STRUCTUEE OF THE LUNGS OF MAMMALS. 63 



the adult Man, when they are moderately distended, to 0*006 

 O'OOS of a millimeter. 



Each alveolus does not possess a separate and independent 

 artery and vein, but the capillary plexus proceeding from a 

 terminal arterial twig is usually distributed to several con- 

 tiguous alveoli before it gives origin to a vein, at the point of 

 transition of the terminal bronchia into its system of alveolar 

 passages. In the smaller bronchia themselves, and immediately 

 beneath the pleura, numerous anastomoses occur between the 

 capillaries of the pulmonary artery and those of the bronchial 

 arteries. 



The lymphatics of the alveoli, according to the observations 

 of Wywodzoff * on the lungs of Horses and Dogs, commence 

 in small anastomosing lacunse destitute of proper coats, 

 situated in the fibrous alveolar wall ; the larger trunks run in 

 the direction of the elastic fibres, and subsequently follow the 

 course of the capillaries, yet not so strictly but that they fre- 

 quently cross the latter, and form large lacunse in their meshes. 

 From these rootlets the deep-seated lymphatics f arise on the 

 one hand, which, accompanying the bronchi and vessels, extend 

 to the root of the lung ; and on the other the superficial lym- 

 phatics, which, lying close beneath the pleura, form a plexus 

 investing the extremities of the lobuli, and in Man partly run 

 towards the tubes, and partly dip in at certain points to com- 

 municate with the deep plexus. 



The inner surfaces of the alveoli, as well as the whole of the 

 infundibula and alveolar passages, lastly, are invested by a 

 continuous, and in the foetus homogeneous, epithelium. In 

 adult Mammals, however, it is not homogeneous. 



In the alveoli of mature foetuses a uniform layer of con- 

 tiguous fiat four to six-angled epithelial cells is found, each of 

 which possesses a membrane and granular contents, with a 

 clear spherical nucleus ; but in all individuals that have respired, 

 even for a short time, a few epithelial cells may be discovered 

 considerably enlarged and brighter, and with their granular 



* Wiener Medizinische Jahrbucher, Band xii., p. 1. 



t These also receive their lymph supply from the bronchia, and have 

 already been noticed. 



