CHAP, ii.] TUE CONTRACTILE TISSUES. 03 



the artificial stimulation cannot fully replace the natural one and 

 sooner or later the muscle like the nerve suffers degeneration, loses 

 all irritability and ultimately becomes replaced by connective 

 tissue. 



The Influence of Temperature. 



We have already seen that sudden heat 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, and the same might 

 be said of cold when sufficiently intense. It is however much 

 more difficult to generate nervous or muscular impulses by ex- 

 posing 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 

 100C., without discharging any nervous impulses, as shewn by the 

 absence of contraction in the attached muscle. The contractions 

 moreover may be absent even when the heating has not been very 

 gradual. 



A muscle may be cooled to C. or below without any contrac- 

 tion 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. The rigor mortis thus brought 

 about by heat is often spoken of as rigor caloris. 



Moderate warmth, ex. gr. 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 m. to 1 m. per sec. At about the irritability of the nerve 

 disappears altogether. 



When a muscle is exposed to similar cold, ex. gr. 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, 



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

 portion of the woik. 



