MUSCULAR AND NERVOUS SYSTEMS. 227 



nerves, still it is also true, that the resulting dispositions 

 can be modified, and in a great measure controlled by ex- 

 ternal circumstances. 



710. In persons where the two different nervous powers 

 are naturally in the most perfect equilibrium, the one may 

 be made to predominate completely over the other, by the 

 habits, the occupation, or perhaps the condition in life 

 into which the person happens to be thrown. Excessive 

 study, a sedentary life, luxurious -living, especially in re- 

 spect to drinks, habitual melancholy, or a general dispo- 

 sition to give way to the love of ease and indolence, will, 

 either of them, in a longer or shorter time, induce a nerv- 

 ous temperament, though the natural organization of the 

 two systems might have been as perfect as ever a human 

 being enjoyed. 



711. On the contrary, persons in whom the nervous 

 disposition might have naturally inclined to laxity of mus- 

 cular fibre, and excessive sensorial irritability, by the ha- 

 bitual practice of rigid temperance, moderate study, if at 

 all, an active life, requiring the constant use of the mus- 

 cles, together with a train of circumstances in life calcu- 

 lated to inspire cheerfulness and hope by these means, 

 such persons would undoubtedly overcome the natural 

 predominance of the nervous system, and induce a perma- 

 nent state of muscular firmness, which would produce a 

 highly gratifying contrast with that which an apposite 

 mode of living, or train of circumstances would have pro- 

 duced. 



712. Force of Muscular Contraction. The force with 

 which the muscles contract, depend on the size and condi- 

 tion of the muscle, and on the energy of the brain ; that 

 is, the degree of excitement which exists during the time. 



713. The same muscle, or same limb, varies greatly in 

 the strength which it is capable of exerting. If a man 

 naturally and habitually powerful in his muscles, should 

 suffer them to remain inactive for a considerable length of 

 time, they will become absolutely incapable of those strong 

 contractions, which, had they been habituated to constant 

 action, they would have performed with ease. This is a 

 fact that has fallen within the experience of almost every 



