Now all this grand play of diverse emotions and conditions in the psychical functions takes place in a state of health; still, there can be no doubt that it comes from an unstable condition of the nervous elements of the brain, or from changes produced in some manner in these varied and delicate structures. It may be from varying states which are constantly occurring in the blood, in the processes of reception and elimination; or from those delicate chemical operations which must be forever going on in the nerve elements of the brain hemispheres, affecting their recipient and sensitive capacities; or it may be from other unknown causes: and these changes occur much more readily and frequently in some persons than in others. Bearing in mind, now, these conditions of the physical and mental systems, the tendencies to which are inherited, and more or less changeful in character, we may proceed a step farther. This unstable condition, both physical and mental, may exist not only as an inherited condition, but it may be produced, or become greatly increased by causes external to the system, which are brought into contact so as to influence it. A few illustrations may serve to make this more clear: 1. When a person who has not been accustomed to use the muscles of the arm in active and vigorous exercise, lifts, or makes a strong effort to lift, a heavy weight, if the effort is continued any considerable length of time, two conditions result therefrom: First, a state of tremulousness, or spasm of the arm, hand, and especially of the fingers. The largest effort the individual can make toward controlling this irregularity of movement, is insufficient, and these parts remain in a condition of more or less spasmodic action, until the nervous energy is restored. Second, there results a sensation of pain, more or less severe, according to the time the exertion has continued. This would indicate positive injury, to some extent, in the nerve filaments of the muscles which have been so unusually exercised. The degree of effect will be dependent upon the condition of the nerves of the hand and arm at the time the effort is made, and upon the amount of force expended. If the effort is repeated soon, there will result a similar condition, and ultimately the nerve would cease to respond to the call for action in any degree,--the arm would be paralyzed.