412 A MANUAL OF VETERINARY PHYSIOLOGY 



the stimulus which fires off an explosive compound in the muscle 

 substance, resulting in a contraction and the production of work 

 and heat. Such a statement is obviously a mere skeleton, and 

 takes us little nearer to an understanding of what really con- 

 stitutes a muscular contraction. 



Fatigue. — The influence of fatigue on muscular contraction 

 has previously been dealt with. The cause of fatigue is the 

 using up of the contractile substance of the fibres, and the 

 accumulation in the muscle of the chemical products of con- 

 traction. In fact, if a fatigued muscle be washed out with normal 

 saline solution and a little weak alkali circulated through its 

 bloodvessels, it becomes restored, and regains its power of con- 

 traction. A muscle at work in the body is protected from ready 

 fatigue by the ever-circulating blood, which supplies it with food 

 and carries off the waste products of its activity. 



The material in muscles which gives rise to fatigue is probably 

 sarcolactic acid, and by passing a solution of this acid into 

 muscles the typical phenomena of muscle fatigue may be arti- 

 ficially induced. The production of potassium salts may also 

 be a cause of fatigue, in spite of the fact that they are usually 

 found in muscle, yet potassium salts in their action on this tissue 

 rapidly destroy its irritability. 



Attention has been drawn to the fact (p. 390) that muscles 

 are connected by elaborated nerve-endings with sensory nerves, 

 to whose existence the so-called ' muscular sense ' is due. It is 

 therefore conceivably possible that the sensation of general fatigue 

 which arises from excessive muscular exertion is due to a cerebral 

 appreciation of the changes brought about in the muscles as the 

 result of their contracting activity, and may be looked upon as a 

 protective mechanism against straining the machine. On the 

 other hand, muscular activity implies the action of central nerve- 

 cells in which the impulses which give rise to the contractions of 

 the muscles are originated, of the passage of these impulses along 

 the motor nerves, and of their communication to the contractile 

 fibres by the agency of the end-plates. Hence the phenomena 

 of fatigue, if we regard it as a ' weariness ' of the body as a 

 machine, may be really due to a fatigue of the central cells, of the 

 motor nerves, or of the end-plates. We may at once dismiss 

 the motor nerves from our consideration, inasmuch as nerves 

 do not appear to be capable of fatigue. Now, the blood of a 

 fatigued animal contains fatigue products, and if it be transferred 

 into the circulation of a normal animal, all the symptoms of 

 fatigue are produced. If the spinal cord be divided and the 

 distal end stimulated, the hurried respiration of fatigue may be 

 produced as the result of muscular contractions (see p. 142). 

 Possibly, therefore, the phenomena are due to the injurious action 



