MUSCLE 211 



the elasticity of the muscle brings it back to its orio-inal 

 form, at first rapidly, and then more slowly {Practical 

 Physiology). This is seen in muscle relaxing after con- 

 traction. 



The advantages of these_ properties of muscle are, that 

 every muscle, in almost all positions of the parts of the body, 

 is stretched between its point of origin and insertion. When 

 it contracts it can therefore act at once to bring about the 

 desired movement, and no time is lost in preliminary 

 tightening. Again, the force of contraction, acting through 

 such an elastic medium, causes the movement to take place 

 more smoothly and without jerks, and a force acting through 

 such an elastic medium produces more work than when it 

 acts through a rigid medium because less energy is lost as 

 heat. 



The extensibility of muscle is of value in allowing 

 muscles to act without being strongly opposed by their 

 antagonistic muscles, Avhich, however, are actively relaxed 

 through the action of nerves (p. 86). The elasticity of 

 muscles tends to bring the parts back to their normal 

 position when the muscles have ceased to contract. 



The extensibility of muscle is increased when it is stimu- 

 lated, so that the application of a weight causes a greater 

 lengthening than when the muscle is unstimulated. 



(3) Tonus of Muscle. — The tense condition of resting 

 muscle between its points of origin and insertion is not due 

 to passive elasticity, but is caused by a continuous con- 

 traction or tone kept up by the action of the nervous system. 

 If the nerve to a group of muscles be cut, or the spinal cord 

 destroyed, the muscles become soft and flabby and lose their 

 tense feeling. 



Alterations in the tonicity of the muscles is an essential 

 feature in certain diseases. It is lost in infantile paralysis 

 where the cells presiding over the nerves to the muscles are 

 destroyed, and it is increased in the condition of myotonia, 

 a disease the cause of which is not known, in the idiopathic 

 tetany of infants, in animals after removal of the para- 

 thyreoids (p. 603), and after decerebration (p. 113). In 

 i-lecerebration rigidity Sherrington finds that the extension 



