PHYSIOLOGY. 83 



which they could have exerted in their ordinary health, and that 

 not from a more violent passion or emotion of mind, but in every 

 trifling exertion ; even when in a calm and placid state their 

 strength is greater, and it continues greater, when various means, 

 as cold, hunger, and evacuations have been employed to weaken 

 it ; and this can be referred to nothing but the brain, the seat 

 of the disease. Thus, too, we see that fear, a general timidity, 

 very much weakens the tone of every part ; the muscles are so 

 much enervated as to lose manifestly their ordinary powers. 



" I conclude with suggesting a little piece of theory in con- 

 firmation of the whole. If I have insinuated that the nervous 

 power is an elastic fluid, in common to the whole of the nervous 

 system, such is the nature of elastic fluids, that they will con- 

 stantly endeavour to restore themselves to an equilibrium ; wher- 

 ever there is a communication, every part constantly presses 

 upon another, so that there must be a constant energy from 

 each part, and more especially from the brain. But this is 

 enough to prove that the inherent power will be, in all cases, in 

 some measure in proportion to the state of the nervous and 

 animal powers, and here is the application of it." 



C V. If the tonic power of any muscular fibre depends more 

 upon its state of tension (CIII.) than upon the state of the 

 nervous and animal powers (CIV.), such fibres will be more 

 affected by changes of the state of tension, than by changes in 

 the state of the nervous and animal powers ; and on the con- 

 trary, &c. 



" I have brought it to this, that the tonic power may depend 

 upon two causes, which are to be considered separate : the one 

 is merely the tension of the fibre, the other a certain power de- 

 rived from the brain or animal power. These two general- 

 ly concur ; but they may be separate, and in different degrees. 

 And in considering the causes of increase or diminution of 

 tonic power, we may refer them partly to the one and partly to 

 the other, as some depend more upon the state of tension, 

 others more upon the state of the animal power. 1 " 



CVI. The force of contraction, or the vigour of muscular 

 fibres, will be always as the force of stimulus, and the vigour of 

 the animal, nervous, and inherent powers taken together. 



CVII. The mobility of muscular fibres (LXXXIX.) seems 



F2 



