THE LUNGS. 



161 



Fig. 148. — A pulmonary alveolus of the calf; 

 a, larger blood-vessels, which run in the alve- 

 olar septa ; b, capillary net-work ; c, epithe- 

 lial cells. 



incomplete or more complete rings (Fig. 148, a). From them 

 arises an uncommonly close capillary net-work of tubes 

 0.0056 to 0.0113 mm. wide, 

 which are scarcely separated 

 from the atmospheric air by the 

 thin membrane of the alveolar 

 walls (b). The respiratory in- 

 terchange of gases takes place 

 here. These capillaries appear 

 elongated when the lung vesi- 

 cles are strongly expanded. 

 When less expanded they pro- 

 ject, in a tendril-like manner, 

 into the cavity, reminding us 

 of a relative condition in the 

 muscles. The pulmonary veins 



commence with small branches in the interalveolar septa. 

 Gradually combining into larger trunks, they accompany the 

 ramifications of the bronchia and the divisions of the pulmo- 

 nary arteries. 



The bronchial arteries are regarded as the nutritive vessels 

 of the respiratory organ, but there is no very sharp demar- 

 cation between them and the respiratory pulmonary arteries. 



The former supply the walls of the larger blood-vessels, the 

 adjacent lymphatic glands, the connective tissue between the 

 pulmonary lobules and beneath the pleura. Finally, they 

 form the capillary net-works of the various parietal layers of 

 the efferent bronchial system ; but the most superficial net- 

 work of the mucous membrane arises, in a peculiar manner, 

 from the respiratory system of vessels. 



The bronchial veins appear to be quite peculiar. They are 

 conjectured to be only the reflux vessels of the arterial 

 branches from the larger bronchial ramifications, from the 

 lymphatic glands and from the pleura nearest the hilus of the 

 lungs. The venous roots from the walls of the finer bronchi 

 pass, on the contrary, into the respiratory pulmonary veins. 



The lungs are rich in lymphatics, beneath the pleura as well 



