CH. II.] EXTERNAL SENSATIONS. 89 



more hindrances there are to an uninterrupted transmission to 

 the brain, of an external impression, in the same proportion 

 the part is insensible (34). 



45. We are now able to say what is requisite to the develop- 

 ment of an external sensation. 



i. A nerve must be so acted on, that its medulla thereby 

 receives an external impression (31, 32). 



ii. This impression must be propagated into the brain, so 

 far as the origin of the nerve (43, 44) . 



iii. It must there excite the animal movement (a material 

 idea), which naturally arises from this external impression ; and 

 so soon as this takes place, the conceptive force of the soul 

 develops the external sensation (34, 25). 



46. An external sensation derived from a given nerve may 

 be interrupted, or cannot arise. 



i. When the nerve is not acted on, or not sufficiently so, 

 that its medulla receives an external impression (45, i) . All con- 

 ceptions, consequently, which are considered to be such external 

 sensations, but which arise only from the conceptive force with- 

 out an external impression, are not true external sensations : 

 of this kind are imaginations, recollections, anticipations, &c. 



ii. When the external impression does not reach the brain at 

 all, but particularly that point where the material sensation is to 

 be developed. It does not follow because a nerve has been acted 

 on, and an external impression excited, that the latter must 

 necessarily be felt (42), for to this end the impression must 

 find its way uninterruptedly to the brain (45, ii). 



iii. When the material idea, which ought naturally to result 

 from the external impression, cannot arise in the brain (45, iii). 

 The brain may be defective at those points whence the nerve 

 arises, and thus the limb, to which the nerve is distributed, may 

 be rendered insensible to all external impressions, although their 

 existence along their whole course to the brain, be rendered 

 manifest by other animal movements. 



47. Since, in the normal condition of animal organisms, all 

 external impressions do not excite material external sensations, 

 so also there are portions amply supplied with nerves which 

 have little sensation ; so that the amount of sensibility of a part 

 cannot be deduced, from the number of nerves without certain 

 limitations. Nevertheless, these numerous nerves may be of great 



