ON THE EXTERNAL CAUSES OF DISEASE. 
369 . 
arterial and venal circulation in the lungs, the blood being car¬ 
ried no farther than the small arterial branches surrounding the 
air-cells. 
Sir E. Home’s description of a single cell, as it appeared on 
the field of the microscope, is as follows :—When the internal 
cavity of a single cell was exposed, immediately behind its inter¬ 
nal membrane, the branches of the pulmonary artery, injected 
with red wax, w'ere seen ramifying, as arteries do in common; 
these w^ere accompanied by branches of the pulmonary veins, 
larger in proportion than those of the arteries, more numerous, 
and having valves, at apparently regular intervals, to prevent 
regurgitation of their contents. Besides the arteries and veins, 
there were innumerable absorbents opening into the cavity of the 
cell; their valves were at very short distances, and in their 
course in the interstitial substance between the cells, they accom¬ 
panied the veins. When the terminal branches of the pulmo¬ 
nary artery were traced, the injection was found to have stop¬ 
ped someway before the artery’s termination, and the space be¬ 
yond w’as filled with gas. The substance of the lungs, intersti¬ 
tial to the cells, when dried become transparent, and was found 
to be composed of a smaller order of cells, with transparent co¬ 
verings that freely communicated with one another, as well as 
with the cavity of the large cell they surrounded.” 
From this account of the anatomical structure of the lungs, it 
is evident they are calculated not only to receive supplies from 
the atmosphere, but to convey a part of them, wdth the greatest 
rapidity as well as facility, to the heart; since the momentary 
interruption to the passage of the blood from the arteries to the 
veins, and the numerous valves in the absorbents, as well as those 
in the veins, are admirably fitted for that purpose. 
As the object of this paper is not so much to explain the 
phenomena of respiration as it is intended to illustrate our sub¬ 
ject, viz. that in diseases arising from aerial poison the blood 
may be contaminated by the poison being attracted immediately 
into the circulatory current, along with the atmospheric air, we 
have refrained from entering; more minutely into this interestinor 
subject. 
We have seen that not only oxygen (the vivifying portion 
of the atmosphere) but that azote likewise is conveyed into the 
mass of the blood ; and although we are wholly ignorant of its 
real use, yet the quantity of this gas that is consumed by respi¬ 
ration, wdien compared wdth its absolute amount, is so trifling, 
that one should think it must operate upon the well-being 
of life, in a way, and to an extent, that are not yet understood. 
