286 ANIMAL FORCES. [ii. 



thereby, but the sentient actions gradually cease, in conse- 

 quence of the brain being rendered unfit for the performance of 

 its functions, and no material sensations being excited in the 

 brain. Opium acts on the nerves, as on the brain, and renders 

 the vis nervosa as inefficient as the cerebral forces. When 

 opium is applied to the nerves of a decapitated animal, it 

 changes and arrests the movements of the heart, but somewhat 

 more slowly than if taken. According to Whytt, a muscle 

 suddenly loses its irritability, if opium be applied to its nerve. 

 But the power of the vis nervosa to excite indirect nerve- actions, 

 as well as the direct, is abolished by opium, since all the nerves 

 of the body are rendered insensible, as well as those with which 

 it comes in contact; so that external impressions are neither 

 transmitted upwards along the nerves, nor reflected downwards. 

 Very frequently, not a particle of opium or other narcotic 

 poison, reaches the brain in narcotization ; for it acts when it 

 has scarcely come in contact with the stomach, and even when 

 applied externally, or when the heart is excised, and, conse- 

 quently, the circulation of the blood arrested (Whytt). It is 

 often impossible, that narcotics can excite sleep by causing ex- 

 ternal sensations, since their impressions on the stomach and 

 intestines are rarely felt (470), for they deprive the terminal 

 points of the nerves with which they come in contact, of both 

 their sensibility and vis nervosa at the same time, and thus, in 

 a moment abolish the most violent external sensation of a 

 nerve, namely, pain. It would appear, that Whytt (whose 

 experiments we have just quoted) was correct in concluding, 

 that the sleep which opium induced, was much more probably 

 the result of a diminution of the general sensibility of the 

 nerves caused by it, or in other words, of the cerebral forces 

 and the vis nervosa, than of the excited instinct for sleep ; 

 although when the opium acts at the same time on the brain, 

 this instinct may be developed. 



559. The war-instinct, by which animals are motived to use 

 their natural weapons when exposed to danger, is really only 

 another form of the instinct to voluntary movement ; and, con- 

 sequently, its sentient actions may, under suitable circumstances, 

 be nerve-actions, excited by the same external impressions, 

 which, when felt, excite the instinct. Insects, as the earwig 

 and bee, thus use the natural weapons, placed in their abdomen, 



