452 FATIGUE. ENERGY AND KAPIDITY 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 back 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 reflex 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 

 nights, 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 



