EFFECT OF COXSTITUTION. 237 



different parts of the nervous system act exactly in the opposite 

 way to others ; and if anything shoidd act on botli of these, it 

 will produce an opposite efl'ect according as one or other part is 

 more developed and more powerful. Thus, the vagus nerve has 

 the power of rendering the heart's action slow, and tlie accele- 

 rator nerve of quickening it ; and any drug wliicli irritates them 

 both will make the heart's action slow if the vagus be more 

 developed, or quicken it if the sympathetic be stronger. Thus, 

 two horses of unequal strength, pulling in opposite directions, 

 may counterbalance each other ; but if both be struck with a 

 whip at the same moment, the power of the stronger becomes 

 evident and he ])ulls the weaker after him. 



A good exam[)le of this action is given by muscarin, an alka- 

 loid obtained from a poisonous mushroom, Agaricus muscarius. 

 Professor Schmiedebeig, of Strassburg,* has shown that tliis 

 alkaloid produces great irritation of the vagus nerve, so that in 

 frogs the heart will stand still for hours together. When given, 

 to dogs, it sometimes makes the pulse slow, but sometimes- 

 quickens it; and one might therefore be inclined to say that 

 when it produces quickening it cannot be acting on the vagus^ 

 But the explanation of this phenomenon is, that muscarin does 

 not act on the vagus alone, but has also an effect on the vaso- 

 motor nerves, producing dilatation of the vessels and diminu- 

 tion of the blood-pressure in them.f Now, lessened pressure 

 acts as a stimulant to the accelerator, and quickens the heart. 

 In this way muscarin stimulates both vagus and accelerator, and 

 the pulse is rendered quick or slow according as the power of 

 the one or other is greater in the particular dog to whicli it is 

 given. In frogs, the blood-pressure has no great action on the 

 heart, and in them the effect of the vagus is not interfered 

 with. 



Another instance may be given where an apparent difference 

 in the effect of a drug on two animals may be removed by 

 reducing their organs to the same condition. In most animals, 

 the slowing action of the vagus on the heart is constantly 

 exerted during health ; and when it is cut the heart beats much 



* Schmiedeberg und Koppe, Bas Muftcarin, p. 23. 

 t Schmiedeberg, op. cit., pp. 43 and 48. 



