8. 
just as the sound becomes inaudible, be changed to the other 
side, the sound will be distinctly heard—the fresh untired sense 
apparatus of the one side is sensitive to the vibration which the 
tired apparatus of the other side can no longer feel. 
Most of you will remember the soap advertisements of twenty 
years ago to demonstrate the limit of sensation of the retina for 
certain colours. You were told to look fixedly at the word 
“Pears” for some short time, until the retina was fatigued, and 
then to look away from the hoarding to the sky, and you saw 
the same word in the sky in their complimentary colour. Many 
a one has paid a big fee for learning that physiological fact, by the 
loss of a purse or other valuable ; for the pickpocket was near 
studying the student of fatigue, and his fingers had not reached 
the limit of sensation. 
The heart is only a hollow muscle, and is subject to the same 
laws of fatigue. As it is always on duty, Nature has done her best 
to guard against exhaustion, by forcing it to rest six elevenths of 
each second. The lungs too avoid over function by resting more 
than half their time. 
But even these wise provisions of Nature are not always 
sufficient to protect these organisms from irretrievable damage. 
Both the youth, and the city man past middle life, occasionally 
fall victims through a paper chase or a mountain climb. I am 
quite sure that in our best public and private schools greater 
attention is paid to this matter of overtaxing the heart and limbs 
by too long runs, or being too long in the water, than was the case 
at one time. Still, more will have to be done in separating the 
pupils into classes for physical exercise, as is done for mental 
exercise. In the near future this will probably, I may say almost 
certainly, be a much simpler matter than it has been up to the 
present time. 
As regards the city men, many of whom do a minimum of 
muscular work on a maximum of the heavier nitrogenous foods, 
and z# this condition (or rather ow¢ of condition) which this 
entails, if then they attempt a too sudden, or too prolonged active 
exertion their hearts and greater vessels are especially liable to 
suffer from the graver effects of fatigue. Many a one has caused 
a dilated heart, or aneurism, by a mountain climb, or tramping 
too far across the moors in pursuit of game. 
It must be remembered that in severe cases of fatigue certain 
poisons, which have a very deleterious effect on the whole system, 
are formed by the muscles. This has been demonstrated by 
injecting the muscle juice of a fatigued animal into a healthy one 
and thus causing convulsions and other symptoms of ptomaine 
poisoning. These fatigue products certainly predispose the joints, 
the pleura, peritoneum, and probably the membrane of the brain, 
to tubercular changes. 
