GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 



75 



weights, the extension during the latter is progressively greater, until 

 with a given weight the length of the muscle is the same. Under 

 such circumstances, there being no shortening of the muscle, the 

 energy of its contraction manifests itself physically merely as tension. 

 In the successive actions of the muscle represented in the same figure 

 there is to be observed also a combination of a change of length and 



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FIG. 22. EXTENSION CURVES: B B', of the resting; b B', of the contracting muscle. 



a change of tension, the ratio of the one to the other being deter- 

 mined by the amount of the supported weights. When the weight 

 is slight in amount, the shortening of the muscle reaches a maximum 

 and the tension a minimum ; when the weight is large in amount, the 

 reverse conditions obtain. 



THE CONTRACTION PROCESS. METHODS OF INVESTIGATION. 



The contraction of a muscle as it takes place in the living body and 

 under normal physiologic conditions is a complex act, persisting for 

 a variable length of time in accordance with the number of stimuli 

 transmitted to it in a given unit of time, and as determined experi- 

 mentally is the resultant of the fusion of a greater or less number of 

 separate and individual contractions or pulsations. To this enduring 

 contraction the term tetanus has been given. With the aid of ap- 

 propriate apparatus it has become possible to obtain and record 

 single muscle contractions, to analyze and decompose them into their 

 constituent elements, or to combine them in such a manner as to pro- 

 duce practically a normal physiologic tetanus. As in the experi- 

 mental study of the phenomena of a muscle contraction it frequently 

 becomes necessary to remove the muscle from the body of the animal, 

 the muscles of warm-blooded animals are not well adapted for this 

 purpose, owing to the rapid alteration in composition they undergo, 



