156 EXPERIMENTAL PHYSIOLOGY 



Instead of watching the changes with a heart beating sponta- 

 neously, the effect of a single beat may be observed by first applying 

 the Stannius ligature and then mechanically stimulating the ventricle. 

 "With each contraction the mercury will move so as to show that 

 the part stimulated becomes first negative and then positive to the 

 other part. 



Examine the arrangement of a second electrometer which is fitted 

 up to record the movement of the mercury meniscus photographically. 

 The capillary is brightly illuminated in a projecting lantern and 

 placed in front of a projecting lens by means of which a vertical image 

 of the mercury is thrown on to a vertical slit, part of which it covers. 

 A photographic plate is moved by clockwork behind the slit, and so 

 records the vertical movements of the mercury meniscus. 



The result of such an experiment is given in fig. 122. It shows 

 that the electrical variation occurring in a single contraction of the 

 uninjured heart is diphasic in character. In the first phase (i, fig. 

 122) the base was negative to the apex ; in the second phase, n, apex 

 was negative to base. 



Experiment 7. — Paradoxical contraction. Pith a frog and expose the 

 sciatic nerve, which at its lower end will be found to split into two branches : 

 one, the tibial nerve, supplies the gastrocnemius, and the other, the pero- 

 neal nerve, supplies the peroneal 

 muscles. Follow the peroneal 

 nerve a little way down the leg 

 and cut it through. In this way 

 is obtained a piece of the sciatic, 

 ab (fig. 123), with the upper ends 

 of its two main branches, one, b c, 

 running to the gastrocnemius, g, the 

 other, b d, the isolated piece of the 

 peroneal. Place a pair of electrodes 

 connected with the secondary coil 

 under b d. "With each shock sent 

 Fig. 123.— Arrangement of Apparatus to through B d the gastrocnemius con- 

 show the Paradoxical Contraction. tracts. 



This result is due to a part of the electrotonic current set up in 

 the peroneal nerve passing through the fibres to the gastrocnemius as 

 they lie side by side in the sciatic, ab, It is not due to an escape of 

 the current because the result is prevented by tying a ligature round 

 the peroneal immediately above the electrodes sufficiently tightly to 

 injure the fibres without, however, cutting through the nerve, which 

 could therefore still act as an electrical conductor and permit escape 

 of current. Moreover the contraction is not due to the negative 

 variation sent along the peroneal fibres as a result of the stimulation, 

 because it is not produced when the peroneal nerve is stimulated 

 mechanically. 



