MUSCULAR CONTRACTION 905 



economy. A tetanic contraction, or tetanus, is one in which the new elas- 

 tic condition is maintained by processes which require a considerable 

 expenditure of energy. The properties of these two forms of contraction 

 will now be considered in some detail, together with the uses to which 

 they are put by the organism. 



The Tonic Contraction of Skeletal Muscle 



The skeletal muscles of the body are normally maintained in a state 

 of slight tension even when at rest. This condition, to which the word 

 tone or tonus is applied, is due to the action upon the muscle of nerve 

 impulses which come to it over reflex arcs which we will describe in the 

 next chapter. When these reflex arcs are interrupted the muscles loose 

 their tone, that is, their elastic properties change and they can now 

 extend from origin to insertion without developing any tension. Limbs 

 in which the muscles have lost their tone become " loose jointed" and 

 readily assume under the influence of gravity, a variety of postures 

 which a normal limb would not exhibit. Consequently it is easy to dis- 

 tinguish between death and sleep by the postures of the limbs, since the 

 tone of the sleeper's muscles is retained. Tonic contraction is thus seen 

 to be connected with the maintenance of posture in the limbs. The posi- 

 tion of the body, and particularly of the limbs is constantly changing, 

 and with each change the muscles assume new lengths, maintaining 

 meanwhile the same tension which they exerted before. To do this a 

 new elastic state must be set up in the muscle so that the muscle can 

 exist at its new length without exerting a greater tension on its inser- 

 tion. The tone of the muscles consequently may be changed to fit the 

 new position of the joint and to maintain this new posture. It is conse- 

 quently called plastic tonus, (Sherrington 27 ). Since the greatest ten- 

 sion must be exerted by those muscles which support the weight of our 

 limbs, or the weight of the body which falls on the limb, these muscles 

 have developed the greatest powers of tonic contraction. Consequently 

 they become affected most in conditions which tend to increase the tone 

 of the muscles. 



Tonic contraction, as contrasted with tetanus, is characterized by the 

 economy with which the elastic state of the muscle is maintained. Roaf 

 was unable to detect any difference in the respiratory exchange of cats 

 during the highly developed tone of decerebrate rigidity when com- 

 pared with the same animals in which all muscular contraction was abol- 

 ished with curare. Evans has found that the metabolism was less after 

 curare, but the gaseous exchange was clearly much less in the state of tone 

 than if the muscles had been thrown into tetanic activity. Bayliss also 

 found a slight heat production in muscles in decerebrate rigidity, which 

 varied with the degree of tonic contraction, but very much less than 



