226 VETERINARY PHYSIOLOGY 



muscle is made to contract repeatedly, the contractions take 

 place more and more sluggishly. At first each contraction 

 is greater in extent, but, as they go on, the extent diminishes 



as fatigue becomes manifest, and 

 stimulation finally fails to call 

 forth any response (fig. Ill) 

 {Practical Physiology). 



This condition is probably 

 caused by the accumulation of 

 the products of activity in the 



Fig. 111. — Influence of continued ^ i • n i 



Exercise on Skeletal Muscle ^^^^cle, and especially by an 



• —(1) The first trace ; (2) a increase in the hydrogen ion 

 trace after moderate exer- concentration. The Same pheno- 



cise; (3) a trace when fatigue i • i i i 



has been induced. mena may be mduced by the 



application of dilute acids and 

 certain other drugs, and \x\2iy be removed, for a time, by 

 washing out the muscle with salt solutions or very dilute 

 alkalies. These products act first upon the nerve endings 

 in the muscle, and thus, in ordinary conditions of stimulation 

 through the nerve, the muscle is protected against the onset 

 of fatigue. 



As a result of repeated stimulation a condition of 

 incomplete relaxation — contracture — may appear and then 

 gradually wear off. Its onset may explain the stiffness of 

 muscles when first brought into play, and the advantages of 

 ' ' warming up " exercises before an athletic contest. 



Later, as fatigue becomes manifest, a contracture due to 

 the very slow relaxation may be observed. 



(2) Terfiperatiire. — If a muscle be ivarmed above the 

 normal temperature of the animal from which it is taken, 

 all the phases of contraction become more rapid, and the 

 contraction may be at first increased in extent, but is sub- 

 sequently decreased. 



If, on the other hand, a muscle be cooled, the various 

 periods are prolonged. At first the force and extent 

 of contraction becomes greater, a fact which indicates 

 that the development of tension must be of the nature 

 of a change in surface tension (p. 205). As the cooling 

 process goes on, the contraction becomes less and less, until 



