114 THE CONTRACTILE TISSUES. 



coagulation takes place, may vary independently. The rapidity of onset 

 after muscular exercise and wasting disease may, perhaps, be in part depend- 

 ent on an increase of acid reaction, which is produced under those circum- 

 stances in the muscle, for this seems to be favorable to the coagulation of 

 the muscle plasma. When rigor mortis has once become thoroughly estab- 

 lished in a muscle through deprivation of blood, it cannot be removed by 

 any subsequent supply of blood. Thus, when the abdominal aorta has 

 remained ligatured until the lower limbs have become completely rigid, 

 untying the ligature will not restore the muscles to an irritable condition ; 

 it simply hastens the decomposition of the dead tissues by supplying them 

 with oxygen and, in the case of the mammal, with warmth also. A muscle, 

 however, may acquire as a whole a certain amount of rigidity on account of 

 some of the fibres becoming rigid, while the remainder, though they have 

 lost their irritability, have not yet advanced into rigor mortis. At such a 

 juncture a renewal of the blood-stream may restore the irritability of those 

 fibres which are not yet rigid, and thus appear to do away with rigor mortis ; 

 yet, it appears that in such cases the fibres which have actually become 

 rigid never regain their irritability, but undergo degeneration. 



Mere loss of irritability, even though complete, if stopping short of the 

 actual coagulation of the muscle substance, may be with care removed. 



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. 



84. 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 show that the nutrition of a muscle 

 is favorably 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 bloodvessels 

 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 exhaustion caused by a contraction is 

 immediately followed by a reaction favorable 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 contrac- 

 tion is subsequently more than made up for by increased metabolism during 

 the following period of rest. 



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 functional activity. Whether 

 functional activity, therefore, is injurious or beneficial depends on its amount 

 in relation to the condition of the muscle. The muscle is sooner fatigued 

 and exhausted than the motor nerve. 



The sense of fatigue of which, after prolonged or unusual exertion, we are 

 conscious in our own bodies, is probably of complex origin, and its nature, 

 like that of the normal muscular sense of which we shall have to speak 

 hereafter, is at present not thoroughly understood. It seems to be in the first 

 place the result of changes in the muscles themselves, but it is possibly also 

 caused by changes in the nervous apparatus concerned in muscular action, 

 and especially in those parts of the central nervous system which are con- 

 cerned in the production of voluntary impulses. lu any case it cannot be 

 taken as an adequate measmre of the actual fatigue of the muscles ; for a 

 man who says he is absolutely exhausted may, under excitement, perform a 



