SI/P. I. 1 6. 2. THEORY OF FEVER. 499 



principally actuated by the fenforial powers of irritation, and of 

 aflbciation ; but in fome degree or at fome times by thofe of 

 fenfation, and even of volition. There are three kinds of ftim- 

 ulus, which may eafily be occafionally diminifhed, that of heat 

 on the fkin, of food in the ftomach, and of the oxygenous part 

 of the atmofphere, which mixes with the blood in refpiration, 

 and ftimulates the heart and arteries. 



2. When any parts, which are naturally excited into perpetu- 

 aj action by ftimulus, become torpid orlefs active from decreafe 

 of that ftimulus ; there firft occurs a decreafe of the activity of 

 the parts next catenated with them j thus going into cold water, 

 produces a torpor of the capillary veflels of the lungs, as is 

 known by the difficult refpiration, which immediately occurs ; 

 for the fenforial power of aflbciation, which naturally contributes 

 to actuate the lungs, is now leis excited by the decreafed actions 

 of the cutaneous veflels, with which they are catenated. This 

 conftitutes the cold fit of fever. 



There next occurs an accumulation of the fenforial power of 

 irritation in the parts, which were torpid from defect of ftimu- 

 lus, as the cutaneous veflels for inftance when expofed to cold 

 air ; and a fimilar accumulation of the fenforial power of aflbf 

 ciation occurs in the parts which were catenated with the for- 

 mer, as the veflels of the lungs in the example above mentioned. 

 Whence, if the fubduction of ftimulus has not been too great, 

 fo as to impair the health of the part, the activity of the irrita- 

 tive motions returns, even though the ftimulus continues lefs 

 than ufual ; and thofe of the aflbciate motions become confid- 

 erably increafed, becaufe thefe latter are now excited by the pre- 

 vious fibrous motions, which now act as ftrong or ftronger than 

 formerly, and have alfo acquired an accumulation of the fenfo- 

 rial power of aflbciation. This accounts for the curious event 

 of our becoming warm in a minute or two after remaining in 

 water of about 80 degrees of heat, as in the bath at Buxton , or 

 in the cold air of a frofty morning of about 30 degrees of heat. 



But if the parts thus poflefled of the accumulated fenforial 

 powers of irritation and of aflbciation be expofed again to their 

 natural quantity of ftimulus, a great excefs of activity fuper- 

 venes ; becaufe the fibres, which poflefs accumulated irritation, 

 are now excited by their ufual quantity of ftimulus ; and thofe 

 which poflefs accumulated aflbciation, are now excited by double 

 or treble the quantity of the preceding irritative fibrous mo- 

 tions, with which they are catenated ; this conftitutes the hot 

 fit of fever. 



Another important circumftance occurs, when the parts, 

 which are torpid from decreafed ftimulus, do not accumulate a 



quantity 



