38 CEREBRAL FORCES. [i. 



is the same with the tickling which a fine feather, or particle 

 of dust, can excite, for it is a state of the nerve allied to that 

 of pain, and sensations much pleasanter than it require much 

 stronger external impressions. Indeed, the more indifferent 

 external sensations of heat and cold, hardness and softness, 

 moist and dry, of light, of dissolved salts, &c., are so totally- 

 different in the mind, that it is certain the external impressions 

 on the nerves must be different also ; but we know of nothing 

 generally, as to this difference, which may serve as a general rule. 

 41, 42. It is equally impossible to compare the material ideas 

 with the external impressions, or both these with the external 

 sensations {vide Haller^s ' Phys.,' § 556). Every external im- 

 pression does not necessarily excite external sensations (34), 

 although external sensations are the only conceptions it can 

 excite (35). Since an external impression differs from every 

 other in this, that it excites animal operations, and these can 

 be either in the mind as external sensations, or only in the 

 body, and consist simply in animal movements (7) j and since 

 we have only to consider here the operations on the mind of 

 an external impression (33), we must inquire under what con- 

 ditions an external impression develops external sensations, and 

 under what conditions it does not. 



43. If a nerve of special sense be compressed or divided, the 

 sense is lost. If the brain be compressed, sensation ceases in 

 the whole body ; and when the spinal cord is compressed, sen- 

 sation ceases in the part below the point compressed. The 

 reason in all these cases is, that either external impressions are 

 not transmitted to the brain, or, if transmitted, do not excite 

 in it the material ideas requisite to sensation. 



44. That a part be sensitive, it is requisite that it be endowed 

 with nerves capable of receiving those external impressions, 

 which can be transmitted uninterruptedly to the brain, and 

 there excite the material idea of a conception. The more a 

 part is endowed with such nerves, the more readily it receives 

 an external impression ; and the more uninterruptedly it can be 

 transmitted to their origin in the brain (43), the more sensitive 

 it is. The less a part is supplied with such nerves, although 

 it may have many others of a different kind, and the more diffi- 

 cult it is to convey external impressions to them, that is to say, 

 the more they are covered and protected from contact, and the 



