382 



PRINCIPLES OF GENERAL PHYSIOLOGY 



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unaltered ; in fact, there must l>e a change of potential in order to excite, and this 

 change must not be too gradual or it will not excite at all, nor too rapid, as the 

 extremely rapid alternations of the Tesla currents, which are practically inactive 

 in proportion to the energy which they contain. These currents are produced by 

 induction from the rapid natural oscillatory discharge of a condenser, such as 

 a Leyden jar, charged to a high potential by connection to a large induction coil 

 or influence machine. A current of nearly half an ampere, sufficient to light an 

 incandescent lamp in the same circuit, can be sent through the human bndv 

 without exciting nerves therein. It appears, indeed, that what effects are 

 produced by these so-called "high-frequency" currents are merely due to the heat 

 into which they are converted in the tissues through which they flow. 



A convenient method of application of currents of different time course is by 

 the rheonome of von Fleischl, described in Garten's article (1908, p. 406). Keith 

 Lucas (1907, 1) describes a simple form of rheonome, formed by an ebonite 

 diaphragm with a hole, which is moved across another hole in a second shutter, 

 which separates two compartments, each containing saturated solution of zinc 



sulphate, through 



. ... . which the current 



j Kisses by means 

 of a zinc electrode 

 in each compart- 

 ment. 



The use of alter- 

 nating currents of 

 sinusoidal form pre- 

 sents some advan- 

 tages on account of 

 the equality and re- 

 gularity with which 

 they can be made to 

 stimulate. Currents 

 with an approxi- 

 mately sine curve 

 can be obtained by 

 the rotation of a 

 coil in a magnetic 

 field, or vice versa, 

 but to obtain mathe- 

 matically correct 



curves requires very accurate apparatus. The alternating current supplied by central stations 

 has nearly a sine curve and, when available, forms a convenient means of getting very regular, 

 graduated, tetanising stimuli from an induction coil of the Du Bois Reymond type. It can be 

 sent through the secondary coil and the electrodes connected to the primary, or vice versa. In 

 the latter case, of course, a lamp resistance must intervene between the mains and the coil. 

 The strength of the induced currents is varied by altering the distance between the coils. 



The various patterns of electrodes used for applying electrical stimulation to 

 nerves will be found in Garten's article (1908, pp. 331-340). Two additional 

 useful patterns may be mentioned here. The first is that used by Sherrington 

 (1909, p. 382), especially for deep-lying nerves, and consists of a glass T-tube, into 

 which the cut nerve is drawn, the current being applied by two platinum wires, 

 one on each side of the nerve, passing through the side branch (see Fig. 103). 

 They are also very good for superficial nerves, since they prevent drying and can 

 be kept warm by a current of saline over the outside. The electrodes of Keith 

 Lucas (1913, 2, and Fig. 104) are useful when it is necessary to excite nerves 

 immersed in a saline solution, such as sea water or Ringer's solution, without the 

 current spreading to neighbouring parts. The principle on which these electrodes 

 are constructed is that the sectional area of the solution around the nerve is made 

 to change very suddenly at the point where stimulation is desired and the current 

 is made to pass by this course. Electrodes on the same principle, for the 

 exact localisation of stimuli on the excised nerve muscle preparation, are 

 described by the same investigator (1908, p. 114) and their degree of accuracy 

 determined. 



Nerves can also be excited by chemical means, as by crystals of salt or by 



Fio. 103. SHERRINUTON'S ELECTRODES FOR STIMULATINO NERVES. The' 

 nerve is protected from drying, and the electrodes can be sewn up 

 in the wound, or kept warm by a current of warmed saline run over 

 them. 



(Sherrington, 1909, 1.) 



