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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



rises and falls as the muscle contracts and relaxes, and records the movement 

 of the muscle on a surface which passes by the writing-point at a regular 

 speed (see Fig. 38); such a record is called a myogram. 



The Myograph. — The writing mechanism, together with the apparatus 

 which moves the surface on which the record of the movement of a contracting 

 muscle is taken is called a myograph. The writing mechanism has usually the 

 form of a light, stiff lever, which moves very easily on a delicate axis; the 

 lever is so connected with the muscle as to magnify its movements. The point 

 of the lever rests very lightly against a glass plate, or surface covered with 

 glazed paper, which is coated with a thin layer of soot. The point of the lever 

 scratches off the soot, and the movements are recorded as 

 a very fine white line. At the close of the experiment 

 the record is made permanent by passing it through a 

 thin alcoholic solution of shellac. The recording surface 

 in some cases is in the form of a plate, in others of a cyl- 

 inder, and is moved at a regular rate by a spring, pendu- 

 lum, falling weight, clockwork, electric or other motor. 1 

 The record which is traced with the myograph lever 

 by the muscle has the form of a curve. From the height 

 of the curve we can readily estimate the amount that 

 the muscle changes its length, but in order to accu- 

 rately determine the duration of the contraction process 

 and the time relations of different parts of the curve, 

 it is necessary to know the exact rate at which the 

 recording surface is moving. The shape of the curve 

 drawn by the muscle will depend very largely on the 

 rate of the movement of the surface on which the record 

 is taken. This is illustrated by the four records repro- 

 duced in Figure 36. These were all taken from the 

 same muscle within a few minutes of each other and 

 under exactly the same conditions, except that in the 

 successive experiments the speed of the drum on which 

 the record was traced was increased. 



A udauce at these records shows that a knowledge 

 of the rate of movement of the surface on which the record is taken is indis- 

 pensable to an understanding of the time relations of the different parts of the 

 curve written by the muscle. The rate of movement of the recording surface 

 can !><• registered by an instrument called a chronograph. 



The chronograph (g, Fig. 37), consists of one or two coils of wire wound 

 round cores of soft iron, and a little lever bearing a strip of iron, which is 

 attracted to the soft-iron cores whenever they are magnetized by an elec- 

 tric current flowing through the coils of wire about them. When the current 

 ceases to flow and the iron ceases to be magnetized, a spring draws the lever 



1 See O. Langendorfl': Physiologuehe Graphik, Kranz Deuticke, Leipzig, 1891 ; J. S. 

 Brodie : The Essentials of Experimental Physiology, London, 1898. 



Fig. 36.— Records of four 

 contractions of a gas- 

 trocnemius muscle of a 

 frog: a, recording sur- 

 faee at nst; b, surface 

 moving slowly; c, sur- 

 face moving more rapidly ; 

 '/, surface moving even 

 faster. 



