THE RESPIRATORY TRACT. 263 



of the bronchi as far as the lobules. The lobular branches 

 are terminal arteries i.e., they do not anastomose with each 

 other. They break up into very small branches, which encircle 

 the alveoli and supply the capillary plexuses of their walls. 

 These capillaries are very small, and the network so fine that, 

 when injected, the open spaces are not as wide as the vessels 

 themselves. This, however, will vary with the degree of dis- 

 tention of the lung. Between two adjacent alveoli only one 



Pro. 113. Section of human lung injected through the pulmonary artery : a, a, free alveolar margins ; 

 6, small arterial branch ; c, c, alveolar walls seen in transverse section. F. E. Schulze. 



capillary plexus is found, the branches of which are seen to 

 pursue an undulating course, projecting, first, into the cavity 

 on one side, then into that an the other. These unite again 

 into veins which run irregularly through the lobules to unite 

 upon the bronchi and follow their course to the root of the 

 lung. The peculiarities of the pulmonary veins are, 1st, that 

 their united calibre does not exceed (if it equals) that of the 

 arteries ; 2d, that they have no valves. The bronchial vessels 

 supply the coats of the bronchial tubes and the surrounding 

 connective tissue and the pulmonary pleura. 



But the line of demarcation between the bronchial and pul- 

 monary circulations is indistinct on the venous side, as injec- 

 tions thrown into the bronchial arteries fill the pulmonary 



