56 



ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



In the frog the cord extends as far as the ninth vertebra (at the dorsal 

 prominence). From here the tenth vertebra or urostyle passes back- 

 ward to the tip of the animal. It is best to open the abdominal cavity 

 in the median line and to remove the viscera. Make two transverse 

 sections so as to isolate the seventh and eighth vertebrae and roots of 

 the sciatic nerve of one side. Trace these fibers through the pelvic 

 aperture and down upon the dorsal aspect of the thigh. Isolate the 

 gastrocnemius muscle and place the coiled up nerve temporarily upon 

 it, but do not allow it to come in contact with the skin. Remove the 

 muscles of the thigh and cut the femur. Adjust this preparation in 

 the clamp of the moist chamber and lay the nerve across the widely 

 separated needle electrodes. Moisten the nerve with saline solution 

 and replace the glass cover. Test the two electrodes to see whether 

 both are effective. 



The procedure to be followed is the same as that described previously 

 for determining the different periods of the muscle twitch. Having set 



FIG. 35. P DHL'S COMMUTATOR. (Univ. of Penna. Lab. Outlines.) 



the tuning-fork in vibration, the muscle is made to contract by closing 

 the key, the contraction may occupy a single rotation of the drum, 

 and must be repeated a numbe'r of times on far stimulation as well as 

 on near stimulation. Obviously, the current may be diverted into 

 either pair of electrodes by simply changing the position of the wires 

 at the binding-posts outside the moist chamber. A pole changer may 

 also be employed, but since this requires considerable lengths of extra 

 wire the change may be more conveniently effected in the manner sug- 

 gested above. 



Draw perpendiculars to determine the length of the latent period 

 on near and far stimulation. Measure the distance between the elec- 

 trodes in millimeters. The time occupied by the nerve impulse in 

 traversing the measured stretch of nerve, corresponds to the difference 

 in the latent periods. Calculate the velocity of the nerve impulse. 



3. Rate of the Nerve Impulse in Man. A simple method for deter- 

 mining the speed of the nerve impulse in man not being available, the 



