NATURE OF THE NERVE IMPULSE. 115 



unite a motor and sensory trunk directly, to cut the hypoglossal 

 (motor) and the lingual (sensory) and suture, say, the central stump 

 of the lingual to the peripheral stump of the hypoglossal. If stimu- 

 lation of this latter trunk, after union had been established, gave 

 signs of sensation it was considered as proof that the efferent hypo- 

 glossal fibers were now conducting afferently. We now know that in 

 such a case the old hypoglossal fibers degenerate completely, and 

 the new ones that are eventually formed in their place are out- 

 growths from the lingual stump, or at least are not the old efferent 

 fibers, and hence experiments of this kind are not so conclusive 

 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 Temperature. The strength of the impulse and its 

 velocity may be modified in various ways: by the action of 

 temperature, narcotics, pressure, etc. Variations of tempera- 

 ture, 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 irritability as well as the con- 

 ductivity of the nerve fiber is influenced markedly by tem- 

 perature. 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.* By this means 

 we can block the nerve impulses in a nerve trunk for any desired 

 length of time. The exact relationship between the temperature 

 of the nerve and the velocity of the impulse has been studied 

 carefully with the object of determining the temperature coeffi- 

 cient. It has been shown by van't Hoff that the velocity of 

 chemical reactions is increased twofold or more for each rise of 

 10 degrees in temperature, that is, the temperature coefficient 

 for chemical reactions lies between 2 and 3. On the other hand, 

 with most physical processes the temperature coefficient for the 

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



