NERVOUS IRRITABILITY. 777 



Rapid desiccation of nerves by abstracting the water comes under 

 the head of a chemical stimulus. Sugar, urea, glycerin, and many metallic 

 salts likewise act as stimuli and often produce paralysis. 



Nerves may likewise be thrown into activity by the application of 

 the electrical current, whether on employing the constant or induced 

 current. When a constant current is allowed to enter a nerve a single 

 contraction is produced at the moment of application of the current, 

 and no other apparent effect is evident until the current is broken. The 

 breaking of the current again causes a contraction to occur. So, also, in 

 sudden increase or decrease in the strength of a constant current 

 passing through a nerve the same effect will be produced as on making 

 or breaking the current. 



When a constant current which is too feeble to produce a contraction 

 is allowed to pass into a nerve, and the strength of a current then 

 gradually increased, the degree at which the contraction is produced is 

 spoken of as the minimal stimulus. As the current increases in strength 

 the degree of contractions produced, at first rapidly and then more 

 slowly, increases until a maximum is reached ; such a stimulus is spoken 

 of as the maximal stimulus. As a rule, the effect produced by making 

 the current is more powerful than when the current is broken. 



Nerves are more sensitive to electrical stimuli than muscles, and a 

 current which, applied directly to a muscle, may be too feeble to produce 

 contraction may throw the muscle into contraction when allowed to pass 

 through its motor nerve. When a strong current is allowed to pass 

 through a motor nerve for some time and the circuit then suddenly 

 broken, instead of a single contraction the muscle will be thrown into 

 tetanus ; such a condition is especially produced when the positive pole, 

 or anode, is nearest to the muscle, while, when the negative pole, or 

 cathode, is nearest to the muscle, tetanus occasionally follows the making 

 of the current. This effect is to be explained by the production of 

 a condition which is known as electrotonus, which will be alluded to 

 directly. 



When a stimulus is applied to any part of a motor nerve a 

 condition of increased excitation is produced and the impulse travels 

 along the nerve, the direction of the motion depending upon the 

 character of the terminal organs with which the nerve is in communica- 

 tion. When, therefore, a motor nerve is stimulated the impulse travels 

 to the periphery ; when the nerve terminates on a cutaneous surface it 

 travels toward the centre, although it must be understood that nerves 

 maj^ conduct impulses in either direction and even carry impulses 

 simultaneously in different directions without interfering with each 

 other. 



The rate of conduction of nerve impulses is about twenty-seven 



