236 



HISTOLOGY. 



stitutes the pleura. The parietal pleura is the part attached to the body 

 wall; the pulmonary pleura covers the lungs; other subdivisions are the medi- 

 astinal, 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. 



Development of the alveoli. Fig. 269, A, from an embryo of four 

 months, shows a portion of the lung adjacent to the pleura. The terminal 

 subdivisions of the bronchi are lodged in an abundant, vascular connective 

 tissue. They are lined with a simple cuboidal epithelium and are gland- 

 like in form. This appearance is retained until birth when they become 

 distended with air. Then their cuboidal cells are flattened, and many of 

 them are transformed into thin non-nucleated plates (Fig. 269, B). The 



FIG. 269. SECTIONS OF THE VISCERAL PLEURA, pi., AND ADJACENT ALVEOLI, al., FROM THE LUNG OF 



A FOUR MONTHS' EMBRYO, A, AND FROM AN ADULT, B. 



ar., Artery; b. v., blood vessel; cap., capillary; ly., lymphatic vessel; s., surface view 

 of alveolar wall; v., vein. 



connective tissue between the alveoli is compressed into strands scarcely 

 wider than the diameter of a capillary. In fact the capillaries which they 

 contain are in contact with the respiratory epithelium of both of the ad- 

 jacent alveoli. A section of the adult lung is essentially a network of these 

 slender partitions, scattered among which are islands of connective tissue 

 containing the bronchi and vessels. There are also connective tissue 

 septa, dividing the lung into lobules. 



Summary. The lungs develop as a branched entodermal gland with 

 the trachea and bronchi as its ducts. The terminal alveoli become greatly 

 distended and their cells form flat plates adapted for respiration but not 

 for secretion. The lungs have two sets of blood vessels, both capillary 

 in type, the pulmonary and the bronchial vessels. The connective tissue 

 forms a peripheral layer which is part of the pleura, and a large mass at the 



