70 TEXT-BOOK OF PHYSIOLOGY. 



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

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

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

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

 elastic tension of antagonistic groups of muscles. 



Another explanation for the tonicity of muscle is found in the fact 

 that the skeletal muscles of the body receive continuously nerve 

 impulses from the thermogenic centers. The chief function of the 

 tonicity would thus be the production of heat, other functions which 

 the tone subserves being merely secondary. 



Irritability, Contractility. These are terms employed to de- 

 note that property of muscle-tissue in virtue of which it responds by a 

 change of form, becoming shorter and thicker on the application of 

 any external agent which acts as a stimulus. On the withdrawal 

 of the stimulus the muscle again undergoes a change of form, 

 becoming longer and narrower, and returns to its original con- 

 dition. All muscles which possess this capability are irritable and 

 contractile; and all agents which excite the muscle to action are 

 stimuli. The rapid change of form which a highly irritable muscle 

 undergoes in response to the action of a stimulus of short duration is 

 usually termed a twitch or pulsation. With appropriate apparatus 

 it can be shown that the muscle at the time of the twitch becomes 

 warmer and exhibits electric phenomena. The muscle is therefore^ 

 fan apparatus for the conversion of potential into kinetic energy/^ 

 yviz., heat, electricity, and mechanic motion. 



Though usually associated with the activity of the nervous system, 

 and to some extent dependent on it, irritability is nevertheless an 

 independent endowment of the muscle and persists for a longer 

 or shorter period, as shown by many experiments, after all nerve 

 connections have been destroyed. Among the proofs which may be 

 presented in support of this view is the following: The introduction 

 of the drug curara into the body of an animal produces in a short 

 time complete paralysis. Experiment has shown that curara sus- 

 pends the conductivity of the intramuscular terminations of the 

 nerve-fiber and thus separates the muscle entirely from the nerve. 

 Though the animal is incapable of executing a single movement, its 

 muscles respond promptly on the application of a stimulus. More- 

 over, portions of muscles exhibit irritability in which there is no trace 

 of nerve structure. This is the case with the ends of the sartorius 

 muscle of the frog and the anterior end of the retractor muscle of 

 the eyeball of the cat. These and other facts demonstrate the in- 

 dependence of muscle irritability. 



In the living body irritability and nutritive activity, with which 

 it is closely associated, are maintained by a due supply of oxygen, 

 of nutritive material, the removal of waste products, and a nor- 



