296 HISTOLOGY 



monary wings, constitute the visceral pleura; and the similar layers toward 

 the thoracic wall form the parietal pleura. These layers are comparable 

 in development and structure with the corresponding layers of the peri- 

 toneum. Other subdivisions of the pleura are the mediastinal, pericardial, 

 and diaphragmatic pleurae. The lung is connected with the mediastinum 

 by a short and broad stem of connective tissue, across which the bronchi, 

 vessels and nerves extend. This is the root of the lung, and the vessels 

 enter at the hilus. 



The branches which are given off by the stem-bronchus within the pulmonary 

 wings, are formed with great regularity, and they have been carefully studied in many 

 mammals. Very early in development, the human lungs become asymmetrical, and 

 at the stage shown in Fig. 293, B, the three lobes of the right lung and the two lobes 

 of the left lung are already indicated. In the pig the asymmetry is greater, since on the 

 right an unpaired lobe proceeds directly from the trachea; in certain animals, as in the 

 seal, the right and left lungs have symmetrical bronchi. Whether the symmetrical 

 condition is the primary one, and how the bronchi of one lung should be homologized 

 with those of the other, are questions which have been much discussed. For the com- 

 parative anatomy of the bronchi, see Huntington, Ann. N. Y. Acad. Sci., 1898, vol. n, 

 pp. 127-148; for their development, especially in the pig, see Flint, Amer. Journ. 

 Anat., 1906, vol. 6, pp. 1-137. 



The blood vessels of the lungs are derived from several sources. They 

 include the large pulmonary arteries and veins, which are the principal 

 vessels of the lung, and the small but important bronchial arteries and veins. 

 The pulmonary vessels are shown in Fig. 294, which represents the trachea 

 and right lung of a human embryo, seen from the left side; the left lung has 

 been cut away at /. br. 



The pulmonary arteries develop in connection with the pulmonary 

 arches, which are two vessels, one on either side, passing from the ventral 

 aorta to the dorsal aorta. Approximately midway in its course, each of 

 these arches sends a branch to the lung of the corresponding side. Subse- 

 quently the trunk of the ventral aorta becomes spirally subdivided by a sep- 

 tum, so that the portion leading to the pulmonary arches is split off from 

 the rest; the way in which its root becomes connected with the right 

 ventricle only, has been described with the development of the neart. As 

 a result of this subdivision, the pulmonary artery leaves the heart and di- 

 vides into right and left arches, each of which sends a branch to the lung on 

 the same side and then passes on to the dorsal aorta. The connection be- 

 tween the right arch and the right dorsal aorta is soon lost, however, so that 

 the vessel to the right lung (Fig. 294, r. r.} appears to be given off from the 

 main pulmonary artery. The left pulmonary arch enlarges, and until birth 

 it forms a great vessel, known as the ductus arteriosus, which conveys most 

 of the blood from the pulmonary artery into the aorta. The amount of 

 blood which goes to the inactive lungs may be inferred from the relative 

 size of the vessels shown in the figure. Soon after birth, when respiration 



