NERVOUS ACTION. 179 



another nerve to reach the terminal organ ; while the effect 

 produced depends on the intrinsic properties of that organ. 

 It is also interesting to notice that in such an action there is 

 no generation of new force; but the irritant, or stimulus, 

 gives rise to or is converted into a nervous impression, and 

 the impression, after travelling through a circuit, is followed 

 by the action of the terminal organ. 



It is by such reflex action that a flow of saliva follows the 

 introduction of sapid substances into the mouth. The sapid 

 substance irritates the nerves of the mouth, and the impres- 

 sions travel along them to a nervous centre, whence they are 

 reflected along the nerves terminating in the secreting cor- 

 puscles of the gland. So, also, the muscles of the stomach and 

 intestines are excited to regular movements, not by the direct 

 influence of the food, but by impressions originated in nerve 

 extremities, conducted to centres, and thence reflected. 

 Direct irritation of' muscle produces mere spasmodic con- 

 traction of the muscular fibres irritated; but the impressions 

 sent from nervous centres produce a co-ordinated action of 

 numbers of fibres, suited to the accomplishment of some 

 definite end. Thair, no doubt, is a result not thoroughly 

 understood; but it is connected with the circumstance that a 

 nerve-centre always consists of numbers of corpuscles, and 

 gives off a number of fibres. 



Impressions sent to nervous centres are not, however, 

 always reflected to the region whence they came. They may 

 pass to other nervous centres, and lead to changes in all parts 

 of the body; and, when they reach the brain, they produce 

 sensation. 



Here we sere brought into contact with an apparent compli- 

 cation in the functions of the nervous system, its relations to 

 consciousness. These relations are exhibited in two ways, 

 one is the production of sensation by nervous action reaching 

 the brain; the other is the production of nervous action 

 initiated in the brain by volition. It is, however, to be 

 clearly understood that the brain is only a complex 

 arrangement of nerves and nerve-corpuscles, comparable with 

 the simpler nervous centres; and although it both affects and 

 is affected by the mind, there is no reason to believe that its 

 active condition is dissimilar from that of nerves and nerve- 



