142 COMPARISON OF VOLUNTARY AND INVOLUNTARY MUSCLE [CH. XIII. 



not a tetanus. This is an indication that the electrical change is a 

 single one, and not a succession of changes such as occurs in tetanus. 



But though involuntary muscle cannot be thrown into tetanus, 

 it has the property of entering into a condition of sustained contrac- 

 tion called tonus. We shall have to consider this question again in 

 connection with the plain muscular tissue of the arterioles. 



Involuntary muscle when it contracts undergoes thermal and 

 chemical changes similar to those we have dealt with in the case of 

 the voluntary muscles. 



Involuntary muscle is usually supplied with two sets of nerves, 

 one of which (accelerator) increases and the other of which (inhibitory) 

 decreases its activity. The nerve-endings in involuntary muscle 

 require a much larger dose of curare to affect them than the end- 

 plates in voluntary muscle. 



The phenomena of rigor mortis in involuntary muscle have not 

 been so fully studied as in the case of voluntary muscle. It has, 

 however, been shown that the chemical composition of involuntary 

 muscle differs in no noteworthy manner from that of voluntary muscle, 

 and on death the muscle becomes acid ; such products as carbonic 

 acid and sarcolactic acid are formed. In the heart, stomach, uterus, 

 and rectum, post-mortem rigidity has been noted, and it probably 

 occurs in all varieties of plain muscle. 



Swale Vincent has shown that the characteristic proteins (paramyosinogen and 

 myosinogen) occur in both striped and unstriped muscle, and the heat rigor curves 

 of involuntary muscle are practically identical with those obtained by Brodie (see 

 p. 139). He is inclined to think that the two proteins are formed by the breaking 

 down of a compound protein which in living muscle mainly coagulates at 47 J C. 

 This view is taken by Stewart in reference to striped muscle also, but has been 

 very seriously questioned by v. Fiirth. The most striking chemical difference 

 between unstriped and striped muscle is seen in the amount of nucleo-protein which 

 they contain. Plain muscle contains six to eight times the amount found in 

 voluntary muscle ; cardiac muscle contains an intermediate quantity. 



