322 Prof. Helmholtz on the Methods of Measuring 



tary muscles, such as those of the entrails, the iris, the fibres 

 which are diffused over the surfaces of vessels, of the glands, &c. 

 In these cases, the process, as is known, occupies from 100 to 

 1000 times the interval necessary in the former cases, so that 

 we can conveniently observe the single stages. As, however, 

 the commencement of the contraction is, according to this, not 

 sharply defined, we cannot make use of it as the limit of the time 

 to be measured, but we must avail ourselves of the occurrence of 

 a certain stage of the contraction, that is, the moment when the 

 activity of the muscle attains a certain measurable value. We 

 must, however, at the same time assure ourselves that the differ- 

 ences of time, which it is our object to determine, must not be 

 the consequences of an irregular muscular activity ; that, on the 

 contrary, the strength and duration of the contraction shall be 

 exactly the same, whatever portion of the nerve may be excited. 

 Our object therefore can only be attained by series of observa- 

 tions, which shall establish that all the stages of activity take 

 place later when the excitation has to proceed through a greater 

 length of nerve. This is, in point of fact, the case. 



The measurements were performed by the electro-magnetic 

 method. Their conditions require that the time-measuring cur- 

 rent shall commence at the moment when an instantaneous ex- 

 citement of the nerve takes place — the excitation was effected by 

 a second electric current of vanishing duration — and that the 

 time-measuring current shall end at the moment when a certain 

 definite stage of the contraction is attained, that is, at a point 

 when the tension of the muscle has increased to a certain degree. 

 It is so arranged, that the muscle itself by its contraction inter- 

 rupts the current, and must at the same time overcome the 

 resistance of a certain weight, the current being thus broken at 

 the moment when the tension of the muscle is sufficient to over- 

 power the gravity of the mass attached to it. The place of inter- 

 ruption is formed by two pieces of metal which are connected 

 with the two poles of a galvanic battery. As long as they are 

 in contact, the current circulates without hindrance ; as soon, 

 however, as they are separated by the smallest conceivable space, 

 the current ceases instantaneously. Hence it is not necessary to 

 produce a motion of measurable extent, which would incur 

 the loss of time ; the time-measuring current, on the contrary, 

 is interrupted as soon as the muscle commences to move one of 

 the bits of metal, and this occurs as soon as the indicated degree 

 of tension has been attained. That this theoretical deduction 

 corresponds to the reality, I have convinced myself by particular 

 controlling experiments. 



The series of measurements of the interval between excitation 

 and contraction showed all the regularity that could be expected 



