PHYSIOLOGY. 6 



place of application and the muscle to be moved, it is concluded 

 that the contraction of muscular fibres can be excited by a power 

 communicated to them by a motion propagated along the nerves. 

 This power is called the NERVOUS POWER. 



LXXXVIII. The nervous power (LXXXVII.) is com- 

 monly determined to motion by the will. This we suppose to 

 act in the brain only (XXXIII.), and to depend upon sensa- 

 tion, and other modifications of thought ; and this power, which 

 is to be chiefly referred to the mind, and acts in the brain only, 

 we name the ANIMAL POWER. " This term, animal power, has 

 not been common in our systems of physic ; but if it is true, as 

 we have alleged, that, in consequence of will or volition, a motion 

 is commenced in the brain, and that this happens in consequence 

 of other exertions of the nervous power, e. g. in consequence of 

 impressions made upon different parts of the system, which can 

 only act by the intervention of the brain ; if, in all the communi- 

 cations between the different parts, much depends upon the pre- 

 sent state of the brain, we will not be in any difficulty in lodg- 

 ing a particular power there, of which we shall speak under the 

 name of animal power, or energy of the brain. 



" There is a necessity for considering the contractility of 

 muscles as in part depending upon the inherent power, in part 

 upon the nerves, and in part upon the brain. Hence, the terms 

 inherent, nervous, and animal power."" 



LXXXIX. The facility with which the contraction of mus- 

 cular fibres can be excited, and the force with which it can be 

 performed, are to be distinguished. The first we name the MO- 

 BILITY, the last the VIGOUR of muscular fibres. Both have 

 been confounded under the name of Irritability. " Irritability, 

 taken strictly, implies every circumstance of the contraction of 

 muscular fibres which can be produced by the action of other 

 bodies. But there occurs a very considerable difference : 

 certain persons are irritable to much weaker impressions in 

 point of force, or in point of the quality, than others ; hence 

 we say they are more moveable than others ; and this gives 

 the MOBILITY. But we likewise know, that, from any cause 

 applied, the force with which the muscular fibres of one per- 

 son can be contracted is different from that which another 

 VOL. i. E 



