266 
ON THE EXTERNAL CAUSES OF DISEASE. 
are past—and, as if struck by an arrow to the heart, down they 
fall motionless at our feet. 
We have aflso witnessed its effects on quadrupeds, of which 
we shall speak by and by. 
It is only within the last forty years that we have been able to 
investigate, with any degree of precision, the phenomena of re¬ 
spiration. That the blood sustained many changes, was observed 
by Harvey ; but his discoveries were incomplete, until connected 
with those of Priestly and Lavoisier ; and it is not a matter of 
surprise that there are still several questions relating to it yet im¬ 
perfectly understood. 
“ Causa latet, vis est iiotissiraa.” 
The cause is hid from our view, but the effect is known;— 
It is ever present,—ever active,—ever seen.” 
The wonders of the animal frame are beyond our ken, and they 
prove daily that there are more things in Heaven and Earth 
than are dreamt of in our philosophy.” 
“ Ye skilful masters of Machaon's race, 
AA’ho Nature’s mazy intricacies trace, 
And to siiblimer spheres of knowledge rise 
By manag'd fire and late-invented eyes, 
Tell, how your search has here eluded been.” 
At present philosophers have agreed only on two points:— 
first, the disappearance of a portion of oxygen;—second, the 
production of carbonic acid gas. Respecting the vivifying princi¬ 
ple that the blood receives from the atmospheric air, they are 
likewise agreed on; but a great difference of opinion exists re¬ 
specting the disappearance of the oxygen, and the manner in 
which the carbonic acid gas is formed :— 
“ Who shall decide when doctors disagree ?” 
“ Let us, howe'er, the theme as far pursue, 
As learn’d observers know', or think they do.” 
The commonly received and popular theory is, that the oxygen 
of the atmosphere enters through the air-cells of the lungs, 
into contact with the blood in the pulmonary vessels, unites with 
