MUSCLES 281 



when contracting and when relaxing. Because involuntary 

 muscles act so slowly they are very easily thrown into a con- 

 dition of tetanus. Indeed, so slow is their response to stimuli 

 that in some forms of smooth muscle, one stimulus in five 

 seconds is sufficient to prevent the muscle from relaxing at all. 



It is a great gain to us that so many muscles as are present 

 in the entire digestive, blood, and excretory systems can perform 

 their daily and nightly work without any thought or bidding 

 on our part, and without error in time or rate. 



Curiously, however, smooth muscle has a disposition to 

 act with apparent spontaneity. If a bit of the circular 

 muscle of the intestine of some animal be cut out, hung up 

 by one end and stretched by a light weight attached to the 

 other, it will very soon lift the weight and then relax again. 

 The muscle makes these movements over and over again 

 without any stimulus and they continue until the muscle is 

 exhausted or until its unusual exposure results in its death. 



Isolated from the body in this way, the muscle can plainly 

 receive no stimulus from the brain, and, although different 

 kinds of smooth muscle act very differently in this respect, 

 they all have this power of spontaneous movement when en- 

 tirely disconnected from the central nervous system. Whether 

 or not there resides among the muscle cells some nerve in- 

 fluence which is the exciting agent is still an open question. 



Although in many respects striped and unstriped muscles 

 act differently, yet in others their relations are the same. 

 Both seem to require the stimulus of nerves to make them 

 contract although in one case the contraction is voluntary and 

 in the other involuntary. Both can be thrown into tetanus 

 by repeated stimuli. Both are affected by heat and cold in 

 the same way. Both become fatigued from long action. 



CARDIAC MUSCLE 



The muscles of the heart are unlike any others, although in 

 structure as well as in action they have some points of simi- 



