7 6 



TEXT-BOOK OF PHYSIOLOGY. 



with a consequent loss of irritability, when deprived of their normal 

 blood-supply. The excised muscles of cold-blooded animals, par- 

 ticularly of the frog, in which, owing to the relatively slow rate of 

 the nutritive activities, the irritability and contractility endure for a 

 long period of time, even though deprived of blood, are particularly 

 valuable for experimental studies. The muscles generally employed 

 are the gastrocnemius, the sartorius, and the hyoglossus. If kept 

 at a normal temperature and moistened with 0.6 per cent, solution of 

 sodium chlorid, such a muscle will contract for a long period of time 

 on the application of any form of stimulus, but especially the electric. 

 Graphic Record of a Muscle Contraction. Inasmuch as the 

 changes in the form of a muscle during a single contraction take place 

 with extreme rapidity, their succession, peculiarities, and time re- 



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

 ing lever. W. Weight. I. Induction coil. 



lations cannot be determined with any degree of accuracy by the 

 unaided eye. This difficulty can largely be overcome by the employ- 

 ment of the graphic method, the principle of which consists in record- 

 ing the movements by means of a pen on some appropriate moving 

 and receiving surface. To accomplish this object the muscle is at- 

 tached at one extremity by a clamp to a firm support, and at the other 

 extremity to a weighted lever, which is, however, sufficiently light to 

 enable it to take up, reproduce, and magnify its movements. The 

 end of the lever provided with a pen is applied to a smooth surface, 

 such as glazed paper on a cylinder or plate, and covered with lamp- 

 black. If the surface is stationary, the contraction is recorded as a 

 vertical line; if it is placed in movement at a uniform rate by clock- 



