88 HUMAN PHYSIOLOGY 



Hence only th of the total " latent time " of the reflex 

 action is occupied in the passage of the change along the 

 neurons, and '05 second, or th of the whole is taken up in 

 the passage of the change from one neuron to another. 

 Obviously the synapsis between the dendrites of the neurons 

 offers a resistance, and this resistance varies with the con- 

 dition of the neurons involved, possibly with the condition 

 of the dendrites which form the synapses. 



Not only does the time of the resulting action vary with 

 the state of the neurons, but the extent also varies. If the 

 toe of a frog deprived of its brain is pinched, it draws up the 

 leg; but if a dose of strychnine is first administered, even 

 touching the toes causes a violent spasm of every muscle in 

 the body. If, on the other hand, a dose of bromide of 

 potassium has been administered, or if ice be put on the 

 back of the animal, much more powerful stimulation is 

 required to produce any reaction. The activity of the 

 central synapses may be increased or diminished in various 

 ways, and hence it is never easy to predicate the ultimate 

 result of any stimulation of the nervous system. But, other 

 things being equal, the strength of stimulus applied to the 

 first neuron that is the extent of excitation directly affects 

 the extent of the resulting action. (Practical Physiology, 

 Chap. XII.) 



THE NEURO-MUSCULAR MECHANISM. 



The study of the physiology of muscle and nerve leads to 

 the consideration of how the neuro-muscular mechanism 

 acts, so that (1st), the various visceral muscles perform 

 their functions, and (2nd), so that the co-relationship of the 

 animal with its surroundings may be maintained. With the 

 second of these we shall at present deal, leaving the former 

 for consideration when studying the physiology of the viscera. 

 This interaction of the body on the surroundings may 

 be considered first as regards the simpler reactions the 

 so-called reflex actions; and secondly as regards the more 

 complex reactions cominonty classified as voluntary actions. 

 This neuro-muscular mechanism is controlled by three chains 



