116 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



effect of temperature on the velocity of the impulse is shown 

 graphically in Fig. 53. Anesthetics and narcotics,* such as ether, 



Fig. 53. — Figure to show the effect of temperature on the velocity of the nerve impulse. 

 At each teinperature two contractions of the gastrocnemius were recorded, one when the 

 nerve was stimulated close to the muscle, one when it was stimulatexi further away (44 mm.). 

 The horizontal distance between the curves as they rise can be expressed in time by refer- 

 ence to the tuning-forli vibrations (200 per second) given below. For intervals of 10° C. 

 it will be seen that the velocity, as indicated by tlie reciprocals of the distances between 

 the pairs of curves, indicates a coefficient of two. — {Snyder.) 



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 interesting fact that the conductivity of 

 the nerve may be suspended also by deprivation of oxygen, f — 

 that is, by locaj suffocation or asphyxia. A nerve fiber sur- 

 rounded 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 increase(i or decreased or suspended entirely by the 

 action of a galvanic (polarizing) current on account of the develop- 

 ment of a condition of electrotonus. This method of suspending 

 conductivity temporarily has been frequently employed for experi- 

 mental purposes, the arrangement being as represented in Fig. 54. 

 The Refractory Period. — In the case of the heart, the nerve 

 cell, and the muscle it has been shown that for a short period after 

 the tissue enters into a condition of functional activity it is non- 

 irritable toward a second stimulus. This condition of loss of 

 excitability following upon or accompanying functional activit}^ 

 is designated as the refractory period. It is interesting to find that 

 a tissue so irritable as a nerve fiber exhibits the same phenomenon. 

 For a very brief period (0.002 to 0.003 second) after an excitation 



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

 t Baeyers, ibid., 2, 169, 1903. 



