383 
ON THE EXTERNAL CAUSES 
OF DISEASE. 
the recent evolution of heat usually exceeds the previous abstiac¬ 
tion; hence moderate degrees of cold, quickly applied, prove in¬ 
vigorating, by calling forth the action of the caiorific powers; 
but, when it is either more intense, or applied for a longer time, 
the system becomes deprived of the due influence of that princi¬ 
ple which before conduced to the support of its inherent heat, 
notwithstanding respiration continues to be performed as usual, 
and the ordinary quantity of air continues to be taken into the 
lungs. Its operation in this case is sedative or debilitating. 
Whenever the whole or part of the body has been exposed to 
the sedative influence ot cold for a long period, to a great extent, 
it proves destructive to life. When local or general in a less de¬ 
gree, it proves the predisposing cause to various diseases of the 
active kind, determined in their seat by the particular predispo¬ 
sition of the animal. In this case the injury is produced by the 
change from cold to heat. When heat suddenly succeeds to 
cold, the blood is powerfully determined to the surface; and the 
more intense the preceding cold, and the longer its continuance, 
the greater is the accumulation of irritability, and the more vio¬ 
lent are the effects produced by sudden application of heat. 
We have hitherto spoken of cold as the predisposing cause of 
inflammation; the sudden change of cold to heat is therefore 
properly considered as the most pernicious agent in producing 
catarrhal affections; whilst the change from heat to cold is not 
generally .considered to be productive of much injury. c shall 
find, however, on examination, that the effect of currents of cold 
air is as pernicious as heated air. To enter largely into this 
subject would be foreign to the design of the present paper ; the 
following concise explanation will, we hope, be amply sufficient 
to afford a general idea. 
It has been observed that the action of cold upon the living 
system, when generally applied, unless it be extreme in 
degree, is not productive of any bad effects; for the animal 
frame, supported by its inherent heat, can bear a very consider¬ 
able diminution of temperature without injury : and it there¬ 
fore follows, that, if the succeeding change from cold to natural 
warmth were gradually and properly conducted, no injury could 
possibly happen. The popular opinion, that the mischief arising 
from change of temperature depends upon the sudden transi¬ 
tion from great degrees of heat to cold, is one not altogether 
founded on error.* Of this we are certain, that the greater the 
heat of the body, the more susceptible it is of the impression of 
cold; and when the application of cold is sudden, or when the 
body is immersed in water, the pores are strongly closed ; a con¬ 
striction takes place in the extreme vessels on the surface of the 
