254 PRACTICAL PHYSIOLOGY. [LL 



{b.) Cut it as in fig. 177, avoiding injury to the nerves, so that only the 

 nerve twig {k) connects the larger and smaller halves, and in one tongue (Z) 

 terminates a nerve. After excision lay it on a glass plate with a black back- 

 ground, else one does not see clearly the inscription and the course of the 

 nerves. 



(f.) Stimulate the tongue (Z) with fine electrodes about i mm. apart, and 

 contraction occurs in both L and K. This, according to Kiihne, is due to 

 centripetal conduction in a motor nerve. This experiment is adduced by him 

 as the best proof of double conduction in nerve fibres. Mays has shown that 

 the nerve fibre divides and supplies both halves of the muscle. 



{(L) If the muscle be exposed in a curarised frog, on stimulating 

 either half of the muscle with repeated shocks, only that half 

 responds, as the inscription blocks the passage of the muscle-wave. 



{e.) If an uncurarised muscle is used, stimulation of the muscle 

 near its ends causes response only in its own half. Why ? Because 

 there are no nerves there; but stimulation near the inscription 

 causes response in both halves. Why 1 Because they are excited 

 through their nerves, as shown definitely by (c). 



5. Action of a Constant Current — In muscle and nerve, stimulation occurs 

 only at the kathode when the current is made (closed), and at the anode when it 

 is broken {opened)— (V, Bezold). This is most readily seen in fatigued 

 muscles. 



(A.) Engelmann's Experiment. — {a.) Suspend vertically a curarised sar- 

 torius of a frog, and pass a constant current through its upper extremity. 

 On making the current, the muscle moves towards the side of the kathode, 

 because contraction occurs at the kathode on making. At break, it inclines 

 to the anode. 



{b.) Slit up the muscle longitudinally, so that it looks like a pair of 

 trousers, and keep the two legs, as it were, asunder by an insulating medium ; 

 at make, the kathodic half alone contracts ; at break, the anodic half. 



(B, ) Another Method. — Dissect out the sartorius of a curarised frog, but 

 remove it with its bony attachments, clamp it at its centre, and arrange it 

 either vertically as in fig. 191, attaching its ends to two recording levers 

 placed one above it and the other below it, or fix it on a double crank-myo- 

 graph. Pass thin wires from the battery through the two ends of the muscle ; 

 on making the current, the lever attached to the kathode rises before the 

 other, i.e., where the current leaves the muscle. On breaking the current, 

 the anodic lever rises first, showing that the anodic half contracts before the 

 kathodic halt 



LESSON LI. 



OTHER CONDITIONS AFFECTING THE EXCITA- 

 BILITY OF NERVE — CHEMICAL, TEMPERA- 

 TURE, &c. 



1. Unequal Excitability of Diflferent Portions of a Motor 

 Nerve. — Apparatus. — Cell, two keys, wires, commutator, induction 

 coil, either for single or faradic shocks, two pairs of electrodes. 



