STRUCTURE OF THE LUNGS OF MAMMALS. 61 



discover any muscular fibres in the more compact borders of 

 the individual alveolar septa.* 



The respiratory capillaries, the importance of which is so 

 great for the due performance of the function of the lungs, 

 are connected in a peculiar manner with the walls of the 

 alveoli. They proceed from the branches of the pulmonary 

 artery, which at first run with the bronchi imbedded in their 

 tunica adventitia and external fibrous layer, and subsequently 

 in the interstitial connective tissue of the lobuli and alveolar 

 passages, and conduct the blood, after it has become arterialized, 

 to the primary branches of the verne pulmonales which are 

 usually situated on the opposite side of each entire group of 

 alveoli. These venous trunks, in their recurrent course, then 

 usually accompany the pulmonary arteries, or more rarely run 

 separately through the tissue of the lungs. Where the alveolar 

 wall is surrounded by a compact layer of fibrous connective 

 tissue, as at the extreme boundaries of the several lobuli, 

 especially beneath the pleura, the rounded-angled, oval, or cir- 

 cular meshed plexus is so distributed in plane or flat arched 

 expansions on the inner surface of the connective- tissue wall, 

 that the capillaries only lie with a small portion, or at most 

 one half, of their diameter imbedded in the basement mem- 

 brane ; the remainder of their wall projecting into the lumen 

 of the alveolus. 



But where, as in the greater number of instances, the walls 

 of the adjacent alveoli have become fused into a thin mem- 

 brane, the originally double layer of capillaries, each lying on 

 the inner side of the membrane of their own alveolus, but now 

 brought into immediate contact with one another, combine to 

 form a single intricate plexus, owing to the development of in- 

 numerable anastomoses which traverse the common membrane, 

 the meshes of which are very narrow, not exceeding O'OOl of 

 a millimeter in Man, and but slightly narrower in smaller 



* Whilst most anatomical writers have, like myself, been unable to 

 discover smooth muscles in the alveolar walls, their presence has been 

 asserted and described by Gerlach, Gewebelehre, p. 248 ; Moleschott, in 

 his Untersuchungen, Band vi., p. 390 ; Colberg, De penitiore pulmonum 

 structura, Halis, 1863 ; Hirschmann, Yirchow's Arcliiv, Band xxxvi., 

 1866 ; and Piso-Borme, Moleschott's Untersuchungen, Band x., 1867. 



