EXCITATION AND CONDUCTION 59 
Excitability —The excitability of the nerve fiber has 
three criteria: (1) the degree of irritability, ie., the 
ease with which it can be stimulated; (2) velocity of 
the nerve impulse, i.e., the speed with which the state 
of excitation travels from one point to another; (3) 
the direction of the nerve impulse. All nerves are 
classified into two general functional types: efferent 
and afferent, the former conducting away from the 
nerve center (brain, etc.), the latter toward the center. 
We shall consider somewhat in detail in this chapter 
what relation the metabolic condition bears to these 
three phenomena in the nerve. 
Degree of excitability —Not all nerves can be stimu- 
lated equally well by the same strength of stimulus. 
The threshold value—the minimum strength of stimulus 
which can call forth functional activity—is different 
in different nerves. Not only have the different nerves 
different degrees of excitability, but the same nerve can 
be made excitable in different degrees under a variety of 
conditions. If we study metabolic activity in nerves 
under different conditions which we know affect the 
state of excitability, we find that there is a very close 
relation between metabolism and excitability. 
a) If the sciatic nerve is removed from a frog, it 
exhibits electrical phenomena for many hours. Since 
electrical changes are characteristic of living nerves 
only, we consider that the isolated nerve does not die for 
many hours. Such a nerve, although it shows large 
electrical responses, is nevertheless less excitable than a 
fresh one. If measurements are made on an isolated 
nerve at successive time intervals for many hours, we 
find that the carbon dioxide production steadily dimin- 
