254 ANIMAL FORCES. [ii. 



ternal impressions taking the opposite direction, as the concep- 

 tional internal impressions (126), because it is indifferent 

 whether the stimulus be a material idea, or some other irritant 

 in the brain, or on the course of the nerve ; and because the 

 apparent reason of this phenomenon, namely, the natural dis- 

 tinction between the afferent and efferent nerve fibrils remains 

 the same (126, 127). Experiments confirm this doctrine. 



488. The doctrine laid down in § 126, as to the existence 

 of two kinds of fibrils in each nerve, ^ the one transmitting ex- 

 ternal impressions upwards, and the other internal impressions 

 downwards, enables us to explain how it may happen, that in 

 a natural or contranatural condition of the nerve, certain me- 

 chanical machines may be excited into action by the one, and 

 not by the other ; although it may be excited to action by both 

 kinds of impressions. For example, the terminal nerve-fibrils, 

 in a mechanical machine, may not be duly excited by certain 

 external excitants (424), or a natural or contranatural impedi- 

 ment may take place in the fibrils which have to transmit the 

 external impression upwards (425), and, consequently, neither a 

 direct nor indirect nerve-action results (427 — 431). But, if 

 there is no such impediment in the efferent fibrils of the same 

 nerve, a nerve-action may take place in the mechanical machine 

 from an internal impression on the medulla of the nerve, and 

 vice versa. Except on the hypothesis of such a difference in 

 the nerve-fibrils, it is very difficult to understand phenomena 

 of this kind. That such phenomena really take place is un- 

 doubted ; for a limb which has become insensible to all external 

 impressions, and never once manifests nerve-actions, as in the 

 case of a paralysed limb, which neither feels nor even contracts 

 when scourged with nettles, may be stimulated to contractions, 

 independently and even in spite of the will, by irritation of the 

 trunk of its nerve; or when some internal agent causes a 

 morbid change in it ; and these contractions are manifestly 

 nerve-actions of a non-conceptional internal impression. Again, 

 there are instances in which the limb is not moved either by 

 conceptional internal impressions, or other internal stimuli of 

 the nerves, although it is sensible of external impressions on 

 the terminal nerve-fibrils, and nerve-actions are produced 



' See note to § 126, with reference to this point. — Ed. 



