50 EXPERIMENTAL PHYSIOLOGY. 



the muscle lever so that only the beginning of the contraction 

 is recorded. 



Using the base line each time, record the interval of the latent 

 period, first for the far position of the electrodes and then for 

 the near position. This can be done by changing the wires of 

 the secondary over to the electrode desired in each case. Esti- 

 mate the duration of the latent period by means of a tuning 

 fork tracing (Fig. 14). 



The time required for the passage of the impulse along the 

 nerve is the difference between the latent periods resulting from 

 the far and near stimulations. The length of nerve traversed 

 may be taken as the distance between the nearer points of the 

 two electrodes. 



Determine the velocity of the nerve impulse in meters per 

 second. Make four or more sets of records. Tabulate the 

 results. (This expt. may also be done with the Harvard drum 

 as in Expt. 8.) 



FACTORS INFLUENCING NERVE FUNCTION. 



Mechanical. If a nerve is stretched or compressed its power 

 to conduct may be lost. When the compression is neither too 

 severe nor too prolonged conductivity may be re-established. 

 Compression over a broad area requires much greater force to 

 paralyze than when over a narrow region. The first effect of com- 

 pression, frequently, is to increase the excitability. This stage, 

 however, is quickly passed. 



Thermal. Beginning with C. the rate of conduction in- 

 creases as the temperature is raised. Below C. conduction is 

 suspended. At the higher limit of conduction, about 47 C. in 

 the mammal, due to coagulation of the proteins the nerve may 

 become permanently paralyzed. 



Chemical. Many drugs lower or suspend conductivity when 

 applied locally. Ether, chloroform, chloral, cocaine, phenol and 

 alcohol have this power. Conduction will return in time if the 

 drug does not act too long. Lack of oxygen will also destroy con- 

 ductivity in time. 

 *Experiment 22. The Action of Carbon Dioxide, Ether, and 



Chloroform upon Nerve. A gastrocnemius muscle with its 



