NATURE OF THE NERVE IMPULSE. 109 



as they seemed to be at the time when it was supposed that severed 

 nerve fibers can unite immediately, by first intention, without 

 previous degeneration. A similar objection applies to Paul Bert's 

 often quoted experiment. Bert implanted the tip of a rat's tail 

 into the skin of its back. After union had taken place the tail 

 was severed at the base, and the stump now attached to the back 

 was tested from time to time as to its sensibility. Sensation 

 returned slowly. At first it was indefinite, but by the end of a 

 year was apparently normal. 



Modification of the Nerve Impulse by Various Influences 

 Narcosis. The strength of the impulse and its velocity may be 

 modified in various ways: by the action of temperature, narcotics, 

 pressure, etc. Variations of temperature, as stated before, change 

 the velocity of propagation of the impulse, the velocity increasing 

 with a rise of temperature up to a certain point. So also the irri- 

 tability and the conductivity of the nerve fiber are influenced 

 markedly by temperature. If a small area of a nerve trunk be 

 cooled or heated the nerve impulse as it passes through this area 

 may be increased or decreased in strength or may be blocked 

 entirely. Different fibers show somewhat different reactions in 

 this respect; but, speaking generally, the limits of conductivity 

 in relation to temperature lie between C. and 50 C. Cooling 

 a nerve to C. will in most cases suspend the conductivity, but 

 this function returns promptly upon warming.* This fact fur- 

 nishes a convenient means of blocking the nerve impulses in a 

 nerve trunk for any desired length of time. In the same way 

 anesthetics and narcotics, f such as ether, chloroform, cocain, 

 chloral, phenol, alcohol, etc., may be applied locally to a nerve 

 trunk and if the application is made with care the conductivity 

 and irritability may be lessened or suspended entirely at that point, 

 to be restored again when the narcotic is removed. It is an inter- 

 esting fact that the conductivity of the nerve may be suspended 

 also by deprivation of oxygen,! that is, by local suffocation or 

 asphyxia. A nerve fiber surrounded by an oxygen-free atmosphere 

 will slowly lose its conductivity, and this property will be restored 

 promptly upon the admission of oxygen. Compression of a nerve 

 will also suspend its conductivity without permanently injuring 

 the fibers provided the pressure is properly graduated. Lastly, as 

 was explained in a preceding chapter, the conductivity of the nerve 

 may be increased or decreased or suspended entirely by the action 

 of a galvanic (polarizing) current. This method of suspending 

 conductivity temporarily has been frequently employed for ex- 



* Howell, Budgett, and Leonard, "Journal of Physiology," 16, 298, 1894. 

 t Frohlich, " Zeitschrift f . allgemeine Physiol.," 3, 75, 1903. 

 j Baeyers, ibid., 2, 169, 1903. 



