 ing the oesophagus and extending between the 

 heart and the dorsal aorta. It is bounded 

 on either side by the mesothelium of the 

 body cavity, and so has the structure of a 

 broad mesentery of the heart. The pair of 

 mediastinal swellings or 'pulmonary wings' 



project into that portion of the coelom which connects the median 

 pericardial cavity, on either side of the mediastinum, with the 

 peritonaeal cavity. These portions of the coelom become cut off, first from 

 the pericardium and later from the peritonaeum, thus producing two 

 closed sacs, the pleural cavities. Each of these is lined with a continuous 

 layer of mesothelium, which, with the underlying connective tissue, con- 



th.aa 



FIG. 268. RECONSTRUCTION OF A 

 PART OP A HUMAN EMBRYO 

 OP 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 ramus of 

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

 its right ramus; oe., oesopha- 

 gus; p. c., pericardial cavity; 

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

 septum transversum; th. ao., 

 thoracic aorta; tr., trachea. 



