60 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



stimulate. Consequently, the response of muscle B is the result of impulses which 

 are produced in nerve B by the current of action of muscle A. 



5. Action Current of Nerve. Cut nerve A near the muscle and 

 place it alongside nerve B, so that the two overlap for a distance of 

 about 2 cm. Stimulate nerve A briefly and note that muscle B con- 

 tracts. 



Annotation. Nerve tissue shows a difference in potential in accordance with 

 the progressive activation of its segments. This current of action in nerve A is 

 responsible for the excitation of nerve B and the contraction of the muscle. The 

 fact that it is not due to a direct escape of the faradic current may be proved by 

 placing a ligature upon nerve B. This effectively destroys the response of the 

 muscle. 



6. Action Current of the Frog's Heart. The fact that the beating 

 heart generates an action current may be easily proved by exposing the 

 heart of a rabbit during deep narcosis and placing the nerve of a muscle- 



FIG. 37. THE RHEOSCOPIC FROG PREPATATION. 



Muscle A, stimulated through its nerve at S, generates an action current which causes 



muscle B to contract. 



nerve preparation lengthwise upon its surface. The muscle then re- 

 sponds to every systole of this organ. 



While the uninjured heart of a frog does not show this phenomenon 

 very readily, it may be obtained in the following way : Excise the heart 

 of a pithed frog and permit it to continue its activity upon a plate. 

 Cut off its apex and apply to it the cut surface of the sciatic nerve. 

 Place the more distant segment of this nerve upon the surface of the 

 basal portion of the ventricle. The muscle then twitches with each 

 beat of the heart. 



7. Paradoxic Contraction. The sciatic nerve of the frog divides near 

 the knee into its peroneal and popliteal branches. The fibers con- 

 stituting these two minor nerves are not formed by a splitting of the 

 distal axons, but arise separately from the spinal cord and merely lie 

 alongside each other in the nerve itself. Isolate the outer or peroneal 

 branch and divide -it near the knee. Stimulate its central end with a 

 brief faradic current. When a certain strength of current is employed 



