THE CONTRACTION OF MUSCLE 



551 



Muscular " Tone." During life, the muscles, never fully relaxed, 

 are kept in a state of incipient contraction, or " tonus " a condition 

 dependent upon their connection with the central nervous system. 

 The muscles are ceaselessly influenced by their nervous centres, which 

 in turn are excited by messages reaching them from all parts of the 

 body, particularly the skin, joints, and the muscles themselves. 



This tonus makes for a general " wakefulness " a readiness to 

 contract on the part of the muscle. It also plays a considerable 

 part in the production of heat within the body, being reflexly in- 

 creased by sensations of cold, and relaxed by sensations of warmth. 

 It is also affected by mental states, such as excitement, anger, fear. 



NaCI-7! 



' 



Fia. 292. SARTORIUS OF FROG, 'STIMULATED AT INTERVALS OF THIRTY SECONDS 

 ALTERNATELY WITH GALVANIC CURRENT AND INDUCTION SHOCKS AT x . (G. R. 

 Mines.) 



Spontaneous Movements. -Amphibian muscles in saline solutions 

 exhibit spontaneous movements (Fig. 289). These movements will 

 continue in concentrated curare solution, indicating that the source 

 of the movements lies in the contractile substance of the muscle 

 fibres. This is also proved by experiments on muscle containing no 

 nerve endings, e.g., the non-neural regions of the sartorius muscle. 

 Potassium chloride at first increases the movements, often causing a 

 very rapid rhythm (Fig. 260), and then stops them entirely. Its effect 

 on the excitability of the muscle towards galvanic currents of long 

 duration, which is increased in this condition of the muscle, is a further 

 exaltation and then depression (Fig. 291). The addition of calcium 

 chloride leads to an immediate diminution or cessation of the move- 

 ments and a fall in the excitability towards galvanic currents (Fig 292). 



