558 A TEXTBOOK OF PHYSIOLOGY 



rigidity passes off. In wasting diseases, rigor may not appear at all, 

 or appear early and pass off quickly. 



Rigor is mainly due to the formation of lactic acid in the absence- 

 of oxygen. The lactic acid thus formed alters the normal reaction of 

 the muscle, and brings about coagulation of the muscle proteins. If 

 the accumulation of lactic acid be prevented, either by the presence 

 of oxygen or by perfusion of the muscle with saline, rigor does not 

 supervene. 



Rigor may also be induced in muscles by plunging into boiling 

 water. In this case, " heat rigor " takes place. The proteins of the 

 muscles are coagulated without any attendant chemical changes, such 

 as acid formation. Similarly, soaking in distilled water brings about 

 a "water rigor." A water pressure of 400 atmospheres produces 

 rigor of the muscles of terrestrial and shallow-water animals. 



Smooth Muscle. Although smooth muscle is generally found in 

 organs which perform slow movements, in some animals noted for 

 quickness and grace, such as the squid, there exists nothing but smooth 

 muscle. In the higher animals it gives motility to organs over 

 which there is no voluntary control; hence the name " involuntary " 

 muscle. 



In addition to a slow rate of contraction, with a long latent period, 

 smooth muscle is characterized by the fact that it cannot be thrown 

 into complete tetanus. Its contraction is of the nature of a single 

 twitch. 



Smooth muscle, like striated muscle, responds to a gradation of 

 stimuli, and shows thermal, chemical, and electrical changes. In 

 chemical composition it is probably much the same as striated muscle. 

 It undergoes both chemical and heat rigor. 



Smooth muscle possesses the property of tonus a condition 

 of sustained muscular contraction, which is influenced in the direc- 

 tion of further contraction or relaxation by the nerve-supply. Aug- 

 mentor or accelerator nerves increase tonus. inhibitory nerves relax. 

 This double nerve-supply is a characteristic of smooth muscle. 



Smooth muscle is also characterized by the property of rhyth- 

 micity periods of contraction alternate with periods of rest. This 

 seems to be a function of the muscle itself. Such is observed in the 

 stomach, intestine, bladder, spleen, and other organs. 



Associated with smooth muscle are local nervous networks or 

 plexuses, such as Auerbach/s and Meissner's plexuses in the intestine. 

 These endow the muscle with the property of peristalsis co-ordinate 

 and recurring waves of contraction, preceded by waves of relaxation 

 which together force the contents along the muscular tube 



