EXCITABILITY. 119 



as the irritant which is able to evoke a pulsation of 

 definite strength is weaker. Each of these methods 

 when practically applied has advantages and disad- 

 vantages. The former is capable of detecting very 

 minute differences in the excitability, but it can only 

 do this within certain narrow limits ; for when the 

 excitability sinks, the limit for a definite irritant is 

 soon reached, after which no further pulsation at all 

 results ; and when the excitability rises, the muscle 

 attains its maximum contraction, above which it is 

 incapable of further contraction. Changes above or 

 below either of these limits are, therefore, beyond 

 observation so long as the irritant remains the same. 

 The best way to apply the second method practically is 

 to find that strength of irritant which exactly suffices 

 to produce a just observable contraction of the muscle. 

 This assumes the power of graduating the strength of 

 the irritant at pleasure. If inductive currents are used 

 to effect irritation, this graduation may be made with 

 the greatest precision by altering the distance between 

 the primary and secondary coils of the apparatus. In 

 du Bois-Reymond's sliding inductive apparatus, repre- 

 sented in fig. 13, p. 35, the secondary coil is, there- 

 fore, attached to a slide which may be moved forward 

 in a long groove. This arrangement is used in order 

 to find the particular distance of the secondary coil 

 from the primary which results in a just observable 

 contraction of the muscle ; and this distance, which 

 can be measured by means of a scale divided into 

 millimetres, is regarded as the measure of excitability. 1 

 6. If a recently prepared nerve, as fresh as possible, 

 is placed on a series of pairs of wires, and the excita- 

 1 See Notes and Additions, No. 3. 



