STRUCTURE OF THE MEDUS&. 25 
ing this stimulus. I may add that when nerve-cells 
are collected into ganglia, they often appear to 
discharge their energy spontaneously ; so that in 
all but the very lowest animals, whenever we see 
apparently spontaneous action, we infer that ganglia 
are probably present. Lastly, another important 
distinction must be borne in mind—the distinction, 
namely, which is to be drawn between muscle and 
nerve. A stimulus applied to a nerveless muscle 
can only course through the muscle by giving rise 
to a visible wave of contraction, which spreads in 
all directions from the seat of disturbance as from a 
centre. A nerve, on the other hand, conducts the 
stimulus without sensibly moving or undergoing 
any change of shape. Now, in order not to forget 
this distinction, I shall always speak of muscle- 
fibres as conveying a visible wave of contraction, 
and of nerve-fibres as conveying an invisible, or 
_ molecular, wave of stumulation. Nerve-fibres, then, 
are functionally distinguished from muscle-fibres— 
and also from protoplasm—by displaying the pro- 
perty of conducting invisible, or molecular, waves 
of stimulation from one part of an organism to 
another, so establishing physiological continuity 
between such parts, without the necessary passage of 
waves of contraction. 
