Chap. 10 MOVEMENT MUSCLES 161 



fatigued. Fatigue is a loss of contractility, apparently from accumulation of 

 the waste products of metabolism. Symptoms of muscle fatigue are easily pro- 

 duced. Hold your arm out straight; at first it is steady, then it trembles, and 

 finally you cannot prevent its sinking down in exhaustion. 



Tonus. This is the continuous partial contraction of muscle cells arising 



Fig. 10.6. Sensory nerve fibers with their end plates spread upon the surface of 

 a fiber of an eye muscle. The sensory end plates can be stimulated by conditions 

 within the muscle and changes sweep over the sensory nerve fibers as they do over 

 the motor ones (Fig. 10.5). Muscles are supplied with both kinds of nerves. A 

 muscle can receive a message and can also send one. (Courtesy, Maximow and 

 Bloom: Textbook of Histology, ed. 6. Philadelphia, W. B. Saunders Co., 1952.) 



from muscle sense of position. Sense of position is closely associated with 

 environment and habit. An aviator may lose his "sense of right side up." A 

 cat's feet feel for the floor or the ground surface to which they are accustomed 

 (Fig. 10.1 ). Tonus of skeletal muscles of the legs and trunk occurs in sitting, 

 standing, and walking. In general tonus does not require as much energy as 

 ordinary contraction. 



.>u.- 





Fig. 10.7. Capillaries surrounding skeletal muscle fibers in a dog's tongue. A, 

 longitudinal section; B, cross section. The abundance and intimacy of capillaries 

 with the muscle cells reveal an elaborate provision for the exchange of oxygen 

 and carbon dioxide and a rich supply of food. (Courtesy, Nonidez and Wind!e: 

 Textbook of Histology, ed. 2. New York, McGraw-Hill Book Co., Inc., 1953.) 



