THE NERVOUS SYSTEM 241 



and then allowed to fall, it will be noticed that the limb does 

 not drop instantly, like a lifeless member, but a certain amount 

 of rigidity remains in its muscles, which resists or breaks the 

 force of its descent. Again, when a substance like melted 

 sealing-wax, or a heated coin, falls upon the hand, the limb is 

 snatched away at once, even before the feeling of pain has 

 been recognized by the brain. When jolted in a rapidly mov- 

 ing car, we involuntarily step forward or backward, so as to 

 preserve the centre of gravity of the body. 



45. These and similar acts are executed by the same mechan- 

 ism as that previously described in the case of paralysis from 

 an injury of the spinal cord. The muscles thus called into 

 play are those which are ordinarily under the sway of the will, 

 but which in these cases act through this reflex action of the 

 cord, altogether independently of the will. A healthful reflex 

 activity produces an elasticity, or " tone," in the voluntary mus- 

 cular system, which in a great measure explains the existence 

 in the young and vigorous of a feeling of buoyancy and reserve 

 power. Its possessor is restlessly active, and it may appropri- 

 ately be said of him, "he rejoiceth as a strong man to run a 

 race." But this reflex energy may be deficient. This is true 

 when the blood is poor and wanting in its solid ingredients, or 

 the circulation is feeble; the muscles, then, are flabby and 

 weak, and the person himself is said to be " nerveless," or in- 

 disposed to exertion. Shivering from cold and trembling from 

 fear may, in part, be referred to a temporary loss of tone, 

 resulting from a powerful impression upon the brain. (Bead 

 Note 6.) 



6. On Nervous Health, or Tone. " That state of general vigor, which 

 we call ' tone,' depends upon the healthy action of the nervous centres. 

 It consists in the habitual moderate contraction of the muscles, due to a 

 constant stimulus exerted on them by the cord, and is valuable less for 

 itself than as a sign of a sound nervous balance. Tone is maintained 

 partly by healthful impressions radiated upon the spinal cord through the 

 nerves from all parts of the body, and partly by the stimulus poured 

 down upon it from the brain. So it is disturbed by whatever conveys 

 irritating or depressing influences in either direction. A single injudi- 



45. Result of healthful reflex activity ? When mny the reflex energy be deficient? 



