SECT. XXXIV. i..a. OF VOLITION. 32 p 



eye, pain and inflammation and heat are produced from the ex- 

 cefs of ftimulus ; but violent hunger, hemicrania, or the clavus 

 hyftericus, are attended with coldnefs of the extremities, and de- 

 fe& of circulation. When we are expofed to great cold, the 

 pain we experience from the deficiency of heat is attended with 

 a quiefcence of the motions of the vafcular fyftem ; fo that no 

 inflammation is produced, but a great defire of heat, and a trem- 

 ulous motion of the fubcutaneous mufcles, which is properly a 

 convulfion in confequence of this pain from defect of the (timu- 

 lus of heat. 



It was before mentioned, that as fenfation confifts in certain 

 movements of the fenforium, beginning at fome of the extremi- 

 ties of it, and propagated to the central parts of it j fo volition 

 confifts of certain other movements of the fenforium, commenc- 

 ing in the central parts of it, and propagated to fome of its extrem- 

 ities. This idea of thefe two great powers of motion in the ani- 

 mal machine is confirmed from observing, that they never exiit 

 in a great degree or univerfally at the fame time ; for while we 

 firongly exert our voluntary motions, we ceafe to feel the pains 

 or uneafmefles, which occafioned us to exert them. 



Hence during the time of fighting with fifts or fwords no 

 pain is felt by the combatants, till they ceafe to exert themfelves. 

 Thus in the beginning of ague- fits the painful fenfation of cold 

 is dimiuiihed, while the patient exerts himfelf in the (hivering 

 and gnafhing of his teeth. He then ceafes to exert himfelf and 

 the pain of cold returns ; and he is thus perpetually induced 

 to reiterate thefe exertions, from which he experiences a tem- 

 porary relief. The fame occurs in labour- pains, the exertion of 

 the parturient woman relieves the violence of the pains for a 

 time, which recur again foon after fhe has ceafed to ufe thofe 

 exertions. The fame is true in many other painful difeafes, as 

 in the ttrangury, tenefmus, and the efforts of vomiting j all 

 thefe difagreeable fenfations are diminifhed or removed for a 

 time by the various exertions they occafion, and recur alter- 

 nately with thofe exertions 



The re (lie (The fs in fome fevers is an almoft perpetual exertion 

 of this kind, excited to relieve fome difagreeable fenfations ; the 

 reciprocal oppofite exertions of a wounded worm, the alternate 

 emproithotonos and opifthotonos of fome fpafmodic difeafes, and 

 the intervals of all convulfions, from whatever caufe, feem to be 

 owing to this circumftanceof the laws of animation ; that great 

 or univerfal exertion cannot exift at the fame time with great 

 or univerfal fenfation, though they can exifl reciprocally ; which 

 is probably refolvable into the more general law, that the whole 

 fenforial power being expended in one mode of exertion, there 



VOL. I. T T is 



