84 HUMAN PHYSIOLOGY 



in time between the two trials must be attributed to the 

 employment of an extra length of nerve in the first case. 

 By a simple calculation it appears that the rate of trans- 

 mission in the nerve of a frog at room temperature is in 

 the neighborhood of 100 feet per second. This is the 

 speed of an express train running 70 miles an hour. 

 Higher rates seem to be reached in the warm-blooded 

 animals but the maximum is less than the velocity of 

 sound waves in the air, and is insignificant by com- 

 parison with the speed of light. 



The term nerve-impulse which we usually use to denote 

 the energy carried along a nerve does not bind us to any 

 particular theory as to its nature. Whatever it is, it is 

 rapid enough to insure prompt reactions in animals of 

 ordinary size. Furthermore, it is not appreciably ex- 

 hausting to the fibers concerned in forwarding it. 

 Muscles fatigue with use, as we have seen; nerves seem 

 nearly if not quite proof against fatigue. The reader 

 should recognize that this statement is limited to the 

 fibers on which our attention for the present is fixed. 

 There are other elements in the nervous system which 

 probably suffer wear and tear when in use. The sus- 

 ceptibility of these structures to fatigue will be dis- 

 cussed at another time. 



Origin of Nerve -impulses. We can now proceed to 

 investigate the place of origin of the nerve-impulses 

 by which muscles are thrown into action. We have 

 pictured a cluster of muscle 'fibers whose end-plates 

 are supplied by the branches of a single nerve fiber. 

 Whence does this fiber 'come? The answer will depend 

 upon the position ojf the muscle in the case so we will 

 assume a definite example. Suppose that it is the biceps 

 of the arm. A motor fiber ending in this muscle could 

 be traced back to the intricate commingling of nerves 

 in the neck to which we give the name of the cervical 

 plexus. The fiber could, theoretically at least, be 

 shown to have come by an unbroken course through 



