148 INFLUENCE OF ACTIVITY. [BOOK i. 



irritability may be lowered and (up to a certain limit) raised at 

 pleasure. From the epoch however of interference with the nor- 

 mal blood- stream there is a gradual diminution in the responses 

 to stimuli, and ultimately the muscle loses all its irritability and 

 becomes rigid, however well the artificial circulation be kept up. 

 This failure is probably in great part due to the blood sent through 

 the tissues not being in a perfectly normal condition ; but we have 

 at present very little information on this point. Indeed with 

 respect to the quality of blood thus essential to the maintenance 

 or restoration of irritability, our knowledge is definite with regard 

 to one factor only, viz. the oxygen. If blood deprived of its oxygen 

 be sent through a muscle removed from the body, irritability, so 

 far from being maintained, seems rather to have its disappearance 

 hastened. '-In fact, if venous blood continues to be driven through 

 a muscle, the irritability of the muscle is lost even more rapidly 

 than in the entire absence of blood. It would seem that venous 

 blood is more injurious than none at all. If exhaustion be not 

 carried too far, the muscle may however be revived by a proper 

 supply of oxygenated blood. 



The influence of blood-supply cannot be so satisfactorily studied 

 in the case of nerves as in the case of muscles ; there can however 

 be little doubt that the effects are analogous. 



86. The influence of functional activity. This too is more 

 easily studied in the case of muscles than of nerves. 



When a muscle within the body is unused, it wastes ; when 

 used, it (within certain limits) grows. Both these facts shew that 

 the nutrition of a muscle is favourably affected by its functional 

 activity. Part of this may be an indirect effect of the increased 

 blood-supply which occurs when a muscle contracts. When a 

 nerve going to a muscle is stimulated, the blood vessels of the 

 muscle dilate. Hence at the time of the contraction more blood 

 flows through the muscle, and this increased flow continues for 

 some little while after the contraction of the muscle has ceased. 

 But, apart from the blood-supply, it is probable that the ex- 

 haustion caused by a contraction is immediately followed by a 

 reaction favourable to the nutrition of the muscle ; and this is a 

 reason, possibly the chief reason, why a muscle is increased by use, 

 that is to say, the loss of substance and energy caused by the 

 contraction is subsequently more than made up for by increased 

 metabolism during the following period of rest. 



Whether there be a third factor, whether muscles for in- 

 stance are governed by so-called trophic nerves which affect their 

 nutrition directly in some other way than by influencing either 

 their blood-supply or their activity, must at present be left 

 undecided. 



A muscle, even within the body, after prolonged action is 

 fatigued, i.e. a stronger stimulus is required to produce the same 

 contraction ; in other words, its irritability may be lessened by 



