146 INFLUENCE OF TEMPERATURE. [BOOK i. 



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, especially when the 

 temperature is thus brought near to 0, slackens all the molecular 

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

 longed, 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 

 kept frozen however for some' few minutes, or when exposed for a 

 less time to temperatures of several degrees below zero, their 

 irritability is permanently destroyed. When after this they are 

 thawed, they are at first supple and as we have seen may be made 

 to yield muscle plasma ; but they very speedily enter into rigor 

 mortis of a most pronounced character. 



85. The influence of blood-supply. When a muscle still 

 within the body is deprived by any means of its proper blood- 

 supply, as when the blood vessels going to it are ligatured, the 

 same gradual loss of irritability and final appearance of rigor 

 mortis are observed as in muscles removed from the body. Thus 



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

 portion of the work. 



