MUSCULAR AND NERVOUS SYSTEMS. 231 



nerves, still it is also true, that the resulting disposi- 

 tions can be modified, and in a great measure controlled 

 by external circumstances. 



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 w T hich the person happens to be thrown. Ex- 

 cessive study, a sedentary life, luxurious living, especial- 

 ly in respect to drinks ; habitual melancholy, or a gen- 

 eral disposition to give way to the love of ease and in- 

 dolence, will, either of them, in a longer or shorter time, 

 induce a nervous temperament, though the natural or- 

 ganization of the two systems might have been as per- 

 fect as ever a human being enjoyed. 



On the contrary, persons in whom the nervous dispo- 

 sition might have naturally inclined to laxity of muscular 

 fibre, and excessive sensorial irritability ; by the habitu- 

 al practice of rigid temperance, moderate study, if at all, 

 an active life, requiring the constant use of the muscles, 

 together with a train of circumstances in life calculated 

 to inspire cheerfulness and hope by these means, such 

 .persons would undoubtedly overcome the natural pre- 

 dominance of the nervous system, and induce a perma- 

 nent state of muscular firmness, which would produce a 

 highly gratifying contrast with that which an opposite 

 mode of living, or train of circumstances, would have 

 produced. 



Force of -Muscular contraction. The force with 

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

 dition of the muscle, and on the energy of the brain ; 

 that is, the degree of excitement which exists during the 

 time. 



The same muscle, or same limb, varies greatly in the 

 strength which it is capable of exerting. If a man, nat- 

 urally and habitually powerful in his muscles, should suf- 

 fer 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 al- 



