32 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



containing the nerve. The wires from the battery are connected with binding-posts, a, 6 

 (Fig. 9), at opposite sides of a circular groove containing a saturated solution of zinc sul- 

 phate. Strips of amalgamated zinc connect the binding-posts with the fluid, and so com- 

 plete a circuit which offers much resistance to the passage of the current. From the centre 

 of the block containing the groove rises an upright bearing a movable horizontal bar. from 

 each extremity of which an amalgamated zinc rod. e and/, descends and dips into the zinc- 

 sulphate solution. The zinc roils are connected with binding-posts on the movable bar, and 

 from these wins pass to the electrodes on which the nerve rests. The bar revolves on a 

 pivot on the top of the upright, and thus the zinc rods can be readily approached to 

 or removed from the zinc strips, the poles of the battery. When the zinc rods hold a 

 position midway between these poles, the current all passes by the way of the fluid. As 

 the bar is turned, so as to bring the zinc rods nearer and nearer the two poles of the bat- 

 tery, the current divides, and more and more of it passes through the path of lessening 

 resistance of which the nerve is a part. When the zinc rods are brought directly opposite 

 the poles of the battery nearly all the current passes by the way of the nerve. If the bar be 

 turned more or less rapidly, the current is thrown into, or withdrawn from, the nerve more 

 or less quickly. 



By this arrangement we can not < ml v observe that the nerve fails to be irritated 

 when the current is made to enter or leave it gradually, and when it is flowing 

 continuously through it, but that sudden variations in the density of the cur- 

 rent flowing through the nerve, such as are caused by quick movements of the 

 bar, although they do not make or break the circuit, serve to excite. This 

 experiment shows that electricity, as such, does not irritate a nerve, but that a 

 sudden change in the density of the current, whether it be an increase or 

 decrease, produces an alteration in the nerve-protoplasm which excites it to 

 action and causes the development of what we call the nerve-impulse. 



Du Bois-Reymond's Law. — Du Bois-Revmond formulated the following 

 rule for the irritation of nerves by the electrical current : " It is not the abso- 

 lute value of the current at each instant to which the motor nerve replies by a 

 contraction of its muscle, but the alteration of this value from one moment to 

 another; and, indeed, the excitation to movement which results from this change 

 is greater the more rapidly it occurs by equal amounts, or the greater it is in 

 a given time" 



We shall have occasion to see that this rule has exceptions, or rather that 

 there is an upper as well as lower limit to the rate of change of density of the 

 electric current which is favorable to irritation. 



Similar observations may be made with other forms of irritants. Pres- 

 sure, if brought to bear on a nerve gradually enough, may be increased to the 

 point of crushing it without causing sufficient irritation to excite the attached 

 muscle to contract, although, as has been said, a very slight tap is capable of 

 stimulating a nerve. Temperature, and various chemicals, likewise, must be so 

 applied as to produce rapid alterations in the nerve-protoplasm in order to act 

 as irritant-. The same rule would seem to hold good for the nerve-cells of the 

 central nervous system. It is a matter of daily experience that the nervous 

 mechanisms through which sensory impressions are perceived are vigorously 

 excited by sudden alterations in the intensity of stimuli reaching them, and but 

 little affected by their continuous application; the withdrawal of light, a sudden 



