MUSCLE 569 



means of which a record of the work done by one of the flexor 

 muscles of a finger in raising a weight many times in succession 

 can be obtained) (Mosso and Maggiora, Lombard) (p. 621) have 

 shown that fatigue after voluntary effort is due to central changes, and 

 not entirely to changes in the muscles and nerves themselves, for 

 electrical stimulation, either of a * tired ' muscle or of its nerve, is 

 readily responded to at a time when voluntary contraction is im- 

 possible. Now, there is no reason to suppose that the nerve-fibres 

 in the central nervous system differ essentially from those of peripheral 

 nerves, and therefore no reason for placing the seat of the fatigue 

 anywhere in the pyramidal fibres, except perhaps in their terminal 

 fibrils in the anterior horn, which correspond to the endings of the 

 peripheral fibres in the muscles. The only other portions of the 

 voluntary motor path besides these terminal fibrils that are likely to 

 become easily fatigued are the nerve-cells of the cortex and the cord. 

 These central structures we must consider the weakest links in the 

 chain, the next weakest link being the motor endings in the muscles, 

 and the strongest the nerve-fibres. The motor-endings do not, in 

 general, break down 

 in voluntary contrac- 

 tion, because the cen- 

 tral apparatus be- 

 comes sooner 

 fatigued. It is not 

 inconsistent with 

 these facts that a FlG I77 ._ VERA TRIA CURVE. 



muscle fatigued by (Frog . sgastroC nemius.) The <rve shows a peak, the 

 direct electrical Stimu- lever falling a little before the sustained contraction begins. 



lation, can still be 



voluntarily contracted. For it has been shown that the voluntary 



excitation is more effective than any artificial stimulus. 



(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 con- 

 spicuously alter the form and time-relations of the muscle- 

 curve have most interest. Of these ver atria is especially 

 important. When a small quantity of this substance 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 altera- 



