264 
ON THE EXTERNAL CAUSES OF DISEASE. 
of the atmosphere was removed, the poison was not absorbed. 
Here, then, we have two subjects of inquiry : — 
l5^. The mode and channel by which aerial poison is imbibed 
by the absorbent vessels on the surfaces of the body. 
2d. The mode and channel by which it is attracted into the 
circulating current with the atmospheric air in the lungs. 
With respect to the first question, it will not be necessary to 
enlarge on it, as Mr. Vines will no doubt explain that process in 
The Veterinarian, in the continuation of his ‘^Series of Es¬ 
says on the Bloodvessels and Absorbents.” And, as it regards 
the second mode, we will endeavour to explain it as well as we 
are able. 
Healthy arterial blood contains a portion of pure air (not oxy¬ 
gen alone, as is generally taught), which it receives in the lungs, 
and this is diffused in the other ingredients which enter the cir¬ 
culation through the medium of the thoracic duct. 
Pure air is necessary for all the purposes of life. The blood 
requires it in a pure state, as much as the stomach requires pro¬ 
per and nutritious food ; or, in other words, the qualities of the 
air must be suited to the condition of the vital power in the 
lungs, as those of the food to the sensibility of the stomach. 
If the contrary takes place, disease in either instance must be 
the consequence. 
This will lead us briefly to consider the phenomena of respira¬ 
tion. 
By the action of the atmospheric air, the blood, which has 
been rendered impure and black in the extreme circulation, un¬ 
dergoes a remarkable change in the lungs, being converted into 
scarlet or arterial blood;— 
“ Its ros}' beauty gone ; 
Poor by expense, and pains with labour grown, 
Is in the lungs enrieh’d ; which reinspire 
The languid liquors, and restore their fire.” 
By the influence of the atmospheric air, the blood is fitted for 
all the functions of life; and it is a physiological fact, that, if the 
air is deprived of any of its constituent parts, the secretions be¬ 
come altered, and many of the functions, especially those of the 
