SECT. XXXIV. i. DISEASES OF VOLITION. 481 



foe divided into thofe from excefs of motion, and 

 thofe from deleft of motion ; which diftin&ion is 

 of great importance in the knowledge and the cure 

 of many diieafes. For as the pains from excefs of 

 motion either gradually fublide, or are in general 

 fucceeded by inflammation ; fo thofe from deredt of 

 motion either gradually fubfide, or are in general 

 fucceeded by convulfion, or madnefs. Thefe pains 

 are eaiily diftinguifhable from each other by this 

 circumftance, that the former are atr^n Jed with heat 

 of the pained part, or of the whole body; whereas 

 the latter exifts without increafe of heat in the 

 pained part, and is generally attended with coldnefs 

 of the extremities of the body ; which is the true 

 criterion of what have been called nervous pains. 



Thus when any acrid material, as muff or lime, 

 falls into the eye, pain and inflammation and heat 

 are produced from the excefs of ftimulus ; but vio- 

 lent hunger, hemicrania, or the clavus hyftericus, 

 are attended with coldnefs of the extremities, and 

 defecl: of circulation. When we are expofed to 

 great cold, the pain we experience from the defi- 

 ciency of heat is attended with a quiefcence of the 

 motions of the vafcular fyftem ; fo that no inflam- 

 mation is produced, but a great defire of heat, and 

 a tremulous motion of the fubcutaneous mufcles, 

 which is properly a convulfion in confequence of 

 this pain from defecl: of the itimulus of heat* 



It was before mentioned, that as fenfation con- 

 fifts in certain movements of the fenforium, beginr 

 ning at fome of the extremities of it, and propa- 

 gated to the central parts of it ; fo volition con- 

 fids of certain other movements of the fenforium, 

 commencing in the central parts of it, and propa- 

 gated to fome of its extremities. This idea of 

 thefe two great powers of motion in the animal 

 machine is confirmed from obferving, that they ne- 

 ver 



