148 INFLUENCE OF TEMPERATURE. [BOOK i. 



all irritability and ultimately its place is taken by connective 

 tissue. 



84. The influence of temperature. We have already seen 

 that sudden heat (and the same might be said of cold when 

 sufficiently intense), applied to a limited part of a nerve or muscle, 

 as when the nerve or muscle is touched with a hot wire, will 

 act as a stimulus. It is however much more difficult to gene- 

 rate nervous or muscular impulses by exposing a whole nerve or 

 muscle to a gradual rise of temperature. Thus according to most 

 observers a nerve belonging to a muscle 1 may be either cooled 

 to C. or below, or heated to 50 or even 100 C., without dis- 

 charging any nervous impulses, as shewn by the absence of con- 

 traction in the attached muscle. The contractions moreover may 

 be absent even when the heating has not been very gradual. 



A muscle may be gradually cooled to C. or below without 

 any contraction being caused; but when it is heated to a limit, 

 which in the case of frog's muscles is about 45, of mammalian 

 muscles about 50, a sudden change takes place : the muscle falls, 

 at the limiting temperature, into a rigor mortis, which is initiated 

 by a forcible contraction or at least shortening. 



Moderate warmth, e.g. in the frog an increase of temperature 

 up to somewhat below 45 C., favours both muscular and nervous 

 irritability. All the molecular processes are hastened and facili- 

 tated: the contraction is for a given stimulus greater and more 

 rapid, i.e. of shorter duration, and nervous impulses are generated 

 more readily by slight stimuli. Owing to the quickening of the 

 chemical changes, the supply of new material may prove insuffi- 

 cient ; hence muscles and nerves removed from the body lose their 

 irritability more rapidly at a high than at a low temperature. 



The gradual application of cold to a nerve produces effects 

 which differ according to the kind of stimulus employed in testing 

 the condition of the nerve ; but it may be stated in general that a 

 low temperature, especially one near to 0, slackens all the molecu- 

 lar processes, so that the wave of nervous impulse is lessened and 

 prolonged, the velocity of its passage being much diminished, e.g. 

 from 28 metres to 1 metre per sec. At about the irritability 

 of the nerve disappears altogether. 



When a muscle is exposed to similar cold, e.g. to a tempera- 

 ture very little above zero, the contractions are remarkably pro- 

 longed ; they are diminished in height at the same time, but not 

 in proportion to the increase of their duration. Exposed to a 

 temperature of zero or below, muscles soon lose their irritability, 

 without however undergoing rigor mortis. After an exposure of 

 not more than a few seconds to a temperature not much below 

 zero, they may be restored, by gradual warmth, to an irritable con- 

 dition, even though they may appear to have been frozen. When 



1 The action of cold and heat on sensory nerves will be considered in the later 

 portion of the work. 



