384 



PRINCIPLES OF GENERAL PHYSIOLOGY 



B 



because of their more internal position and consequent short circuiting of the 

 stimulus to a certain extent, or, possibly, owing to differences in their own state 

 of excitability. This being so, a very weak stimulus would excite some and not 

 others, thus causing contraction of a portion only of the fibres of the muscle. 

 Although these latter may respond by a maximal contraction, the fact alone does 

 not, however, prove that the impulse in the nerve fibre was a maximal one, since it 

 might be only just sufficient to cause contraction. 



A very ingenious method was devised by Adrian for the investigation of this 

 and similar questions. All methods of experiment agree in showing that the 

 disturbance, as it passes along the fibres of a normal nerve, suffers no diminution 

 in intensity. If, on the contrary, the nerve is narcotised by the application of an 

 anaesthetic, such as alcohol or morphine, a disturbance, started at one end. 

 decreases in magnitude progressively as it travels along and, if the length or the 

 degree of narcosis is sufficiently great, it is completely wiped out. Since the 

 diminution in the disturbance is a regularly progressive one, it is clear that a 



smaller disturbance will be able 

 to pass along a shorter distance 

 of narcotised fibre without 

 annihilation than one which 

 was larger to start with. In 

 practice, it is more convenient 

 to vary the degree of narcosis, 

 or period during which the 

 anaesthetic is applied. The 

 following description from 

 Adrian's paper (p. 393) will 

 assist the reader : " The point 

 to be decided is whether a dis- 

 turbance which has passed 

 through an area of decrement 

 and entered normal tissue again 

 is equal to or less than a dis- 

 turbance which has been set up 

 in normal tissue, peripheral to 

 the area of decrement, and has 

 consequently escaped any reduc- 

 tion. The following arrange- 

 ment was adopted. Two 

 muscle nerve preparations 



RECOVERY OF A NERVE IMPULSE AFTER IT HAS (sciatic gastrocnem i us) are 

 PASSED THROUGH A REGION OF DECREMENT. taken under exactly similar < >n- 



ditions. In one of these (Fig. 



105, A) two equal lengths of nerve, d and d', are narcotised. These lengths are separ- 

 ated by a variable length of normal nerve and the conductance of a disturbance 

 through one or both of them can be tested by stimulating electrodes I. and II., placet 1 

 as shown in the figure. The other preparation is narcotised at the same rate o\ er 

 a length, D, which is equal to the two lengths, d and oT, together. Conduction through 

 D is tested by electrode III. Under these conditions the decrement suffered by 

 the disturbance from II., in passing through d, will be equal to that suffered l>y 

 the disturbance from III. in passing through the first (central) half of D. If the 

 disturbance does not increase in size when it leaves d, it will enter d' in exactly 

 the same condition as a disturbance which enters the peripheral half of D. In 

 this case the depth of narcosis required to extinguish the disturbance altogether 

 will be the same in both preparations, and stimuli at electrodes II. and III. will 

 become ineffective at the same time. On the other hand, the disturbance from 

 electrode I. will enter d' unreduced and therefore conduction from I. to the muscle 

 will persist for some time after the failure at II. 



"Fig. 105, B may help to make this clearer. Ordinates are intended to 

 represent the size of the disturbance at different points along the nerve during the 



To Muscle 



<o II 



FIG. 105. ADRIAN'S DIAGRAM TO ILLUSTRATE 



THE 

 HAS 



