7 
way. This plant abounds in several of the canals of Alexandria, 
which at certain seasons become dry, and the beds of these 
canals, which quickly become burnt as hard as bricks by the 
action of the sun, are used as carriage roads. When the water is 
re-admitted the plants again resume their growth with redoubled 
vigour. I was surprised to find that John Hunter had explored 
this field so fully, and knew so much about the methods plants 
adopted to secure rest and thus prevent fatigue. ‘‘ Most plants,” 
says he, “have their periods of growth, and periods of rest. 
Some plants close their leaves, others their flowers, at particular 
hours of the day or night; and with such regularity does this 
period of rest take place that more than one vegetable physiologist 
has proposed to construct from them a floral clock. 
Before we proceed to speak of fatigue in Animal life it would 
be well, perhaps, to define, or further explain the term. Speaking 
of the muscular system, fatigue is not the same as strain. Strain 
may produce a dangerous lesion in a moment, fatigue may take 
hours, or days, or even longer before any serious change in the 
tissues takes place. A man may attempt to lift a very heavy 
weight, or to do other very heavy work, and suddenly, in a 
moment, damage his heart irretrievably. On the other hand a 
man may walk for a long time at his usual pace, and may produce 
such a state of fatigue as may permanently affect his health, and 
for the rest of his life leave him with a somewhat weakened 
muscular and nervous system. Strain is sudden and physical, an 
altogether unnatural process in the organism. On the other 
hand fatigue is physiological ; it is the result of a normal process 
carried too far. You might call it over-function. It is of course 
impossible, in most cases, to state the exact point at which fatigue 
starts. Nor can we in all cases trust our senses for guidance. 
— Weber's Law Explained. 
Probably most of the better known examples of fatigue of 
special nerves are known to you all. How quickly after some 
strong odours the olfactory nerve reaches its limit of sensation, 
and becomes temporarily paralysed by fatigue. Two or three 
short sharp sniffs of an ordinary musk plant will rapidly exhaust 
the nerve, and prevent its detecting the same perfume again until 
its fatigue is recovered from by resting a few minutes. If salt is 
held in the mouth for a short time the nerves of taste become 
fatigued and fail to respond. ‘That the ears are similarly affected 
by the auditory nerves becoming fatigued is sufficiently obvious 
from our use of such terms as “ deafening sounds.” 
Sir Michael Foster says the auditory nerve is capable of rapid 
fatigue. If the sound be continued long, with almost any note, 
the sensation diminishes, and finally disappears, and the exhaustion 
comes on more rapidly with high notes than with low, especially 
with very high. If the tuning fork be held to one ear, and then, 
