The Muscular System 125 



Fig. 38. — Muscle curves showing tetanus. A,B,C,D, Incomplete; E, complete. 

 (From Zoethout and Tuttle: Textbook of Physiology.) 



Tonus. — At all times, the individual muscle is in a state of partial 

 contraction known as tonus or tone. This is brought about by the con- 

 tinuous emission of impulses by the central nervous system which causes 

 a small number of the fibers to contract. This contraction is actually 

 a form of tetanus and keeps the resting muscle in a state of resilience. If 

 the nerve supplying the muscle is injured in some manner and the muscle 

 is not kept in a state of tonus, the muscle fibers become flaccid and de- 

 generate. 



The AIl-or-None Law. — Depending upon the strength of the stimu- 

 lus, a single muscle fiber either does or does not contract. If it does 

 contract, it does so to its full ability. This is a demonstration of the 

 all-or-none law. There is no gradation in the amount of contraction 

 dependent upon the degree of stimulation. At first, this observation may 

 appear to be at variance with observed facts. It is well known that 

 skeletal muscle varies in the strength of its contraction. Actually, how- 

 ever, this variation is due to the fact that varying numbers of muscle 

 fibers are brought into play as the strength of the stimulus is increased. 

 Clearly this law applies only to the individual muscle fibers or cells. 



Fatigue. — If an isolated muscle is continuously stimulated through 

 its attached nerve, the response will at first gradually increase to a 

 maximum and then drop. Finally no response will be elicited no matter 

 how strong the stimulus. If now, the stimulus instead of being applied 

 to the nerve is applied directly to the muscle, the contractions will 

 resume. Obviously the muscle is not fatigued. From other experi- 



