PHYSIOLOGY. 105 



ferent parts of the nervous system, but by the intervention of 

 the brain ; it is from hence also probable, that the brain, by 

 its organization, is fitted to propagate the motions arising in 

 one part to the other parts of the nervous system ; and, as 

 these mechanical communications produce different effects, ac- 

 cording to the state of the brain itself, we, upon the whole, 

 conclude, that the brain is a corporeal organ susceptible of vari- 

 ous conditions, and thereby of considerable influence in most 

 of the phenomena of the nervous system. 



CXIX. The action of the brain, in moving the several parts 

 of the body, is excited by various causes, or by the same causes 

 in different circumstances. 



1. It is especially excited by the WILL, directing the motion 

 of certain parts as means to an end. 



As the motion of certain parts is adapted to various purposes, 

 we are conscious of willing these purposes, as they occasionally 

 occur, and so far also of the motion of the parts concerned in 

 them ; but, where the motion of the parts is connected with 

 one sensation, or a few only, the motions required follow these 

 sensations without our being conscious of specially willing 

 them ; and, unless we have continued the practice of adapting 

 the motions to different purposes, we lose the power of doing so, 

 and the motions become unavoidably connected with those sen- 

 sations which, for a long time, had also given occasion to them. 

 In most of the instances of what are called VOLUNTARY MO- 

 TIONS, we are conscious of willing the end proposed, more than 

 the motions excited ; and, of the motions produced, we are con- 

 scious chiefly of those of a whole member, or of the general 

 effect, and very little of the many particular motions that con- 

 cur to produce it. We are never conscious of the particular 

 muscles employed. 



"Unless we have continued the practice of adapting the 

 motions to different purposes, we lose the power of doing so. 

 An example of this is afforded by the motions of the two eyes. 

 That both their axes are directed to one individual point does 

 not happen by any necessary law ; because many children 

 squint without any faulty conformation of the organ of the eye ; 

 but the sense of the advantages which arise from their uniform 

 motion, occasions its being very generally employed, and we 



