GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 



79 



and lymph in the muscles be increased by massage. This suggests that the 

 power of the cell to give off its waste products to the blood is sufficiently 

 rapid to keep pace with the ordinary production, but not with the more rapid 

 formation taking place during fatiguing work. This would seem to be the 

 case in spite of the fact that circulation of the blood in the muscles is increased 

 during action. When muscles are stimulated to action by impulses coming 

 to them from the central nervous system, the muscles in the walls of the blood- 

 vessels of the muscle are also irritated by their vaso- dilator nerves, and, relax- 

 ing, permit a greater flow of blood through the muscle ; when the muscles 

 cease to be excited the muscles in the vessel walls are gradually contracted 

 again through the action of the vaso-constrictor nerves, and the blood-supply 

 to the muscle tissue is correspondingly lessened. This arrangement would 

 seem to suffice for the bringing of nutriment and oxygen and the removal of 

 waste matters under ordinary conditions. 



Normally the muscles are never completely fatigued. It would seem 

 that as the muscles tire and their irritability is lessened, the central nerve- 

 cells which send the stimulating impulses to them have to work harder, 

 and that the nerve-cells give out sooner than the muscles. On the other 

 hand, certain experiments seem to show that the nerve-cells recover from 

 fatigue more rapidly than the muscles do, so that it is an advantage to 

 the organism that they should cease to excite the muscles before muscular 

 fatigue is complete. With the decreasing irritability of the muscle, a feeling 

 of discomfort in the muscle and an increasing sense of effort are experienced 

 by the individual, both of which tend to cause a cessation of contraction, and 

 prevent a harmful amount of work. That such an arrangement would be of 

 service was apparent in the experiments of Maggiora, in which he found that 

 if muscles are forced to work after fatigue has developed, the time of recovery 

 is prolonged out of all proportion to the extra work accomplished. 



Fatigue of Nerves. Muscle-, gland- and nerve-cells in fact, almost every 

 form of protoplasm if excited to vigorous long-continued action, deteri- 

 orate and exhibit a decline of functional activity. As we have seen, in 

 the case of muscle there is a using up of energy-holding compounds and 

 a production of poisonous waste matters, and these two effects induce the con- 

 dition known as fatigue. A priori, we should expect similar changes to occur 

 in the active nerve-fibre ; almost all the experimental evidence is, however, 

 opposed to this view. The form of activity which is most characteristic of 

 muscle is contraction ; that which is most characteristic of nerve is conduction. 

 In the case of the muscle it is exceedingly difficult to distinguish between the 

 effects produced by the processes associated with the change of form and those 

 which result from the transmission of the excitatory change. There is li 

 doubt but that fatigue is associated with the former; whether it is associated 

 with the latter is not known. In the case of the nerve, where the transmit 

 process may be studied by itself, conduction does not seem to fatigue I 



Apparently the same may be said of the processes which result in the 



