36 rHYSIOLOGY. 



The Nature of a Nervous Impulse. Of the nature 

 of a nerve impulse we know but little. It is convenient 

 to compare the nervous system, with its conducting fibers 

 and central ganglia, to a telegraph system. And electric- 

 ity is the most convenient stimulus for exciting nerve im- 

 pulses. Yet a nerve impulse is very different from an 

 electric current. A nerve fiber is a poor conductor of 

 electricity. An electric current may travel along a copper 

 wire at the rate of between 100,000 and 200,000 miles a 

 second, while a nerve impulse in a motor nerve travels 

 only 1 70 feet in a second. 



Transmission of Motor Impulses. When a motor fiber is stimu- 

 lated in the middle of its course we observe only one effect, the 

 shortening of the muscle at its lower end. But there is every reason 

 to believe that the nerve current, or impulse, runs along the nerve in 

 both directions from its starting point. But while the action of the 

 muscle at the peripheral extremity manifests the existence of the cur- 

 rent, there is nothing at the central extremity to give such evidence. 



Transmission of Sensory Impulses. Similarly, when a sensor 

 nerve fiber is stimulated at some intermediate point, we have a sensa- 

 tion in the brain due to the current brought by the afferent fiber, and 

 which we refer to the outer end of the nerve fiber. Probably a nerve 

 impulse passed from the point of stimulation to the outer end of the 

 fiber ; but as there is nothing at the outer end of the nerve fiber to 

 interpret it, we get no evidence of such impulse except, by refined 

 physiological tests. 



Harmony in Muscle Action. In throwing a stone a 

 number of muscles are used. Each one of these must 

 shorten in the right way and at the right time or the throw 

 will not be accurate. Each muscle shortens under the 

 influence of a nerve impulse started by the brain and 

 brought by a motor nerve. If any muscle shortens an 

 instant too soon, or a little too strongly, the stone goes to 



