STRUCTURE OF THE AIR-PASSAGES AND THE LUNGS. 203 



considerable power of resistance, they are often found retaining their characteristic 

 arrangement in the expectoration of patients suffering from pulmonary diseases. 

 This is an infallible sign that the pulmonary tissue is undergoing destruction. (3) 

 The branches of the rich capillary network pass rather toward the lumen of the 

 alveoli. The respiratory epithelium of the alveoli is a single layer of squamous 

 epithelium. In it may be found scattered nucleated, protoplasmic cells (i) , which 

 are transformed later into small (from 7 to 15 //), non-nucleated, bright (2) or 

 dark platelets. Finally, several of the latter unite to form larger (from 22 to 45 //) , 

 non-nucleated plates. (3) Here and there incomplete fissures may be seen in 

 these plates, which indicate previous interspaces between the platelets. The plates 

 have been transformed from original cuboidal cells by the stretching of the lungs 

 during respiration. 



See estimates the number of alveoli at 809^ millions, and their respiratory 

 area at 81 square meters (54 times as great as the surface of the body). The 

 alveoli are grouped together by connective tissue into distinct pulmonary lobules. 



The blood-vessels of the lungs belong to two distinct systems : 



A. The system of the pulmonary vessels (the lesser circulation) . The branches 

 of the pulmonary artery follow those of the air-passages, and are so closely applied 

 to the latter that their pulsations may be communicated to the contained air. 

 The capillaries arising from these branches form a rich network of moderately 

 fine tubules. The pulmonary veins, whose branches likewise accompany the air- 

 passages, are collectively narrower than the pulmonary artery, as a result of the 

 loss of water that the blood undergoes in the lungs. 



B. The system of the bronchial vessels conveys the nutrient material for the 

 respiratory organs. The bronchial arteries, following the bronchi, give to them 

 branches, as well as to the lymphatic glands at the hilus of the lungs, the large 

 trunks of the pulmonary vessels (vasa vasorum), and the pulmonary pleura. 

 Numerous anastomoses occur between the branches of the bronchial and pul- 

 monary arteries. Part of the vessels arising from the capillaries communicate 

 with the beginnings of the pulmonary veins ; and for this reason any considerable 

 stagnation of blood in the lesser circulation causes a like stagnation in the circula- 

 tion in the bronchial mucous membrane, with resulting bronchial catarrh. An- 

 other part of the bronchial capillaries forms special veins, which, as bronchial veins, 

 traverse the posterior mediastinum, and empty into the trunks of the azygos 

 veins, the intercostal veins, or the superior vena cava. The veins from the smaller 

 bronchi, and even from the bronchi of the fourth class, empty collectively into 

 the pulmonary veins ; and the anterior bronchial veins also communicate with the 

 pulmonary vessels. 



The interstitial tissue of the lungs is rich in lymphadenoid tissue and is 

 traversed by a network of fine lymph-channels. A coarser, irregular system of 

 lymph- vessels surrounds the pulmonary lobules, larger bronchi, and blood-vessels. 

 These lymph-channels and vessels become injected when animals are made to 

 inhale powdered, soluble dyes. The coloring-matter penetrates the viscid inter- 

 stitial substance between the epithelium, though according to Klein through small 

 pores that are present. 



In the walls of the pulmonary alveoli the finest lymph-tubules form a delicate 

 system of canals lying in the spaces between the blood-capillaries. These canals 

 exhibit enlargements at the points of intersection. Lymph- vessels extend along 

 the bronchi, forming a dense, longitudinally meshed network in the mucosa and 

 submucosa, and finally reaching the lymphatic glands at the roots of the lungs. 



The rapidity with which fluids are absorbed in the lungs, even when introduced 

 in considerable quantities, is remarkable. Landois has often seen this after in- 

 jecting water into the trachea of living animals, and Peiper has demonstrated it 

 for many other substances. Even blood is taken up in like manner, Nothnagel 

 having found blood-corpuscles in the interstitial pulmonary tissue from three to 

 five minutes after injection into the trachea. 



In the pulmonary pleura, which is exceedingly rich in elastic fibers, the net- 

 works of superficial pulmonary lymph-vessels begin as free stomata. In like 

 manner the lymph-vessels of the parietal pleura communicate by means of sto- 

 mata in many places (on the diaphragm only in certain localities) with the pleural 

 cavity; according to Klein even with the free surface of the bronchial mucous 

 membrane. The lymph, vessels of the veins of the lesser circulation lie between 

 the media and the adventitia. 



The nerves of the lungs, bronchi, trachea, and larynx have ganglia. 



It appears that the function of the unstriated muscle-fibers in the trachea and 



