(24 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
muscle has been cut from the body of a freshly 
killed animal ; so long as it is not interfered with in 
any way, so long will it remain quite passive. But 
every time a stimulus is supplied to it, either by 
means of a pinch, a burn, an electrical shock, or a 
chemical irritant, the muscle will give a single 
contraction in response to every stimulation. And 
it is this readiness of organic tissues to respond to a 
suitable stimulus that physiologists designate by 
the term “ excitability.” 
Nerves, no less than muscles, present the pro- 
perty of being excitable. If, together with the 
excised muscle, there had been removed from the 
animal’s body an attached nerve, every time any 
part of this nerve is stimulated the attached muscle 
will contract as before. But it must be carefully 
observed that there is this great difference between 
these two cases of response on the part of the 
muscle—that while in the former case the muscle 
responded to a stimulus applied directly to its own 
substance, in the latter case the muscle responded to 
a stimulus applied at a distance from its own 
substance, which stimulus was then conducted to the 
muscle by the nerve. And in this we perceive the 
characteristic function of nerve-fibres, viz. that of 
conducting stimuli to a distance. The function of 
nerve-cells is different, viz. that of accumulating 
nervous energy, and, at fitting times, of discharging 
this energy into the attached nerve-fibres. The 
nervous energy, when thus discharged, acts as a 
stimulus to the nerve-fibre; so that if a muscle is 
attached to the end of afibre, it contracts on receiv- 
