54 HUMAN PHYSIOLOGY. 



The Physical Properties of Muscle Tissue. The consistency of 

 muscle tissue varies considerably, according to the different states of 

 the muscle. In a state of tension it is hard and resistant ; when free 

 from tension, it is soft and fluctuating, whether the muscle is con- 

 tracting or resting. Tension alone produces hardness. The cohesion 

 of muscle tissue is less than that of connective tissue, and is broken 

 more readily. Cohesion resists traction and pressure, and lasts as 

 long as irritability remains. 



The elasticity of a muscle, though not great is almost perfect. 

 After being extended by a weight, it returns to its natural form. The 

 limit of elasticity, however, is soon passed. A weight of 50 or 100 

 grams will overcome the elasticity so that it will not return to its 

 natural length. In inorganic bodies the extension is directly propor- 

 tional to the extending weight, and the line of extension is straight. 

 With muscles, the extension is not proportional to the weight. 

 While at first it is marked, the elongation diminishes as the weight 

 increases by equal increments, so that the line of extension becomes 

 a curve. In other words, the elasticity of a passive muscle aug- 

 ments with increased extension. On the contrary, the elasticity of 

 an active is less than that of a passive muscle, for it is elongated 

 more by the same weight, as shown by experiment. 



Tonicity is a property of all muscles in the body, in consequence 

 of being normally stretched to a slight extent beyond their natural 

 length. This may be due to the action of antagonistic muscles, or to 

 the elasticity of the parts of the skeleton to which they, are attached. 

 This is shown by the shortening of the muscle which takes place when 

 it is divided. Muscular tonus plays an important role in muscular 

 contraction. Being always on the stretch, the muscle loses no time 

 in acquiring that degree of tension necessary to its immediate action 

 on the bones. Again, the working power of a muscle is increased by 

 the presence of some resistance to the act of contraction. According 

 to Marey, the amount of work is considerably increased when the 

 muscular energy is transmitted by an elastic body to the mass to be 

 moved, while at the same time, the shock of the contraction is 

 lessened. The position of a passive limb is the resultant also of the 

 elastic tension of antagonistic groups of muscles. 



Muscle excitability or contractility are terms employed to denote 

 that property of muscle tissue in virtue of which it contracts or 

 shortens in response to various excitants or stimuli. Though usually 



