562 



PHYSIOLOGY. 



increases it. The stimuli to excite plain muscle are chemical, 

 mechanical, and the opening and closing of a constant current. 

 Organs containing unstriped muscle frequently exhibit involuntary 

 rhythmical movements and a tendency to sustained tonic contrac- 

 tion. The force of the uterus in expelling the child and that of the 

 bladder in expelling the urine show that plain muscle can do con- 

 siderable work. Ordinarily, organs made up of nonstriated muscle 

 are only faintly sensitive. These rhythmic contractions and relaxa- 

 tions, like the tonic contractions, are independent of the. action of 

 nerves, but are modified by it. Unstriped fiber does, like striped 



Fig. 207. Curve of Contraction of the Unstriped Muscle of Mtiller 

 in Dog. (LAULANI&.) 



The intervals on the line T are seconds. 



fiber, increase its height of contraction by increasing the strength 

 of the stimulus. It can not be thrown into a state of tetanus by a 

 series of stimuli. Eapid stimuli simply increase the force and rate 

 of individual contractions. Smooth muscle, 'as a rule, contains nerve- 

 plexuses and ganglion-cells. It has two kinds of functionally different 

 nerves, motor and inhibitory. Both sets of nerves are connected with 

 nerve-cells in their course, such as the plexuses of Auerbach and 

 Meissner in the intestinal tract. 





