360 DISEASES CLASS IV. 1. 1. 



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

 the sensorial power of association; but they are only catenated 

 with those of the stomach, which are not in this case associate 

 motions but irritative ones. The common use of the word as- 

 sociation for almost every kind of connection has rendered this 

 subject difficult; from which inaccuracy I fear some parts of this 

 work are not exempt. 



B. Jlssociate Motions of three Kinds. 



Those trains or tribes of associate motions, the introductory 

 link of which consists of an irritative motion, are termed irrita- 

 tive associations; as when the muscles of the eyelids close the 

 eye in common nictitation. Those whose introductory link con- 

 sists of a sensitive motion, are termed sensitive associations; as 

 when the pectoral and intercostal muscles act in sneezing. 

 And lastly, those whose introductory link consists of a voluntary 

 motion, are termed voluntary associations; as when the muscles 

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



C. Associations affected by external Influences. 



Circles of associate motions, as well as trains and tribes of 

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

 sist of ethereal fluids, and which, by penetrating the system, act 

 upon it perhaps rather as a causa sine qua non of its movements, 

 than directly as a stimulus; except when they are accumulated 

 in unusual quantity. We have a sense adapted to the perception 

 of the excess or defect of one of these fluids; I mean that of 

 elementary heat; in which all things are immersed. See Class 

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

 no power to evade their influence, so we have no sense to per- 

 ceive it; these are the solar, and lunar, and terrestrial gravitation, 

 in which also all things are immersed; the electric aura, which 

 pervades us, and is perpetually varying. See Class IV. 1.4. 

 5; the magnetic fluid, Class IV. 1, 4. 6; and lastly, the great 

 life-preserver oxygen gas, and the aqueous vapour of the atmo- 

 sphere. See Class IV. 1. 4. 6. and 7. and 2. 



Of these external influences those of heat, and of gravity, 

 have diurnal periods of increase and decrease; besides their 

 greater periods of monthly or annual variation. The manner 

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

 some effect is produced, is spoken of in Sect. XXXII. 3. and 6. 



