THE RESPIRATORY SYSTEM 321 



respiratory epithelial plates; and (3) the delicate basement membrane 

 upon which the respiratory epithelium rests together with an extremely 

 small amount of fibrous and elastic tissues (see diagram, Fig. 219.) 



The veins begin as small radicles, one from the base of each alve- 

 olus (Fig. 2 13). These empty into small veins at the periphery of the 

 lobule. These veins at first run in the interlobular connective tissue 

 away from the artery and bronchus. Later they empty into the 

 large pulmonary trunks which accompany the bronchi. 



The broncJilal arteries break up into capillary networks in the walls 

 of the bronchi, supplying them as far as their respiratory divisions, 

 beyond which point the capillaries belong to the pulmonary system. 

 The bronchial arteries supply the walls of the bronchi, the bronchial 

 lymph nodes, the walls of the pulmonary vessels, and the pulmonary 

 pleura. Of the bronchial capillaries some empty into the bronchial 

 veins, others into the pulmonary veins. 



b 



Blood ■ C 



Fig. 21Q. — Diagram of Tissues Interposed Between Blood and Air in Alveolus, a, 

 Respiratory epithelium; b, fibro-elastic tissue; c, endothelium of capillary. As b does 

 not form a continuous membrane, the capillary wall is in many places in direct apposi- 

 tion with the respiratory epithelium, so that only two layers, a and b, are interposed 

 between blood and air. 



Lymphatics. — The lymphatics of the lung begin as small lymph 

 spaces in the interalveolar connective tissue. These communicate 

 with larger lymph channels in the interlobular septa. Some of these 

 empty into the deep pulmonary lymphatics, which follow the pul- 

 monary vessels to the lymph glands at the root of the lung. Others 

 empty into the superficial pulmonary lymphatics, which form an 

 extensive subpleural plexus connected with small subpleural lymph 

 nodes, whence by means of several larger vessels the lymph is 

 carried to the lymph nodes at the hilum. 



Nerves. — ^Bundlesof medullated and non-medullated fibres accom- 

 pany the bronchial arteries and veins. Small sympathetic ganglia 

 are distributed along these nerves. The fibres form plexuses in the 

 fibrous layer of the bronchi, from which terminals pass to the muscle 

 of the bronchi and of the vessel walls and to the mucosa. Free end- 

 ings upon the epithelium of bronchi, air passages, and alveoli have 

 been described. 



Development of the Respiratory System 



The epithelium of the respiratory system develops from entoderm, the con- 

 nective-tissue elements from mesoderm. The first differentiation of respiratory 



