66 TEXT-BOOK OF PHYSIOLOGY. 



impulse to the muscle. By this means it is protected from absolute 

 exhaustion. 



5. Nutrition. The irritability of a muscle which conditions the con- 

 traction process is dependent on the maintenance of its nutrition; 

 hence a continuous and sufficient supply of nutritive material and 

 a rapid removal of waste products are essential conditions for the ex- 

 hibition of normal contractions. A diminution of blood supply or 

 an accumulation of waste products sooner or later impairs the irritability 

 and diminishes the vigor and extent of the contraction. Various 

 drugs e.g., veratrin, barium, etc. introduced into the circulation and 

 finding their way into the muscle modify the contraction process, as 

 shown by a very great increase in the duration of the relaxation period. 

 The Isometric Myogram. With the object of obtaining a curve of 

 the increase and decrease in the tension of a muscle during a single con- 

 traction, with the exclusion as far as possible of a change in length, the muscle 

 may be made to contract against a strong spring or similar resistance practically 

 though not absolutely sufficient to prevent shortening. To this method 

 the term isometric has been given, and the curve so obtained is an isometric 

 myogram or a tonogram. The recording portion of the lever is prolonged 



some distance so that the very slight 

 upward movement at its axis, close to 

 which the muscle is attached, will be 

 considerably magnified. That the 

 ordinate value of an isometric curve 

 may be known, the apparatus must be 

 graduated by subjecting the spring to 

 a series of weights playing over a pul- 

 ley supported by the muscle clamp. 



The curve of the variation in ten- 



FIG. 29. a . DIAGRAM OF ISOTONIC; b, s i on obtained by the isometric method 

 Z< 4Sg? MUSCLE CURVES - ( " S is shown in Fig. 29, b, in which the two 



curves are contrasted. The form of the 



curve indicates that the muscle attains its maximum of tension more 

 rapidly than its maximum of shortening; that the tension endures for a 

 certain period of time unchanged; that the fall in tension takes place more 

 rapidly than the muscle lengthens. 



The Myogram Due to the Make and the Break of a Galvanic Current. 

 The contraction of the muscle which has heretofore been recorded has 

 been caused by the momentary action of an induced current. The con- 

 traction of the muscle which is caused by the action of a constant or galvanic 

 current presents features which are somewhat different and, as it serves to 

 illustrate the difference in the effects of a constant or galvanic and an induced 

 or interrupted current, a myogram of a contraction due to the make and 

 break of a galvanic current is introduced at this place. The effects which 

 are observed in a muscle during the passage of both feeble and strong 

 currents have been alluded to in a previous section. (See page 57.) In 

 Fig. 30 these effects are graphically represented. It will be observed that 

 on the closure of the circuit at c the muscle at once contracted and so long as 

 the current was flowing, the muscle remained in a more or less contracted 



