A MANUAL OF PHYSIOLOGY. 



therefore very well be a factor in inducing fatigue of the central 

 nervous mechanisms in addition to the formation of fatigue 

 products, and the using up of necessary material in these 

 mechanisms themselves. Contrariwise, active and long-continued 

 mental exertion may occasion muscular fatigue (Fig. 244). 



(d) The Influence of Drugs on the Contraction of Muscle. The 

 total work which a muscle can perform, its excitability and the 

 absolute force of the contraction, may all be altered either in the plus 

 or the minus sense by drugs. But in connection with our present 

 subject those drugs which conspicuously alter the form and time- 

 relations of the muscle-curve have most interest. Of these veratrine 

 is especially important. When a small quantity of this substance 



FIG. 244. INFLUENCE OF MENTAL FATIGUE ON MUSCULAR CONTRACTION. 



i, series of contractions of flexors of middle finger before, and 2, series of con- 

 tractions immediately after, a period of three and a half hours' hard mental work. 

 In both cases the muscles were stimulated directly every two seconds by an 

 electrical current, and caused to raise a certain weight till temporary exhaustion 

 occurred. In the first series fifty-three contractions were found possible, in the 

 second only twelve (Maggiora). 



is injected below the skin of a frog, spasms of the voluntary muscles, 

 well marked in the limbs, come on in a few minutes. These are 

 attended with great stiffness of movement, for while the animal can 

 contract the extensor muscles of its legs so as to make a spring, they 

 relax very slowly, and some time elapses before it can spring again. 

 If it be killed before the reflexes are completely gone, the peculiar 

 alterations in the form of the muscle-curve caused by veratrine will 

 be most marked. The poisoned ^muscle, stimulated directly or 

 through its nerve, contracts as rapidly as< a normal muscle, while the 

 height of the curve is about the same, but the relaxation is enormously 

 prolonged (Fig. 245). This effect seems tojgbe to a considerable 

 degree dependent on temperature, and it may .temporarily disappear 

 when the muscle is made to contract several times without pause. 

 Adrenalin, barium salts, and, in a less degree, those of strontium and 



