44 HUMAN PHYSIOLOGY 



3. Physical Characters of Muscle. 



1. Muscle is translucent during life, but, as death stiffen- 

 ing sets in, it becomes more opaque. 



2. Muscle is markedly extensile and elastic. A small force 

 is sufficient to change its shape, but when the distorting force 

 is removed it returns completely to its original shape, pro- 

 vided always that the distortion has not overstepped the 

 limits of elasticity. 



When a distorting force is suddenly applied to muscle 

 e.g. if a weight is suddenly attached the distortion takes 

 place at first rapidly, and then more slowly, till the full effect 

 is produced. If now the distorting force is removed the 

 elasticity of the muscle brings it back to its original form, 

 at first rapidly, and then more slowly. (Practical Physi- 

 ology, Chap. VI.) 



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 at once act to bring about the 

 desired movement, and no time is lost in preliminary tighten- 

 ing. Again, the force of contraction, acting through such an 

 elastic medium, causes the movement to take place more 

 smoothly, and without jerks. Experimentally, too, it has 

 been ascertained that a force acting through such an elastic 

 medium produces more work than when it acts through a 

 rigid medium. 



The extensibility of muscle is of value in allowing a group 

 of muscles to act without being strongly opposed by their 

 antagonistic group. For instance, suppose the extensor 

 muscles of the arm were not readily extensile, when the 

 flexors acted, a large amount of their energy would have to 

 be employed in elongating the extensors. Similarly the 

 elasticity of the muscles tends to bring the parts back to 

 their normal position when the muscles have ceased to 

 contract. It must not, however, be imagined that, in all 

 movements of one set of muscles, the antagonistic muscles 

 are relaxed, although they may be elongated. Often they 

 are in a state of activity so as to guide the movements being 

 produced (see p. 60). 



