CH. IV.] SUBSTITUTION OF SENTIENT ACTIONS. 299 



SECTION II. ON THE SUBSTITUTION OF SENTIENT ACTIONS FOR 



NERVE-ACTIONS. 



580. There are three principal kinds of nerve-actions in the 

 mechanical machines : firstly, those of primary internal impres- 

 sions, not caused by conceptions (419); secondly, those of 

 unfelt reflected external impressions ; and thirdly, those of direct 

 external impressions (418). 



581. The nerve-actions of primary non-conceptional internal 

 impressions may be excited by the impressions of conceptions, 

 since the two kinds develop the same animal movements (541). 

 The non-conceptional internal impressions may be divided into 

 the contra-natural, to which class the experimental belong, and 

 the natural. The experimental internal impressions are those 

 made on the brain, spinal cord, or nervous trunks, by various 

 stimuli in experimental researches. There is no nerve-action 

 caused by impressions of this class, which is not induced also 

 as a sentient action ; and it is because we know them as the 

 latter, that they surprise us so much when excited artificially 

 (486). Nerve-actions are often excited in the usual condition 

 of an animal, by contra-natural internal impressions, as when 

 the cerebral origin of the nerves, the spinal cord, or the trunk 

 of a nerve is irritated by stimuli, which are not conceptions, 

 and so cause nerve-actions; as, for example, when efiused 

 fluid in the brain partly paralyses, partly causes spasmodic 

 action in the extremities ; or when an acrid humor is determined 

 to the spinal cord ; or it has been injured ; or a tumour or growth 

 on nerves causes contra-natural movements in the parts regu- 

 lated by the afi'ected nerves. These are also similar to move- 

 ments which occur as sentient actions in the natural condition, 

 or at least, as contra-natural sentient actions. Thus, a fright 

 will paralyse or convulse the limbs, as much as a paralytic 

 stroke from effusion; the convulsions excited by an acrid 

 humor determined to the nerves are excited also by anger or 

 anxiety; and the most violent convulsions may accompany 

 intense passion in sensitive persons, &c. 



582. When reflected unfelt external impressions produce 

 nerve-actions, they act in the same way as internal impressions, 

 and the same animal movements are excited. Consequently, 

 the observations made in the last paragraph apply equally to 



