3<Ja DISEASES Class IV. i. i, 



ted with each other reciprocally, as being all of them excited 

 by the fenforial power of aflbciation 5 but they are only catenated 

 with thofe of the ftomach, which are not in this cafe aflbciate 

 motions but irritative ones. The common ufe of the word af- 

 lbciation for almoft every kind of connection has rendered this 

 fubject difficult -, from which inaccuracy I fear fome parts of this 

 work are not exempt. 



B. Jjfociate Motions of three Kinds* 



Thofe trains or tribes of aflbciate motions, the introductory 

 links of which confifts of an irritative motion, are termed irrita*- 

 tive aflbciations 5 as when the mufcles of the eyelids clofe the 

 eve in common nictitation. Thofe, whofe introductory link 

 confifts of a fenfitive motion, are termed fenfitive aflbciations 5 as 

 when the pectoral and intercoftal mufcles act in fneezing. 

 And laftly, thofe whofe introductory link confifts of a voluntary 

 motion, are termed voluntary aflbciations ; as when the mufcles 

 of the lower limbs act in concert with thofe of the arm in fencing. 



C. Affociattotts affecled by external Influences* 



Circles of aflbciate ' motions, as well as trains and tribes of 

 them, are liable to be affected by external influences, which con- 

 fift of ethereal fluids, and which, by penetrating the fyftem, act 

 upon it perhaps rather as a caufa fine qua non of its movements, 

 than directly as a hamulus ; except when they are accumulated 

 in unufual quantity. We have a fenfe adapted to the perception 

 of the excefs or defect of one of thefe fluids ; I mean that of 

 elementary heat ; in which all things are immerfed. See Clafs 

 IV. 1 4. 1. But there are others of them, which as we have 

 no power to evade their influence, fo we have no fenfe to per~ 

 ceive it •, thefe are the folar, and lunar, and terreftrial gravitation } 

 in which alfo all things are immerfed 5 the electric aura, which 

 pervades us, and is perpetually varying. See Clafs IV. 1. 4. 

 5 ; the magnetic fluid, Clafs IV. 1. 4. 6 \ and laftly, the great 

 life-preferver oxygen gas, and the aqueous vapour of the atmos- 

 phere, fee Clafs IV. 1. 4. 6. and 7. and 2. 



Of thefe external influences thofe of heat, and of gravity, 

 have diurnal periods of increafe and decreafe \ befides their 

 greater periods of monthly or annual variation. The manner 

 in which they act by periodical increments on the fyftem, till 

 Jfome effect is produced, is fpoken of in Sect. XXXII. 3. and 6 r 



D« AJfici attorn 



