392 



RESPIRATION. 



[CHAP. xxix. 



Fig. 206. 



Bronchial termination in the 



between neighbouring passages, and even between contiguous 

 lobules. 



The diameter of the lobular passages is from T -^ to -^fo of an 

 inch; and that of the cells from -^fa to 

 -g-i-o of an inch, according to our measure- 

 ments. In a preparation of the lung of 

 the calf, given us by our friend Professor 

 Retzius, they measure -Q^-Q', and Dr. W. 

 Addison makes them from -^^ to 3^ of an 

 inch. 



The skeleton of the lobule is thus an 



bu"frpa s sa|e) d br s anching towards elastic membrane, elaborately arranged, so 

 that the air may be brought into contact 

 with it, for the most part, on both its 

 surfaces. Over this membrane the whole 



venous blood of the body is made to course in a continual stream, 

 so as to be submitted to the action of the air. 



Vascular Element. The pulmonary artery, conveying the venous 

 blood to the lungs, is about as large as the aorta, and is furnished 

 with a triple valve at its origin from the right ventricle of the 

 heart. It soon divides into a right and left branch, which enter 

 the lungs at their root, and ramify as far as the lobules, in company 

 with the bronchia. Arrived at the lobules, the small branches of 

 the pulmonary artery do not enter in company with the lobular 

 passages, but distribute themselves over the lobules in the inter- 

 lobular fissures, penetrating at various points between the air- 

 cells, and occupying tubular channels in the angles where three or 

 more cells meet. These channels are formed of the same yellow 

 elastic tissue which constitutes the lobular passages and air-cells, 

 and their wall blends with the proper coat of the terminal twigs of 

 the pulmonary artery which occupy them. The capillaries of the 

 lobule are given off both from the twigs which meander over the 

 lobule, and from those which penetrate it, and they form a network 

 which covers the walls of all the cells of the lobule, as well as of the 

 lobular passages, anastomosing with some twigs of the bronchial 

 arteries, where the passages are continuous with the terminal 

 bronchia. This network empties its blood into venules which lie 

 first in intercellular channels, similar but intermediate to those 

 which lodge the arterial terminations ; and these venules collecting 

 the now aerated blood from the interior, and also from the surface 

 of the lobules, converge to larger veins, which lie in the interlobular 

 spaces, and tend towards the root of the lungs, but for the most 



