54 THE DATA OF BIOLOGY. 



that they have, in a high degree, the power of getting at the 

 olfactory nerves by penetrating their mucous investment. 

 Again, the facts which photography has familiarized us with, 

 show that those nervous impressions called colours, are 

 primarily due to certain changes wrought by light in the 

 substance of the retina. And though, in the case of hearing, 

 we cannot so clearly trace the connexion of cause and effect, 

 yet as we see that the auditory apparatus is one fitted to 

 intensify those vibrations constituting sound, and to convey 

 them to a receptacle containing liquid in which nerves are 

 immersed, it can scarcely be doubted that the sensation of 

 sound proximately results from molecular re- arrangements 

 caused in these nerves by the vibrations of the liquid : know- 

 ing, as we do, that the re-arrangement of molecules is in all 

 cases aided by agitation. Perhaps, however, the best 



proof that nerve-force, whether peripheral or central in 

 origin, results from chemical change, lies in the fact that 

 most of the chemical agents which powerfully affect the 

 nervous system, affect it whether applied at the centre or 

 at the periphery. Various mineral acids are tonics — the 

 stronger ones being usually the stronger tonics; and this 

 which we call their acidity implies a power in them of act- 

 ing on the nerves of taste, while the tingling or pain fol- 

 lowing their absorption through the skin, implies that the 

 nerves of the skin are acted on by them. Similarly with 

 certain vegeto-alkalies which are peculiarly bitter. By their 

 bitterness these 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 is not a regular 

 one, since opium, hashish, and some other drugs, which work 



* When writing this passage I omitted to observe the verification yielded 

 of the conclusion contained in § 15 concerning the part played in the vital 

 processes by the nitrogenous compounds. For these vegeto-alkalies, minute 

 quantities of which produce such great effects in exalting the functions {e. </,, 

 a sixteenth of a grain of strychnia is a dose), are all nitrogenous bodies, and, 



I 



