46 CEREBRAL FORCES. [i. 



themselves (46, 50). The material ideas of external sen- 

 sations, and their operation on the body, are the strongest 

 when they are recent and not habitual, provided other circum- 

 stances are the same. 



The External Senses. 



55. The nerves are the organs of external sensations, but 

 only by means of the brain (43). Those parts of organisms 

 wherein the nerves are distributed that are susceptible only of 

 special external impressions, and consequently only of special 

 external sensations, are termed organs of the {external) senses. 

 In man, these are five; other animals have fewer, a few have 

 probably more. 



56 — 64. The senses are — 1st, touch ; which has its seat in the 

 tips of the fingers, but it is mixed up with general sensation ; 

 2d, taste, subservient to nutrition ; 3d, smell, in many animals 

 much more acute than in man ; 4th, hearing ; 5th, sight. The 

 anatomical relations are to be found in works of anatomy ; the 

 metaphysical questions, as for example, why we do not hear 

 two tones, or see two images, or perceive the rays of light and 

 the undulations of the air, or feel the forms of salts, are dis- 

 cussed at length in works of metaphysics, and therefore need 

 not detain us. 



The Sensational Conceptions. 



It has so far been shown how material ideas are produced 

 in the brain by means of external impressions. In this way the 

 mind receives conceptions corporeally, necessarily, and involun- 

 tarily (27), in consequence of the animal force of the nerves 

 developing external sensations. But the mind can also produce 

 voluntarily, in itself, many kinds of conceptions, and through 

 these, material ideas are formed in the brain, as an effect of the 

 conceptive force, and by conceptions only, without the inter- 

 vention of any external impressions (27). This other kind of 

 material ideas so produced must be defined, before their in- 

 fluence on the animal economy can be explained. Since, 

 however, some of these voluntary conceptions induce only 

 material ideas of a kind that do not manifest externally to 

 the brain any perceptible efiects in the animal economy, and 



