448 MANUAL OF PHYSIOLOGY. 



rents passing through the galvanometer, the thickness of the lines 

 indicating their force. The dotted lines o are connected with 

 points where the electro-motive force is equal, and therefore no 

 current exists. 



The electro-motive force of the muscle-current in a frog's gra- 

 cilis has been estimated to be about .05-.08 of a Daniell cell. It 

 gradually diminishes as the muscle loses its vital properties, and 

 is also reduced by fatigue. The electro-motive force rises with 

 the temperature from 5 C. until a maximum is reached at about 

 the body temperature of mammals. 



These muscle-currents are very weak if the uninjured muscle 

 be examined in situ, the tendon being used as the transverse sec- 

 tion ; they soon become more marked after the exposure of the 

 muscle, and if the tendon be injured they appear at once in almost 

 full force. In animals quite inactive from cold the muscles nat- 

 urally are but slowly altered by exposure, etc., and the muscle- 

 currents do not appear for a considerable time, which is shortened 

 on elevating the temperature. It has, therefore, been supposed 

 that in the perfectly normal state of a living animal there are 

 no muscle-currents so long as the muscle remains in the passive 

 state. 



ACTIVE STATE OF MUSCLE. 



A muscle is capable of changing from the passive elongated 

 condition, the properties of which have just been described, into 

 a state of contraction or activity. Besides the change in form, 

 obvious in the contracted state of the muscle, its chemical, elastic, 

 electric, and thermic properties are altered. The capability of 

 passing into this active condition is spoken of as the irritability 

 of muscle. This is directly dependent upon its chemical condi- 

 tion, and therefore related to its nutrition and to the amount of 

 activity recently exerted, which, it will hereafter appear, changes 

 its chemical state. 



Under ordinary circumstances, during life, the muscles change 

 from the passive state into that of contraction in response to cer- 

 tain impulses communicated to them by nerves, which carry im- 

 pressions from the brain or spinal cord to the skeletal muscles. 



