52 THE DATA OF BIOLOGY. 



which are peculiarly bitter. These by their bitterness, show 

 that they affect the extremities of the nerves ; while by their 

 tonic properties, they show that they affect the nervous 

 centres the most intensely bitter among them, strychnia, 

 being the most powerful nervous stimulant. However true 

 it may be that this relation in not a regular one, since opium, 

 hashish, and some other drugs, which work marked effects on 

 the brain, are not remarkably sapid however true it may be 

 that there are relations between particular substances and 

 particular parts of the nervous system ; yet such instances 

 do but qualify, without negativing, the general proposition. 

 The truth of this proposition can scarcely be doubted when, 

 to the evidence above given, is added the fact that various 

 condiments and aromatic drugs are given as nervous stimu- 

 lants ; and the fact that anaesthetics, besides the general effects 

 they produce when inhaled or swallowed, produce local effects 

 of like kind when absorbed through the skin ; and the fact 

 that ammonia, which in consequence of its extreme molecular 

 mobility, so quickly and so violently excites the nerves be- 

 neath the skin, as well as those of the tongue and the nose, 

 is a rapidly-acting stimulant when taken internally. 



Whether we shall ever know anything more of this nerve- 

 force, than that it is some species of molecular disturbance 

 that is propagated from end to end of a nerve, it is impossi- 

 ble to say. "Whether a nerve is merely a conductor, which 

 delivers at one of its extremities an impulse received at the 

 other ; or whether, as some now think, it is itself a generator 

 of force which is initiated at one extremity and accumulates 

 in its course to the other extremity ; are also questions which 

 cannot yet be answered. All we know is, that forces capable 

 of working molecular changes in nerves, are capable of 

 calling forth from them manifestations of activity dis- 

 charges of some force, which, though probably allied to elec- 

 tricity, is not identical with it. And our evidence that nerve- 

 force is thus originated, consists not only of such facts as the 

 above, but also of more conclusive facts established by direct 



