CONTRACTILE TISSUES 451 



It shows that there is a particular rate of performance of work at which the total 

 efficiency is maximal ; above and below this rate, the efficiency falls. 



In connection with the heat developed in tetanic contraction, the fact described 

 by A. V. Hill (1913, 4, p. 317), that the heat-production per unit of tension is 

 independent of the frequency of stimulation between 17 and 100 per second, 

 is of interest. It indicates that "the rise of tension is due to the presence of 

 chemical substances, liberated in conjunction with heat, by the processes called 

 forth by excitation. The presence of a definite amount of these substances in the 

 neighbourhood of certain surfaces or interfaces in the muscle, calls forth the same 

 amount of tension independent of the exact rate at which the stimuli occur. 

 These chemical substances are removed or destroyed at a rate proportional to their 

 concentration at any moment ; and, therefore, if they are produced (and removed) 

 at a greater rate by an increased frequency of excitation, their concentration in 

 the muscle must be increased proportionally. This increased concentration, 

 however, is accompanied by an increased tension, and, therefore, the tension 

 developed remains proportional to the rate of heat-production." 



FATIGUE 



If a muscle is caused to work at a greater rate than the lactic acid produced 

 can be replaced by the oxidation process, it becomes " fatigued," that is, incapable 

 of full activity, or even of any at all. Naturally, this result comes on more rapidly 

 in the absence of oxygen. 



From the experiments of Fletcher and Hopkins, and of Peters, referred to 

 above, the amount of lactic acid found in a muscle, stimulated to fatigue, only 

 amounts to about one-half of that obtained in heat rigor. The power of con- 

 traction ceases before the whole of the "excitable substance" is used. up. 



This fact suggests that the lactic acid has a toxic effect, or that the process is 

 of the nature of a balanced, reversible one. Certain experiments by Lipschiitz 

 (1908) show that the spinal cord of the frog, after fatigue in absence of oxygen, 

 can be restored to a certain extent by perfusion with Ringer's solution carefully 

 deprived of oxygen ; so that it seems probable that the same fact would be found 

 in the case of muscle. It would be interesting to know whether more lactic acid 

 could be formed by stimulation, if that produced were washed away as formed. 

 As mentioned above, such experiments would also throw light on the question of 

 the formation of lactic acid from carbohydrate in the muscle. 



It is important to note that fatigue of voluntary contraction, as investigated 

 by the ergograph, or similar method, is not situated in the muscle tissue itself. 

 Artificial stimulation of the motor nerve can still cause contraction when fatigue 

 to voluntary inner vation has set in. We have already seen that direct stimulation 

 of muscle will cause contraction after the synapse between nerve and muscle has 

 lost its excitability. 



The effect of the first stimuli after a period of rest is usually less than that of 

 the subsequent ones ; thus a series of stimuli gives, first, a rise in height of 

 contraction (Buckmaster, 1886), then a period of maximal height, and, finally, a 

 diminution owing to fatigue. It appears that the presence of a small quantity 

 of products of activity is favourable. We shall see immediately that this 

 phenomenon is especially marked in the heart muscle. 



SPECIAL CONTRACTILE TISSUES 



The Heart. There are certain important characteristics of muscular structures 

 which are particularly well shown by the heart muscle, while there are other 

 characteristics which have, as yet, been investigated in the case of this organ only, 

 although they have, in all probability, a general application. It may be noticed 

 that certain of these were, at one time, supposed to be peculiar to heart muscle, 

 although later work showed them to be also present in nerve and in voluntary 

 muscle. In the following pages, some facts concerning the general properties 

 of the muscle as a contractile tissue will be mentioned ; its function as a 

 pump for the maintaining of the circulation of the blood, together with the 



