CHAP, ii.] THE CONTRACTILE TISSUES. 127 



all irritability and ultimately its place is taken by connective 

 tissue. 



For some time the irritability of the muscle, as well towards 

 stimuli applied directly to itself as towards those applied through 

 the impaired nerve, seems to be diminished ; but after a while a 

 peculiar condition (to which we have already alluded, 73) sets 

 in, in which the muscle is found to be not easily stimulated by 

 single induction-shocks but to respond readily to the make or 

 break of a constant current. In fact it is said to become even 

 more sensitive to the latter mode of stimulation than it was 

 when its nerve was intact and functionally active. At the same 

 time it also becomes more irritable towards direct mechanical 

 stimuli, and very frequently fibrillar contractions, more or less 

 rhythmic and apparently of spontaneous origin, though their 

 causation is obscure, make their appearance. This phase of 

 heightened sensitiveness of a muscle, especially to the constant 

 current, appears to reach its maximum, in man at about the 

 seventh week after nervous impulses have ceased, owing to injury 

 to the nerves or nervous centre, to reach the muscle. 



79. 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 motor 

 nerve l or muscle to a gradual rise of temperature. 



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 

 musclesabout 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 mole- 

 cular processes, so that the wave of nervous impulse is lessened 

 and prolonged, the velocity of its passage being much diminished, 



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

 portion of the work. 



