GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 57 



of the cold becomes greater the pain is replaced by numbness, both the irrita- 

 bility and power of conduction of the nerve being reduced. Gradual cooling 

 of motor nerves or muscles, and gradual heating, even to the point of death 

 of the tissue, fails to excite contractions. It is stated that if a frog whose 

 brain has been destroyed is placed in a bath the temperature of which is very 

 gradually increased, the heating may be carried so far as to boil the frog without 

 active movements having been called out. If a muscle be heated to 45° C. 

 for frogs and 50° C. for mammals, it undergoes a chemical change, which is 

 accompanied by a form of shortening different from the contraction induced by 

 irritants. This form of contraction, though extensive, is feeble and is asso- 

 ciated with a stiffening of the muscle, known as rigor ccdoris (see p. 164). 



In general it may be said that raising the temperature above the usual tem- 

 perature of the animal increases, while cooling decreases, the irritability of the 

 nerves and muscles. This statement requires to be amplified, because the 

 character of the stimulus has a marked effect upon the result. Cooling 

 the nerve increases its irritability for mechanical and chemical stimuli, for 

 the constant current if it lasts at least 0.005 sec, for condenser discharges, 

 and for sine currents of at least 0.005-0.01 sec. duration : heating; the nerve 

 increases its irritability for these forms of electrical excitation when of 

 shorter duration, and also for induced currents. 1 If a nerve be excited by 

 charging or discharging a condenser through it, the size of the condenser 

 plays an important part, because it determines the duration of the stimulus ; 

 for example a slow, prolonged rate of discharge may excite a nerve at 4° 

 C. and fail to excite one at 30° C, while a rapid, brief fall of energy will 

 excite a nerve at 30° C. and fail to excite one at 4° C. 2 Xot only does 

 temperature influence the ability of the nerve to take on the change which is 

 associated with the development of what we call the nerve impulse, but it 

 alters its power of recovery. This appears in experiments in which the 

 ability of the nerve to respond to two rapidly following stimuli is tested by 

 different temperatures. A nerve, like the heart-muscle, shows a " refrac- 

 tory period" for a short interval after excitation, and during this period it is 

 incapable of responding to stimulation. The length of the interval varies 

 with the temperature. If the two stimuli are separated by an interval of 

 0.001 sec, the second stimulus will be effective at 15° C, but it will fail at 

 3° C. ; at this temperature, even with an interval of 0.006 sec., the second 

 stimulus will be without effect, as much as 0.01— 0.02 sec. being needed for the 

 recovery of nerve at this low temperature. 3 



Cold, unless excessive and long continued, though it temporarily suspends, 

 does not destroy the irritability; while heat, if at all great, so alters the 

 chemical constitution of the cell-protoplasm as to destroy its life. 



The higher the temperature the more rapid the chemical changes of the 

 body and the less its power of resistance ; low temperature, on the other hand, 



1 Gotch and Macdonald: Journal <>/ Physiology, 1896, xx. p. 247. 



2 Waller: Ibid., 1899, x.xiv. p.L 



8 Gotch and Burch: Ibid., 1899, xxiii. |>. 'J-J ; Boycott : Ibid., 1899, xxiv. p. 144. 



