PHYSIOLOGY. 137 



nervous fluid suffers any change, but what is exactly correspond- 

 ent to its state in the brain and extremities ; and therefore the 

 only difference of the state of the nerves to be taken notice of, 

 is, their being more or less free (XXX.). 



C XXXV II I. In the sentient extremities of the nerves 

 (XXIX. 3.) a difference of the state of the nervous fluid arises 

 from the several causes mentioned (LVI. 2, 3, 4, and 5.), which 

 give a different degree of sensibility ; and it is probable, that 

 these different states of the sentient extremities are analogous 

 to the different degrees of excitement in the brain. 



C XXXIX. The moving extremities, or muscular fibres 

 (XXIX. 4.), may also be in a different condition with respect 

 to excitement. It is probable that their constitution is such 

 as to admit of a higher degree of excitement than any other por- 

 tion of the nervous system ; and that upon this their contractil- 

 ity depends. But, whatever is in this, we perceive very clear- 

 ly, that the condition of muscular fibres may be varied by 

 causes affecting their tonic power (CI.), or their vigour and 

 mobility (LXXXIX.), and by the effect of custom (CXIV.); 

 and it is probable, that the states produced by these causes are 

 analogous to the different degrees of excitement in the brain 

 (CXXX.), and in the sentient extremities (CXXXVIIL); 

 and thus the several parts of the nervous system (XXIX.), as 

 they consist of the same kind of matter (XCIV.), are also sub- 

 ject to similar conditions. 



CXL. The beginning of motion in the nervous system is 

 most commonly accompanied with sensation, and the force of 

 this, in producing its several effects, is more or less, 1 . Accord- 

 ing to the force (XLIL), quality (LXXVIL), and novelty 

 (LIV.) of impression. 2. According to the sensibility of the 

 sentient extremity and brain (LVI.)'. 3. According to the state 

 of attention (XL VI I .). These several causes often concur, fre- 

 quently balance one another, and are always to be considered 

 together. " If a forcible impression is made upon a very sensi- 

 ble system, the effect is considerable. If the impression is less 

 forcible, and the system less sensible, the effects will be much 

 less. A weaker impression upon a very sensible system, will 

 have considerable effect ; and a powerful one upon a less sen- 

 sible system, may have very little." 



