ON THE EXTERNAL CAUSES OF DISEASE. 
The experiments of Spallanzi render it certain that reptiles 
absorb azote; and Humbolt and Provencal have ascertained 
that the same effect is produced by fishes. 
The experiments of Allen and Pepys, to determine the change 
produced in the air by respiration, form a valuable confirmation 
to those already stated. These experiments were made solely on 
the respiration of birds. After one hour and twelve minutes re¬ 
spiration, the amount of gases employed being originally 
Oxygen. Azote. Carb. Acid, 
Cubic inches.... 245.59 . 61.45 - 
There remained.. 195.61 . 90.11 . 21.27 
shewing a loss of oxygen beyond the volume converted into car¬ 
bonic acid, and a gain of 28.70 of azote. 
From what has been adduced, it appears sufficiently obvious 
that the atmosperic air is received into the circulating current; 
but the mode and channel by which it is conveyed still re¬ 
mains to be explained. For, if the explanation of the phe¬ 
nomena rests on chemical principles, those principles cannot be 
depended upon unless they accord, in all respects, with the ana¬ 
tomy and physiology of the lungs, by which the assumed process 
takes place. 
The ramifications of the pulmonary vessels are described by 
Percivall and others, as accompanying those of the bronchii, 
and, like them, divide and subdivide, grow’ smaller and augment 
in number as they approach the air-cells, upon the internal sur¬ 
face of which they terminate in extremely delicate, thin, and 
transparent capillary tubes. From this description it is evident 
that no immediate contact can take place between the air and 
the blood; for the thin transparent side of the vessel, if not that 
of the air-cell itself, must ever be interposed ; so that, whatever 
this influence be, it must take place through these membranes. 
We now beg to refer our readers for an explanation of the 
phenomena to Sir E. Home’s and Bauer’s examination into the 
structure of the cells of the human lungs, with a view to ascer¬ 
tain the office they perform in respiration*. The first new fact 
discovered in the course of this inquiry w^as, that although the 
common minute injection used by anatomists for filling the 
bloodvessels, when throwm in by the trunk of the pulmonary 
artery, while the cells of the lungs are empty, returns again by 
the trunks of the pulmonary veins ; yet, w'hen throwm in by 
the veins, it is not returned by the trunks of the arteries. 
Another fact w’as discovered—that, during the moinentary dis¬ 
tention of the air-cells, an interruption is produced between the 
* Philobuphical TransHcfioiis, 182 ?. 
