OH. xili.] FATIGDB. 157 



contraction passing into the circulation and poisoning the central 

 nervous system. It is a matter of common experience that one's 

 mental state influences markedly the onset of fatigue and the 

 amount of muscular work one can do. This aspect of the question 

 has been specially studied by Mosso ; he invented an instrument 

 called the ergograph. The arm, hand, and all the fingers but one 

 are fixed in a suitable holder ; the free finger repeatedly lifts a 

 weight over a pulley, and the height to which it is raised is 

 registered by a marker on a blackened surface. 



By the use of this instrument he has arrived at the conclusion 

 that the state of the brain and central nervous system generally 

 is a most important factor in fatigue, and that the fatigue products 

 produced in the muscles during work cause most of their injurious 

 effects by acting on the central nervous system and diminishing 

 its power of sending out impulses. 



One of the most striking of Mosso's experiments illustrates in 

 a very forcible manner the fact that the central nervous system 

 is more easily fatigued than the nerve endings in muscle. A 

 person goes on lifting the weight until, under the influence of the 

 will, he is unable to raise it any more. If then without waiting 

 for fatigue to pass off, the nerves going to the finger muscles are 

 stimulated artificially by induction shocks, they once more enter 

 into vigorous contraction. 



Mosso has also shown that the introduction of the blood of a 

 fatigued animal into the circulation of a normal one will give rise 

 in the latter to all the symptoms of fatigue. The blood of the 

 fatigued animal contains the products of activity of its muscles, 

 but still remains alkaline; the poisonous substance cannot there- 

 fore be free lactic acid ; and lactates do not produce the effect. 

 Lactic acid is doubtless one only of the products of muscular 

 activity ; we have at present no accurate knowledge of the 

 chemical nature of the others. 



Rigor Mortis. 



After death, the muscles gradually lose their irritability and 

 pass into a contracted condition. This affects all the muscles of 

 the body, and usually fixes it in the natural posture of equilibrium 

 or rest. The general stiffening thus produced constitutes rigor 

 mortis or post-mortem rigidity. 



The cause of rigor is the coagulation of the muscle-plasma, 

 which is more fully described in the next section. This coagula- 

 tion results in the formation of myosin, and is gradual in onset. 



