258 PRACTICAL PHYSIOLOGY. [LL 



resistance of the tissue, this being lowered by warmth and raised by cold ; in 

 ordtT to get rid of this purely physical change, it is essential that a large 

 resistance should be introduced into the exciting circuit. This is most 

 simply done by using non-i>olarisable electrodes with threads attached to the 

 ends of the electrodes kept moist by normal saline solution. The threads are 

 now arranged so as to touch the nerve where it lies on the tube, one thread 

 being placed so that the contact shall be on the edge of the cooling tube 

 nearest the muscle. The simplest method of exciting the nerve is by means 

 of a weak galvanic current. For this purpose the rheochord is used and a 

 weak current employed of such direction that it shall descend the nerve and 

 thus excite this at the cathodic contact on the distal edge of the glass tube. 



In order to ensure that the galvanic current is always of the same duration, 

 it is desirable to close the current by an automatic arrangement, either a 

 revolving drum carrying a striker which shall at each revolution strike a 

 stretched wire, or a metronome ; but the influence of the temperature alteration 

 may be obtained- without this arrangement, the closure being effected by a 

 Pohl's reverser without cross lines as a double make mercurial key worked by 

 the hand. 



The nerve-muscle preparation having been made and the muscle attached 

 to an appropriate lever, so as to record its contraction uj)on a very slowly 

 moving surface, an intensity of current is ascertained, which, with the nerve 

 at the normal temperature of the room, is only just adequate to evoke a very 

 weak minimal muscular response whenever the circuit is closed. 



The temperature of the nerve is now raised by allowing water at 30° C. to 

 pass through the tube, when the response will disappear ; the temperature is 

 now lowered by allowing water at 10^ C. or less to flow — the response is now 

 very marked. Localised cold thus increases the excitability of nerve to this 

 form of stimulus. Similar eflects can be obtained with the condenser dis 

 charge, with mechanical and with chemical stimuli. If the induction 

 current is used instead of the galvanic current, a reverse effect is obtained, 

 the nerve-muscle preparation responding better when the excited nerve is at 

 30'' C. ; and this favourable influence of warmth persists even when a very 

 large external resistance is introduced into the circuit. 



B. Upon Muscle. 



The sartorius muscle of the frog is used for this experiment, the threads of 

 the exciting electrodes being placed upon the broad "nerveless" pelvic end 

 of the muscle under which the tube of the cooling arrangement is lixed. It 

 is then found that the muscle responds better when cooled to every form of 

 stimulus applied to the cooled region, including the induction current. If 

 the electrodes be shifted to the "nerved" portion of muscle, the response, 

 being indirect, is disfavoured by cold when the induction current is used.— 

 (Communicated by Fro/essor Gotch.) See also Journal of Fhys,^ XII. 



