FUNCTIONS OF THE SPINAL ROOTS. 197 



tinuity of a nerve-fibre, such as would not interfere with 

 the passage of an electric current, will not suffice for the 

 transmission of a nervous impulse. For instance if a damp 

 string be tied around a nerve, or if it be cut and its two moist 

 ends placed in contact, no nervous impulse will be trans- 

 mitted across the constricted or divided point, although 

 an electrical current would pass readily. An electrical 

 shock may be used like many other stimuli to upset the 

 equilibrium of the nerve molecules and start a nervous im- 

 pulse, which then travels along the fibre, but is just as dif- 

 ferent from the stimulus exciting it, as a muscular contrac- 

 tion is from the stimulus which calls it forth. 



The nerves, however, have certain interesting electrical 

 properties from the study of which we learn some little 

 about a nervous impulse. As already mentioned whenever 

 a nervous impulse is started in a nerve an electrical change, 

 known as the " negative variation" or "action current," is 

 started at the same time, from the same point, and travels 

 along the nerve at the same rate. Hence we conclude that 

 the new internal molecular arrangement in a nerve-fibre 

 which constitutes its active as compared with its resting 

 state, is also one which changes the electrical properties of 

 the fibre. It is an outward sign and the only known one 

 of the internal change. Now it is found that the action 

 current travels along the nerve (in the frog) at the rate of 28 

 meters (92.00 feet) in a second and takes .0007 second to 

 pass by a given point: accordingly at any one moment it 

 extends over about 18 mm. (0.720 inch) of the nerve- 

 fibre. Moreover, when first reaching a point it is very 

 feeble, then rises to a maximum and gradually fades away 

 again. Taking it as an indication of what is going on in 

 the nerve, we may assume that the nervous impulse is a 

 molecular change of a wavelike character, rising from a 

 minimum to a maximum, then gradually ceasing, and about 

 18 millimeters in length. 



The Rate of Transmission of a Nervous Impulse. 

 This can be measured in several ways, and is far slower than 

 that of electric currents. It agrees as above stated with 

 that of the negative variation, being 28 meters (92.00 feet) 



