24 JELLY-FISn, STAR-FISn, 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-^fer^s, viz. that of 

 conducting stimuli to a distance. The function of 

 nerve-ce^^s 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 a fibre, it contracts on receiv- 



