ACTIVE STATE OF MUSCLE. 451 



\\ith 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 section ; 

 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 natu- 

 rally 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. 

 The influence of the will is, then, the common stimulus which 

 excites most skeletal muscles to action* But we find that there 

 are many other influences which, when applied to a muscle, can 

 also bring about the same change. These influences are called 

 stimuli. 



We commonly utilize a nerve belonging to a muscle in order 

 to throw it into the contracted state, but the great majority of 

 stimuli can bring about the change when applied to the muscle 

 directly. Since the nerves branch in the substance of the muscle, 



