48 HUMAN PHYSIOLOGY. 



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 proportional 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 augments 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 rdle 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 and 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 associated with the activity 

 of the nervous system, it is nevertheless, an independent endowment, and 

 persists after all nervous connections are destroyed. If the nerve terminals 

 be destroyed, as they can be by the introduction of curara into the system, 

 the muscles become completely relaxed and quiescent. The strongest stimuli 

 applied to the nerves fail to produce a contraction. Various external stimuli 

 applied directly to the muscle substance produce at once the characteristic 

 contraction. The excitability of muscle is therefore an inherent property, 

 dependent on its nutrition, and persisting as long as it is supplied with proper 



