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TEXT-BOOK OF PHYSIOLOGY 



A combination of these different appliances constitutes a myograph and the 

 curve of contraction a my o gram. (See Fig. 23.) 



It is necessary for the purpose of placing the excised muscle under me- 

 chanic conditions similar to those found in the body and for the registration of 

 its movements under varying conditions to give the lever mass. This is accom- 

 plished by attaching weights to it beneath the muscle. 



FIG. 23. MYOGRAPH. K. Recording cylinder. M. Moist chamber. L. 

 Recording lever. W. Weight. I. Induction coil. 



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

 the successive changes in the length of a muscle during a single contraction 

 and at the same time avoiding changes in tension and therefore an accelera- 

 tion of the lever, the weight attached to the lever should be applied close to its 

 axis, in accordance with the isotonic method. The curve of contraction thus 

 obtained is known as an isotonic myogram. 1 



With the muscle arranged as previously described and stimulated 

 directly with a single induced electric current, the contraction will be re- 

 corded in the form of a curve similar to that represented in Fig. 24, in which 

 the horizontal line represents the abscissa of time; a, the moment of stimula- 

 tion; and b c d, the degree of shortening and the subsequent relaxation at each 

 successive moment. The undulating line shows the time relations, the 

 distance from crest to crest representing hundredths of a second. The 

 curve may be divided into three portions: 



1 The weighting of the lever or the loading of the muscle is accomplished in several ways: (i) 

 The weight is attached to the lever just beneath or in the immediate neighborhood of the point of 

 attachment of the muscle. This is known as the " loaded method" and has the effect of extending 

 the muscle beyond its normal length previous to the moment of its stimulation and contraction. 

 (2) The weight is attached to the lever at the same point as in the foregoing method, but by 

 means of a support beneath the lever, the weight is prevented from extending the muscle previous 

 to the moment of its stimulation and contraction. This is known as the "after-loaded" method. 

 In either case a certain momentum is imparted to the weight, which continues after the muscle 

 has ceased to act, both when shortening and relaxing, and so imparts to the recording lever addi- 

 tional movements which vitiate the true character of the curve. (3) The weight is attached to a 

 small pulley on the axis of the lever and therefore at some distance from the point of attachment 

 of the muscle. The advantage of this method lies in the fact that the initial tension of the muscle 

 induced by the load remains practically constant throughout the contraction period and hence 

 acceleration of the movement of the lever is prevented. This is known as the "isotonic method." 



