73 CEREBRAL FORCES. [i. 



braiu cannot so duly stimulate the origin of the sensitive nerve as 

 to induce the propagation outwards of the impression (129, ii.). 

 Supposing that the material external sensation makes its ap- 

 propriate impression in the brain at the point of origin of 

 another nerve^ and not of that which brought it to the braiu; 

 this would be the material idea of another conception of the 

 mind, and the sentient action which it excites through 

 the nerve would not be similar to those produced directly by 

 the external sensation (131). Again, supposing the external 

 sensation only makes such an impression on the origin of the 

 nerve, that it cannot be propagated downwards from the brain 

 along the same nerve, no corresponding direct sentient action 

 can be produced thereby (129, ii, iii). Further, — since ex- 

 ternal impressions do really occur which develope an external 

 sensation in the mind (a material external sensation in the 

 brain), and yet do not thereby excite the ordinary direct sen- 

 tient actions (Haller^s ' Physiology,^ ^ 384), although the 

 origin of the nerve actually possesses the material external sen- 

 sation, which is proper to it, and which constitutes the internal 

 impression on the origin of the nerve (121), — we could not 

 explain why the direct sentient action of the external sensation 

 does not take place through the same nerve, unless we assume 

 that there are two classes of fibrils in the nerve, according to 

 the doctrines previously stated (126, 127). The case is explained 

 by assuming that the fibrils of the nerve which receive the 

 cerebral impression, and transmit it downwards, are rendered 

 incapable, in consequence of there being some of the impedi- 

 ments to the reception of the impression described above 

 (127, 128). (As when a limb, for example, which the mind 

 governs through the nerve that receives the impression, remains 

 motionless and paralysed in spite of its external sensations, 

 although the same sensation can excite other sentient actions 

 in other portions of the body, e. g., certain voluntary move- 

 ments by means of other spontaneous conceptions, and the 

 material ideas produced by them at the origins of other 

 nerves — § 128). 



iii. The direct sentient action of an external sensation 

 caused through the nerves is also prevented, when the appro- 

 priate impression received at the cerebral origin of the sensitive 

 nerve cannot be transmitted to the point where the sentient 



