CH. XV.] NERVE IMPULSES 173 



The rate of the transmission of nervous impulses discovered by 

 these methods is, in a frog's motor nerve, 28 to 30 metres a second ; 

 in human motor nerves, 33 metres a second ; in sensory nerves, 30 

 to 33 metres a second. 



Direction of a Nerve Impulse. 



Nerve impulses are conducted normally in only one direction : in 

 efferent nerves from, in afferent nerves to, the nerve-centres. But 

 there are some experiments which point to the conduction occurring 

 under certain circumstances in both directions. 



Thus, in the rheotome experiment just described, if the nerve is 

 stimulated in the middle instead of at one end, 

 the electrical change (the evidence of an im- 

 pulse) is found to be conducted towards both 

 ends of the nerve. 



Kiihne's gracilis experiment proves the same 

 point. The gracilis muscle of the frog (fig. 

 191) is in two portions, with a tendinous in- 

 tersection, and supplied" by nerve-fibres that 

 branch into two bundles; excitation strictly 

 limited to one of these bundles, after division 

 of the tendinous intersection, causes both por- 

 tions of the muscle to contract. 



Another striking experiment of the same 

 kind can be performed with the nerve that ?1G ' l 

 supplies the electrical organ of Malapterurus. 

 This nerve consists of a single axis cylinder and its branches ; stimu- 

 lation of its posterior free end causes the " discharge " of the electrical 

 organ, although the nervous impulse normally travels in the opposite 

 direction. 



Crossing of Nerves. 



Some experiments designed to prove the possibility of nervous 

 conduction in both directions were performed many years ago by 

 Paul Bert. He grafted the tip of a rat's tail either to the back of 

 the same rat, or to the nose of another. When union had been 

 effected, the tail was amputated near its base. After a time, irritation 

 of the end of the trunk-like appendage on the back or nose of the 

 rat gave rise to sensation. The impulse thus passed from base to 

 tip, instead of from tip to base, as formerly. This experiment does 

 not, however, prove the point at all ; for all the original nerve-fibres 

 in the tail must have degenerated, and the restoration of sensation 

 was due to new fibres, which had grown into the tail. Exactly the 

 same objection holds to another series of experiments, in which the 



