in relation to Disease, 37 



to be dealt with as such. Each age, too, has its changes in 

 this respect, as well as every condition of health ; and precau- 

 tions founded on them cannot expediently be neglected, pro- 

 vided they are not so minute as to interfere with other parts of 

 the economy of life, equally essential to the welfare of the whole. 

 In a brief outline like this, it is needless to particularise in- 

 stances. They are familiar to common remark ; cited in me- 

 dical works (though not always so specifically as the subject 

 requires) ; and are very striking in the more extreme cases, 

 where the struggle between the agency from without, and the 

 powers of resistance from within, is most strongly marked. 

 For we must ever revert to those great provisions in the con- 

 stitution against all extreme or sudden changes of external 

 temperature, by the laws which govern the production of ani- 

 mal heat ; the action of the exhalants of the lungs and skin %. 

 and possibly also the secretions of other organs.* No correct 

 results can be obtained as to the agency of heat and cold upon 

 the body, without keeping these powers of balance constantly in 

 view ; and as they again are perpetually undergoing modifica- 

 tions from the various conditions of life, so is there a circle of 

 relations, tending altogether to equality of average, though 

 greatly broken and interrupted in its several parts. 



2. Moisture of the Atmosphere. — We have no evidences of 

 equal provision, as respects the second of the general condi- 

 tions of the atmosphere ; viz., its hygrometrical state. t But, 

 on the other hand, there is every reason to infer that no simi- 

 lar need exists for it. The simple agency upon the body of dry 

 or humid air, is doubtless much more limited in every sense 

 than that of heat and cold ; — restricted, as far as we can see, 

 to certain organs, and less powerful in its influence on these. 

 It is still more difficult also to detach it in observation from 

 the influence of other causes. Sudden and considerable changes 



* Taking the record, seemingly well authenticated, of the two extremes 

 of temperature of the human body, as determined by diseases affecting the 

 blood, we find them to include a range of nearly 40° of Fahrenheit. 



t Unless, indeed, we admit as partially and indirectly such, the apparent 

 relation between perspiration and absorption ; the latter process balancing, 

 by its increase or diminution, any changes the former may undergo from the 

 different conditions of the atmosphere as to moisture or dryness. 



