506 Dynamic Theory. 



times. When by the decay of the motor nerve its excitability is grad- 

 ualty lost, this loss begins at the upper end, or end nearest the center of 

 sensation, and gradually passes down toward the muscle and after the 

 nerve is dead above, it can still be irritated below, till at last when it 

 is dead throughout its length, the irritation of the muscle can be effected 

 by direct application of the electrical current to the muscle which con- 

 tinues alive much longer than the nerve. But when both nerve and 

 muscle are fresh, the muscle is much more susceptible of irritation 

 through the nerve than by the direct application of the electrical stim- 

 ulus. The upper part of a living nerve is more excitable than the lower 

 part. This has been proved in the case of living frogs and living men. 

 When a nerve is cut and the excitability of its lower part is tested, the 

 most excitable point is a little ways from the cut end, at which point 

 the excitability increases for a short time and remains stationary for a 

 while before it begins to decline. Both nerves and muscles become 

 exhausted by over activity and recover somewhat by rest. Evidently 

 in their normal place in the living body the exhausted tissues are re- 

 cuperated from the nutritious matters brought in by the blood, and can 

 be renewed indefinitely and completely, and even more than completely 

 within limits in that way. But in the case of detached nerve or muscle 

 when one excited point is exhausted, its recovery must be at the ex- 

 pense of the other unagitated and unexhausted parts. Each recover}' 

 of these detached nerves and muscles is feebler and less complete than 

 the preceding one, till finally there is no further recovery, all the wearing 

 tissue presumably being consumed. 



Nerve dies in 10 or 15 minutes in a temperature of 44 C, and in a 

 few seconds at 75 C. But at the average temperature of a room the 

 lower ends of a long sciatic nerve of frog may retain their excitability 

 for twenty-four hours longer. The more quickly and suddenly a stim- 

 ulus is applied to a nerve, the greater the excitement. Every sudden pres- 

 sure on a nerve will produce a stimulation of it, but it is possible by a 

 gradually increasing pressure on the nerve to entirely crush it without 

 producing any excitement. It is possible to produce tetanus by gently 

 tapping the nerve with ajittle hammer. If a nerve attached to one 

 gastrocnemius (A) of a frog be placed upon another gastrocnemius (B) 

 so as to touch both a positive and negative point of tension on B, then 

 if B be irritated a pulsation will take place in A. ( Rosenthal. ) The 

 heart of a frog continues to beat for some time after it has been ex- 

 tracted from the body. If the nerve of a muscle is placed on this heart 

 so as to touch its base and point, the muscle pulsates at every beat of 

 the heart. This appears clearly to be a case of electrical induction. If 

 the muscle B is irritated with such rapidity as to produce tetanus in it, 

 tetanus is also produced in A. Induction takes place through the best 



