364 EESPIEATION. 



The entire mass of venous blood is distributed in the lungs 

 by the pulmonary artery for the purposes of aeration. Arte- 

 rial blood is conveyed to these organs by the bronchial arte- 

 ries, which ramify and subdivide on the bronchial tubes, and 

 follow their course into the lungs, for the nourishment of 

 these parts. It is possible that the tissue of the lungs may 

 receive some nourishment from the blood conveyed there by 

 the pulmonary artery ; but as this vessel does not send any 

 branches to the bronchial tubes, it is undoubtedly the bron- 

 chial arteries which supply the material for their nutrition 

 and the secretion of the mucous glands. This is one of the 

 anatomical reasons why inflammatory conditions of the bron- 

 chial tubes do not extend to the parenchyma of the lungs, and 

 vice versa. 



The foregoing anatomical sketch shows the admirable 

 adaptation of the trachea and bronchial tubes to the pas- 

 sage of the air by inspiration to the deep portions of the 

 lungs, and the favorable conditions which it there meets with 

 for an interchange of the elements of the air and blood. It 

 is also evident, from the enormous number of air-cells, that 

 the respiratory surface must be immense. 1 



Carbonaceous Matter in the Lungs. The lungs of most 

 of the inferior animals and the human subject, in early life, 

 have a uniform rose tint ; but in the adult, and particularly 

 in old age, they contain a greater or less quantity of black 

 matter, which may exist in little masses, deposited here and 

 there in the pulmonary structure, or forming lines on the 



1 Hales estimated the combined surface of the air-cells at 289 square feet 

 (Statical Essays, voL i., p. 242) ; Keill at 21,906 square inches (Assays on Several 

 Parts of the Animal (Economy, p. 122); and Lieberkiihn at 1,500 square feet 

 (DDNGLISON'S Human Physiology, 1856, vol. i., p. 278). There are not sufficient 

 data on this point for us to form any thing like a reliable estimate. It is 

 simply evident that the extent of surface must be very great. In passing from 

 the lower to the higher orders of animals, it is seen that Nature provides for 

 the necessity of an increase in the activity of the respiratory process, by a dimin- 

 ished size and a multiplication of the air-cells. 



