MUSCLE 215 



disappears the muscles again become soft, and the body 

 becomes limp. In all probability this latter change is due 

 to a solution of the myosin by a proteolytic enzyme which 

 seems to exist in all the tissues, and to lead to autolysis or 

 self-digestion. It acts in the presence of an acid, and the 

 appearance of sarcolactic acid, therefore, allows it to come 

 into play. 



It is doubtful if a condition of rigor mortis occurs 

 in visceral muscle. The shortening is due to the fall of 

 temperature. 



V. Muscle in Action. 



A. VISCERAL MUSCLE. 



1 . Independence of the Nervous System. — The great character 

 of visceral muscle is its independence of the central nervous 

 system which seems to have granted it an autonomy. As 

 already indicated (p. 54), neuroblasts migrate outwards in 

 great numbers and form plexuses associated with the layers 

 of visceral muscle which exist in the walls of the various 

 viscera. The part played by these plexuses will be considered 

 in discussing the movements of the intestines (p. 332). 



The nerves passing from the central nervous system are 

 of two kinds — augmentors and inhibitors. 



2. Tonus. — When all these nerves are cut, the muscles still 

 manifest a continuous semi-contraction or tone which may be 

 modified by the conditions in which they for the time exist. 

 Thus, stretching of the muscles, e.g. by slowly injecting 

 fluid into the bladder, may cause a decrease of tone and 

 allow the viscus to be distended. But a point is reached 

 at which the tone is increased and a forcible contraction is 

 produced, and this occurs sooner if the injection is made 

 rapidly. 



3. Rhythmic Variations of Tone. — In most situations these 

 muscles manifest a rhythmical increment and decrement of 

 tone, an apparent rhythmic contraction. This is best seen 

 when they are slightly stretched. If a strip of the intestine 

 of a rabbit be placed in Ringer's solution supplied with 

 oxygen and kept at the temperature of the body, and if it be 



