MUSCULAR ACTIVITY 115 



also much slower. Smooth muscle constitutes the operating 

 machinery of the maintenance systems (except the respiratory 

 system), and it is as nicely adjusted to the special requirements of 

 these systems as is skeletal muscle to the needs of external adapta- 

 tion. One striking peculiarity of smooth muscle tissue is illus- 

 trated by the bladder. This organ sometimes contains a large 

 amount of urine, at other times there is little or none in it. When 

 the bladder is empty it is shrunken to a fraction of its size when 

 full. The muscular walls are distended or contracted as the organ 

 is full or empty. These pronounced changes appear to be effected, 

 in part at least, by rearrangement of the cells which make up the 

 muscle coats. When the organ is distended there is a smaller 

 number of layers of cells than when it is contracted. Just how 

 this rearrangement is brought about is not known. 



Another feature in which smooth muscle differs strikingly from 

 skeletal is in the tendency it often shows to carry on spontaneous 

 contractions. Skeletal muscle, as previously emphasized, con- 

 tracts only when subjected to definite stimulation. Smooth 

 muscle, on the other hand, often undergoes periods of rhythmic 

 contraction and relaxation when no obvious sources of stimulation 

 are present. 



Still another peculiarity of smooth muscle is its ability to remain 

 indefinitely in the contracted state without fatigue. This prop- 

 erty is seen in the muscular coats of the small arteries, many of 

 which are never relaxed. They may be more strongly contracted 

 at some times than at others but in health they are always in some 

 degree of contraction. 



There are in the body a number of sphincters, circular bands 

 of smooth muscle which guard the openings of various organs 

 such as the stomach, large intestine, and bladder. These are 

 strongly contracted the greater part of the time, relaxation being 

 for them only an occasional occurrence. They maintain their 

 condition of strong contraction without fatigue and apparently 

 without much expenditure of energy, offering in this regard a 

 sharp contrast to skeletal muscle. 



To excite skeletal muscle sharp stimuli, like the electric shock, 

 are most efficient. Smooth muscle, on the other hand, responds 

 best to slower, more prolonged excitants. A pull or pinch, which 

 will ordinarily fail to cause contraction in such a muscle as the 



