76 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



diately invade the muscle, which may be considered a proof that nerve and 

 muscle protoplasm are not continuous. 



Though no immediate change in the structure of the muscle is observable, 

 the irritability of the muscle soon begins to alter. At the end of a fortnight 

 the irritability of the muscle for all forms of stimuli is lessened. From this 

 time on, the irritability gradually undergoes a remarkable change, the excita- 

 bility for mechanical irritants and for direct battery currents beginning to 

 increase, but the power to respond to electric currents of short duration, 

 as induction shocks, continuing to lessen; indeed, the reactions of the 

 muscle appear to take on more of the character of those of smooth muscle- 

 fibres. The condition of increasing irritability to direct battery currents and 

 mechanical irritants reaches its maximum by the end of the seventh week, 

 and from that time on the power to respond to all forms of stimuli lessens, 

 the excitability being wholly lost by the end of the seventh or eighth month. 

 During the stage of increased excitability fibrillary contractions are often 

 observed. 



As in the case of a nerve so in the muscle the loss of irritability is due to 

 degenerative changes which gradually lead to the destruction of the muscle 

 protoplasm. The cause of the change in the muscle is still a matter of doubt, 

 some regarding it as due to the absence of some nutritive, trophic influence 

 from the central nervous system, while others consider it to be the result of 

 circulatory disturbances, consequent upon the lack of a proper regulation of 

 the blood-supply, due to the division of the vaso-motor nerves. As regards 

 the latter view, it may be said that muscles whose vaso-motor nerves are intact, 

 the vessels being innervated through other nerves than those which supply the 

 muscle-tissue proper, as is the case with some of the facial muscles, undergo 

 similar changes in irritability when their motor nerves are cut. As regards 

 the former view, it may be said that if the muscles be artificially excited, as by 

 electric stimuli, and thus are exercised daily, the coming on of degeneration can 

 be at least greatly delayed. The question as to whether the anabolic processes 

 within the muscle-cell are dependent on the central nervous system, in the sense 

 of there being specific trophic influences sent from the nerve-cells to the mus- 

 cles, is still under discussion and need not be considered further in this place. 

 Without doubt the reflex tonus impulses which during waking hours are all 

 the time coming to the muscles are productive of katabolic changes and, 

 indirectly at least, favor anabolism. 



(c) Effect of Influences which result from the Functional Activity of Nerves 

 and Muscles. Fatigue of Muscles. The condition of muscular fatigue is cha- 

 racterized by lessened irritability, decrease in the rate and vigor with which 

 the muscle contracts and liberates energy, and a still greater decrease in the 

 rate with which it relaxes and recovers its normal form. In a sense, whatever 

 induces such a state can be said to cause fatigue, but it is perhaps best to 

 restrict the term to the form of fatigue which is produced by excessive 

 functional activity. The cause of exhaustion which results from over- 

 work is much the same as the cause of the loss of irritability and power 



