MUSCLES AND PHENOMENA OF CONTRACTION. 131 



Muscle Fatigue. 



One of the most familiar phenomena of our muscles is 

 their fatigue resulting from hard work. This fatigue is 

 caused by an accumulation of waste matter in the muscle 

 substance, owing to its active work. This may be easily 

 demonstrated by taking a frog's muscle, and causing it to 

 contract until it is thoroughly fatigued and refuses to react 

 to further stimuli. If now such a muscle be washed with 

 water which contains a little salt, so as to make the water 

 as nearly like blood serum as possible in that respect, it 

 begins to contract again with apparently renewed energy. 

 The only explanation available is that the salt water in 

 question washed out some of the waste products. Of course 

 this could not be continued indefinitely, for soon the muscle 

 fibre not being nourished would be robbed of all its mate- 

 rial, and so a final exhaustion ensue. This explains why a 

 muscle fatigued revives so quickly when it is moved about 

 freely, and for a few minutes the blood is allowed to stream 

 into it uninterruptedly. This feeling of fatigue, however, 

 does not come directly from the muscles. The muscle fibres 

 are supplied with motor nerves only, and it is probably im- 

 possible to receive a sensory impression from them. There 

 are, however, distributed between the muscle fibres sensory 

 nerves, and from these we derive our muscular sensations. 

 Some of these nerve fibres seem to end in little special end 

 organs which suggest the Pacinian bodies of the mesentery. 

 In fact, some histologists believe that the spindle cells men- 

 tioned on a preceding page* as cells which develop later in- 

 to new muscle fibres, are not muscle cells at all, but are 

 sensory cells connected with nerves, and that from these 

 to a large extent we get our muscular sensations. 



The Blood Supply. 



The vitality of a muscle is very dependent upon its blood 

 supply. If the artery going to a muscle be cut, or if only 

 venous blood be able to reach it, the muscle at once begins 



*See page 119, 



