DEGREE OF IRRITABILITY OF MUSCLES AND NERVES. 113 



perience goes at present the artificial stimulation cannot fully replace the 

 natural one, and sooner or later the muscle like the nerve suffers degenera- 

 tion, loses all irritability, and ultimately its place is taken by connective 

 tissue. 



82. 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 generate nervous or muscular impulses by exposing a whole 

 nerve or muscle to a gradual rise of temperature. 



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

 traction being caused ; but when it is heated to a limit, which in the case of 

 frog's muscles is about 45 C., of mammalian muscles about 50 C., a sud- 

 den change takes place : the muscle falls at the limiting temperature into a 

 rigor mortis, which is initiated by a forcible contraction or at least short- 

 ening. 



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

 somewhat below 45 C., favors both muscular and nervous irritability. All 

 the molecular processes are hastened and facilitated : the contraction is for 

 a given stimulus greater and more rapid, i. e., of shorter duration, and ner- 

 vous impulses are generated more readily by slight stimuli. Owing to the 

 quickening of the chemical changes, the supply of new material may prove 

 insufficient ; 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 tempera- 

 ture is thus brought near to C., slackens all the molecular 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 second. 

 At about C., the irritability of the nerve disappears altogether. 



When a muscle is exposed to similar cold, e. g., to a temperature very 

 little above zero, the contractions are remarkably prolonged ; 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. 



83. 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, if the abdominal aorta be ligatured, the muscles of 

 the lower limbs lose their irritability and finally become rigid. So also in 

 systemic death, when the blood-supply to the muscles is cut off by the cessa- 

 tion of the circulation, loss of irritability ensues, and rigor mortis eventually 

 follows. In a human corpse the muscles of the body enter into rigor mortis 

 in a fixed order; first, those of the jaw and neck, then those of the trunk, 

 next those of the arms, and lastly those of the legs. The rapidity with 

 which rigor mortis comes on after death varies considerably, being deter- 

 mined both by external circumstances and by the internal conditions of the 

 body. Thus, external warmth hastens and cold retards the onset. After 

 great muscular exertion, as in hunted animals, and when death closes 

 wasting diseases, rigor mortis in most cases comes on rapidly. As a general 

 rule, it may be said that the later it is in making its appearance the more 

 pronounced it is, and the longer it lasts ; but there are many exceptions, 

 and when the state is recognized as being fundamentally due to a clotting of 

 myosin, it is easy to understand that the amount of rigidity, i. e.,the amount 

 of the clot, and the rapidity of the onset, i. e., the quickness with which 



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