I 



CH. I.] NERVE-ACTIONS IN GENERAL. 199 



ii. It is possible, that in animals with a sentient brain, 

 movements are transformed from sentient actions into nerve- 

 actions, as is probably the case with the respiratory and other 

 movements (285-86); and vice versa from nerve-actions into 

 sentient actions, as in the sentient actions of the instincts of 

 newly-born animals (269); or become both when previously 

 they were only one or the other, or vice versa, become only 

 one or the other, when previously they were both. To establish 

 the possibility of these changes, each example must be specially 

 considered. 



368, i. A sentient action may be changed into a nerve- 

 action, when the transmission of the external impression to 

 the brain is prevented, which may take place from natural or 

 contra-natural hindrances. (Compare § 51, iii, iv, v.) 



ii. A sentient action from a conceptional impression may be 

 changed into a nerve-action when the conception ceases (136 — 

 139) and other stimuli, having a similar mode of action, are 

 applied to the conducting nerves (123, 360). Thus, the mere 

 physical irritation of an acrid humour acting upon the motor 

 nerves at their origin in the brain or along their course, will 

 excite contractions of the muscles which ordinarily are volitional. 

 The same may take place automatically, as when an external 

 impression which does not reach the brain, stimulates the nerves 

 in the same way as a volitional conception, in consequence of 

 being reflected downwards in the ganglia, or at the points of 

 division of the fibrils, thus producing movements as nerve- 

 actions, which are exactly identical with those excited by voli- 

 tion (48, 151). The closure of the sphincter of the bladder, 

 whereby the urine is retained, is usually a voluntary act ; but 

 when the volition ceases, and even when the contrary state is 

 willed, an irritation in the bladder, which is not felt, causes it 

 to be spasmodically closed, even until death ; in this case, the 

 former volitional action is changed by the vis nervosa of an 

 external impression into a nerve-action. 



iii. A nerve-action may be transformed into a sentient 

 action, if it results from an external impression, when the 

 natural obstacle to the transmission of the latter to the brain 

 is removed (45); if, for example, a limb (as the leg), being 

 deprived of sensation by an injury to the nerve, and being 

 scourged, becomes inflamed, as a nerve-action (207, 357), and 



