242 PRACTICAL PHYSIOLOGY. [XLVIL 



(f.) Instead of induction shocks, use a shock from a Darnell's 

 cell. There is a paradoxical twitch. 



'No paradoxical response is produced hy stimulation other than 

 electrical stimuli, e.g., section of a nerve, salt. It is still produced 

 even if the peroneal nerve be ligatured on the central side of the 

 seat of stimulation. 



6. Frog's Heart-Current (Secondary contraction). 



(a.) Injured Heart. — A quiescent uninjured heart gives no 

 current, but an active heart does, and so does an injured one. The 

 action-current of an injured heart is easily shown when a nerve of 

 a nerve-muscle preparation is placed on a beating rabbit's heart 

 inside the thorax. In the frog, it requires some care to show this. 

 It is easy, however, to obtain a secondary contraction from a 

 beating injured frog's heart. 



Prepare a nerve-muscle preparation or rheoscopic limb. Excise 

 the heart of a pithed frog, and place it on a dry glass plate, removing 

 the surplus blood. Cut off the apex of heart, and to it apply the 

 transverse section of the divided sciatic nerve, letting a part of 

 the longitudinal surface of the nerve rest on the uninjured ventricle. 

 AVith each beat of the heart there is a twitch of the rheoscopic 

 limb or muscle. 



(6.) Action-Cun-ent of Uninjured Frog's Heart. — On placing the 

 nerve of a nerve-muscle preparation along the exposed frog's heart 

 from apex to base, one sometimes gets a muscular response to each 

 beat of the heart, but the experiment does not always succeed. 

 It is easier to do it on a Stanniused heart ; with each contraction 

 of the heart excited artificially, there is a secondary contraction. 



ADDITIONAL EXERCISES. 



7. Kiihne's Nerve-Current Experiment. 



(a.) Invert an earthenware bowl (B), and with wax fix to its base a piece of 

 glass lo cm. square (fig. 167, G.. 



{h.) Make two rolls of kaolin (moistened with normal saline^, about i cm. 

 in diameter and 6 cm. in length (P, P'), bend them at a right angle, and 

 hang them over the glass plate about 6 mm. apart. 



(f. ) Make a nerve-muscle preparation, lay the muscle on the glass plate, 

 and the nerve (N over the rolls of china clay. 



(rf.) Fill a small glass vessel iC' with normal saline, and allow the two 

 fiee ends of the clay to dip into it. With each dip the muscle contracts. In 

 this case the nerve is stimulated by the completion of the circuit of its own 

 demarcation-current, and this in turn indirectly stimulates the muscle, 



8. Kiihne's Muscle-Press — Secondary Contraction from Muscle to Muscle. 

 — Pre2)are t^o sartorius muscles of a frog. Place the end of one muscle 



