MUSCLE -NERVE PHYSIOLOGY. 519 



The power of a muscle is usually measured by the maximum weight which 

 it will support without stretching. In man this is readily determined by weight- 

 ing the body to such an extent that it can no longer be raised on tiptoe : thus 

 the power of the calf -muscles is determined. The power of a muscle thus esti- 

 mated depends of course upon its cross-section. The power of a human muscle 

 is from two to three times as great as a frog's muscle of the same sectional area. 



Fatigue of Muscle. A muscle becomes rapidly exhausted from re- 

 peated stimulation, and the more rapidly, the more quickly the induc- 

 tion-shocks succeed each other. This is indicated by the diminished 

 height of the muscular contractions. 



A fatigued muscle has a much longer latent period than a fresh one. 

 The slowness with which muscles respond to the will when fatigued must 

 be familiar to every one. 



In a muscle which is exhausted, stimulation only causes a contraction 

 producing a local bulging near the point irritated. A similar effect 

 may be produced in a fresh muscle by a sharp blow, as in striking the 

 biceps smartly with the edge of the hand, when a hard muscular swelling 

 is instantly formed. 



As we have seen in discussing the irritability of muscle, the cause of 

 fatigue is twofold, being in part due to its nutritive condition, and in 

 part to the accumulation of poisonous products formed during contrac- 

 tion probably sarcolactic acid, chiefly. In a living animal these poi- 

 sonous products exert their influence not only upon the muscle or mus- 

 cles immediately concerned in contraction, but upon the musculature of 

 the body generally, and the effect remains until they are eliminated 

 from the body. Massage of the muscles increases the passage of them 

 into the general blood-stream and the rapidity of their elimination. 



Under normal circumstances muscles do not become completely fa- 

 tigued, for the reason that the nerve cells which send out the impulses 

 for contraction become fatigued sooner than the muscles themselves do. 

 Nerve cells, however, recover from fatigue more quickly than muscles. 

 These facts are sometimes shown when one feels utterly exhausted and 

 scarcely able to drag one foot after another, yet under a strong effort of 

 will, as from fright, is able to make unwonted- effort. 



Response to Stimuli in Voluntary and Involuntary Muscles. 

 The two kinds of fibres, the striped and the unstriped, hav> charac- 

 teristic differences in the mode in which they act on the application of 

 the same stimulus; differences which maybe ascribed a gi it part to 

 their respective differences of structure, but in some di ^ree, possibly, to 

 their respective modes of connection with the nervous system. When ir- 

 ritation is applied directly to a muscle with striated fibres, or to the 

 motor nerve supplying it, contraction of the part irritated, and of that 

 only, ensues; and this contraction is instantaneous, and ceases on the in- 

 stant of withdrawing the irritation. But when any part with unstriped 

 muscular fibres, e.g., the intestines or bladder, is irritated, the subse- 



