70 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



for mechanical irritants and for direct battery currents (see p. 54) 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 of 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, others consider it to be the result of cir- 

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

 blood-supply, due to the division of the vaso-motor nerves, and still others 

 attribute it to a lack of exercise, it being no longer stimulated to action. As 

 regards the second 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 

 uerves are cut. As regards the first and last views, it may be said that if 

 the muscles be artificially excited, as by electric stimuli, and thus are exer- 

 cised 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 their being 

 specific trophic influences sent from the nerve-cells to the muscles, 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 u-hicJi 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 in part the same as the cause of the loss of irritability and power 

 which follows the cutting off of the blood-supply. The working cell liberates 

 energy at the expense of its store of nutriment and oxygen, and through oxi- 

 dation processes forms waste products which are poisonous to its protoplasm. 

 The fatigue which results from functional activity has, therefore, a twofold 



