THE PHYSIOLOGY OF MUSCULAR WORK 211 



if a muscle be cut across its ends retract in the wound. The 

 practical importance of this elasticity is found in the more prompt 

 performance of work ; the muscles are taut and have not to take in 

 slack when they begin to contract. The extensibility of a muscle 

 is greater during contraction than in the condition of rest. This 

 is a further safeguard to the muscle in any sudden or vigorous 

 contraction against a great resistance ; the shock and strain are 

 lessened and rupture of the muscle is prevented. It is more 

 common to find bones fractured than muscles ruptured by violent 

 contraction. Later it will be shown that in addition to this elas- 

 ticity the muscles have a condition of tone or tonic contraction, 

 which increases their efficiency in the performance of work. 



Muscle consists of 25 per cent, of solids and 75 per cent, of 

 water ; twenty parts of the solids are proteins, and the remaining 

 five parts are extractives and inorganic salts. From the muscle 

 can be expressed a viscid alkaline juice which soon becomes 

 acid and clots. The chief constituents of this muscle plasma 

 are the proteins investigated and named by Halliburton ( 3 ) para- 

 myosinogen and myosinogen ; they correspond respectively to 

 the myosin and myogen of Von Fiirth( 4 ). These proteins in most 

 respects resemble the globulins ; para-myosinogen is coagulated 

 by heat at 47, myosinogen at 63. The myosinogen gives rise to 

 soluble myosin in the process of clotting, and this substance, which 

 is coagulated by heat at 40, is a normal constituent of the muscle 

 plasma of cold-blooded animals. 



The statements made concerning the proteins of muscle plasma 

 may soon need revision, for in a recent preliminary communication 

 Mellanby ( 5 ) has maintained that there is only one protein in muscle 

 and that it is not a globulin. 



A comparison of the protein constituents of the different kinds 

 of muscle brings out an interesting difference. Nucleo-protein is 

 most abundant in plain muscle, and least abundant in volun- 

 tary or striated muscle ; cardiac muscle occupies an intermediate 

 position in this respect. In simple cells nucleo-protein is a typical 

 constituent ; plain muscle is the least and voluntary muscle the 

 most differentiated of the three kinds of muscle ; thus changes 

 in function and structure have been accompanied by a corre- 

 sponding gradation in the amount of nucleo-protein. 



Another point of great interest has been discovered by a com- 

 parison of the temperatures at which the different protein con- 



