MUSCLE-NERVE PHYSIOLOGY 



their tendons and other tissues, are subjected from the resistance ordinarily 

 opposed to their contraction. When no resistance is offered, as when a 

 muscle is cut off from its tendon, not only is no hardness perceived during 

 contraction, but the muscular tissue is even softer and more extensible than 

 in its ordinary uncontracted state. During contraction in each fiber it is 

 said that the anisotropous or doubly refractive elements become less refract- 

 ive and the singly refractive more so, figure 321. 



Chemical Changes in Contracting Muscle. i. The reaction of the 

 muscle, which is normally alkaline or neutral, becomes decidedly acid during 

 contraction, from the development of sarcolactic acid. 2. The muscle gives 

 out carbon dioxide gas and takes up oxygen. The amount of the carbon 

 dioxide given out does not appear to be entirely dependent upon the oxygen 

 taken in, and so doubtless in part arises from some other source. Muscle 

 contracts in an atmosphere of hydrogen, showing that oxygen is present in 

 fixed combination. A muscle, however, contracts for a longer time in an 

 atmosphere of oxygen. 3. Certain imperfectly understood chemical changes 

 occur, in all probability connected with i and 2, in which glycogen is con- 

 verted into dextrose and the latter oxidized. 



Electrical Changes in Contracting Muscle. Resting muscles un- 

 injured in the body have a uniform potential, i.e., are isoelectric. But when 

 removed from the body they are more or less injured and, therefore, show 

 differences of electrical potential between different points on the muscle, 

 called currents of injury or demarcation currents. It is necessary to use 

 non-polarizable and not metallic electrodes in this experiment, as other- 

 wise there is no certainty that the whole of the current observed is com- 

 municated from the muscle itself and not derived from the metallic elec- 

 trodes and arising in consequence of the action of the saline juices of the 

 tissues upon them. Non-polarizable electrodes are usually some modifica- 

 tion of Du Bois Reymond's apparatus, which consists of a cylinder filled with 

 china clay moistened with saline solution, part of which projects as a 

 drawn-out point for contact with the muscle. The rest of the cylinder is 

 filled with a saturated solution of zinc sulphate into which dips a well-amal- 

 gamated piece of zinc connected by wire with the galvanometer. In this way 

 the zinc sulphate and the sodium chloride form a non-polarizable conductor 

 between the zinc and the live muscle. Recently Porter has devised a boot- 

 shaped clay electrode that is burned, and hence has the immense advan- 

 tage of permanency. 



Currents of Injury, or Demarcation Currents. If a segment is cut 

 out of a living gastrocnemius, its cut ends present regions of maximal injury. 

 Such a preparation is called a muscle prism. 



If the points on the surface of a muscle prism be connected with the gal- 

 vanometer by non-polorizable electrodes, it will be found that the currents 

 pass from point to point, as shown in the diagram, figure 324. 



