452 FATIGUE.-ENERGY AND RAPIDITY OF MOVEMENT. 
greater than that which we see manifested in convulsive 
actions, where the movements depend only upon the reflex 
activity of the spinal cord. Thus a slender girl affected with 
a spasmodic affection of the muscles of the spine, which threw 
the hack into an arch of which the head and the heels were 
the two resting-points, has been known to raise a weight of 
9001bs. laid on the abdomen with the absurd intention of 
straightening the body. 
593. The sense of fatigue, which comes-on after prolonged 
muscular exertion, is really dependent upon a change in the 
brain, though usually referred by us to the muscles that have 
been exercised. For it is felt after voluntary motions only; 
and the very same muscles may be kept in refiex action for 
a much longer time, without any fatigue being experienced. 
Thus, we never feel tired of breathing; and yet a forced 
voluntary action of the muscles of respiration soon causes 
fatigue. The voluntary use of the muscles of our limbs, in 
walking or running, soon occasions weariness; but similar 
muscles are used in Birds and Insects, for very prolonged 
flights, without apparent fatigue; and as we find that the 
actions of flight may be performed, after the brain, or the 
ganglia that correspond to it in Insects, have been removed 
(§§ 444, 465), we may regard them as of a reflex character; 
and the absence of fatigue is thus accounted-for. 
594. The energy of muscular contraction appears to be 
greater in Insects, in proportion to their size, than it is in any 
other animals. Thus a Flea has been known to leap sixty 
times its own length, and to move as many times its own 
weight. The short-limbed Beetles that inhabit the ground 
have an enormous power, which is manifested both in their 
movement of heavy weights, and in the resistance they 
overcome with their jaws. Thus the Dung- or “ shard-borne ” 
Beetle can support uninjured, and even elevate, a weight equal 
to at least 500 times that of its body. And the Stag-Beetle 
has been known to gnaw a hole of an inch diameter, in the 
side of an iron canister in which it had been confined. The 
rapidity of the movements of Insects is also most extraordi¬ 
narily great, and is especially seen in the vibrations of their 
wings. It would be impossible to form an estimate of the time 
occupied by these, were it not for the musical tones they pro¬ 
duce ; and it may be calculated from these that the wings of 
