Sect. XXXIV. i. i. Ot VOLITION. 329 



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- 

 fect of circulation. When we are expOfed to great cold, the 

 pain we experience froni 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 

 convulfioh in confequence of this pain from defect of the ftimu- 

 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 ; fo volition 

 'confifts of certain other movements of the fenforiurn, commenc- 

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

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

 mal machine is confirmed from obferving, that they never exift 

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

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

 or uneafinefles, which occafioned us to exert them. 

 , Hence during the time of fighting with fifts or fwords no- 

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

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

 is diminifhed, while the patient exerts himfelf in the fhivering 

 and gnaihing 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 tempo- 

 rary 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 the has ceafed to life thofe 

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

 in the itrangury, tenefmus, and the efforts of vomiting ; all 

 thefe difagreeable fenfations are dimimihed or removed for a 

 time by the various exertions they occafion, and recur alternately 

 with thofe exertions. 



The reftlefihefs in fome fevers is an almoft perpetual exertion 

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

 reciprocal oppofite exertions of a wounded worm, the alternate 

 emproflhotonos and opiflhotonos of fome fpafmodic difeafes, and 

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

 owing to this circumftance of the laws of animation j that great 

 or univerfal exertion cannot exift at the fame time with great 

 or univerfal fenfation, though they can exift reciprocally •> which 

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

 fenforial power bein£ expended in ©ne mode of exertion^ ther^ 



Vol. I. Tx is 



