68 



VETERINARY PHYSIOLOGY 



current in the nerve, and this, of course, causes a contraction. 

 But here the effects of closing and opening the current are 

 practicall}^ fused, and hence the influence of the anode and 

 cathode, and of closing and opening, need not be considered 

 (fig. 27) (Practical Physiology). 



It must, of course, be remembered that in an induction 

 coil the opening of the primary circuit produces a more power- 

 ful current in the secondary coil than the closure of the 



primary circuit, and therefore a more 

 powerful stimulation of the nerve (fig. 

 28). 



1. Relationship of the Excita- 

 I tion to the Strength of the 

 ■ Stimulus. — A nerve is made up of a 



series of axons placed side by side. 

 The difference in the effect of a weak 

 and of a strong stimulus as indicated 

 by the contraction of the muscle 

 supplied may be due either to a 

 graded effect of the stimulus on every 

 fibre or to the number of fibres 

 stimulated by the different strengths 

 of stimulus. The cutaneus dorsi 

 nerve of the frog is composed of only 

 ten fibres, and, as the strength of 

 stimulus is steadily increased, the 



Fig. 28.— Course of Elec- 

 tric Current in primary 

 circuit (lower line) , and in 

 secondary circuit (upper 

 line) of an induction coil. 

 Observe that in the 

 secondary the make (up- 

 stroke) and break (down- 

 stroke) are combined, and 

 that a stronger current 

 is developed in the 

 secondary circuit upon 

 breaking than upon mak- 

 ing the primary circuit. 



resulting contractions increase in ten 

 stages. 



The conclusion is that the 

 result depends upon the number of 

 fibres stimulated, and that, when a 

 stimulus excites a fibre, it does so to call forth its full 

 action — the stimulation of each axon is either all or nothing. 

 Variations in Excitability. — The influence of the 

 galvanic current upon the excitability of nerve has been 

 alread}^ considered (Electrotonus, p. 63). 



Many other factors modify its excitability. It may be 

 increased by a slight cooling, but it is decreased at lower 

 temperatures. It is increased by warming up to a certam 

 point. Drying at first increases excitability, then abolishes 



