98 PHYSIOLOGY. 



that is to say, it admits of the action of the animal power more 

 readily ; and if this depends in any degree upon the afflux of a 

 certain matter, we must suppose that the change in the muscular 

 fibre consists in a higher degree of excitement being produced ; 

 and from its being attended with an increased bulk, the excite- 

 ment produced is also of a more permanent kind. On the other 

 hand, exercise may be the cause of debility, for the circumstances 

 of which you must consult CXI II., and reconcile that with 

 CXIV. in the manner I formerly endeavoured to explain. 



" 5. The vigour of the moving extremities will be different, 

 according to a difference in the state of the brain which may 

 be communicated to them. It is necessary to attend to this. 

 In the case of voluntary motion, we very speciously allege, 

 that, in every contraction of the muscles, an action of the 

 animal power, or energy of the brain, constantly takes place ; 

 but in what are called the involuntary motions, whether they 

 are originally such, or are so in consequence of habit, we sup- 

 pose that no such exertion of the animal power does occur, but 

 that the contraction is in consequence of the stimulus acting 

 upon the inherent power, which is agreeable to the common 

 doctrine of Dr. Haller; but I have before pointed out what 

 limits this requires. I have said that the involuntary muscles 

 are still under the influence of the brain, and depend upon its 

 constant energy ; which we infer not only from various stimuli 

 applied to the brain having considerable influence upon the action 

 of every muscular fibre, but from the muscles being liable to 

 palsy, in consequence of the compression of nerves. The in- 

 herent power, therefore, the vigour of muscular fibres, is greater 

 or less according to the state of the brain communicating its 

 power constantly or occasionally ; accordingly, in the case of 

 mania, where the tone is increased to a considerable degree, or 

 in the case of fever, where it is as remarkably diminished, we 

 seek for the causes of both only in the present state of the 

 brain. 



" These are the causes that we can assign for the different 

 states of vigour in our moving fibres ; but I have said that the 

 moving fibres are also to be considered with regard to their mo- 

 bility. We are, therefore, to consider the causes of more or less 

 mobility (irritability and torpor ; see p. 93.). These have oc- 



