LECT. XVI. HOW INDUCED. 297 



Muscular Contraction, how induced. Volition, mecha- 

 nical actions, heat, and electricity, determine muscular con- 

 traction by their action upon the nerves; for this effect 

 does not take place if the nerves be tied, or if their struc- 

 ture be in any way altered. There is, then, evidently a 

 force propagated along the nervous filament to the muscu- 

 lar fibre. We are, likewise, compelled to admit, that the 

 fibre possesses the aptitude for, and the property of, con- 

 tracting under the action of the nervous force ; and we 

 cannot explain' why, for so long a time, there prevailed, 

 and still prevail in physiology, theories, either exclusively 

 ascribing the contraction of muscles to the action of the 

 nervous force, and denying to the muscular fibre the power 

 of contracting per se ; or ascertaining that the power of con- 

 traction resides in the fibre and is independent of the ner- 

 vous force. In the same manner, that elasticity is the pro- 

 perty of bodies by means of which molecules are capable 

 of being put into vibration ; so is it necessary that some 

 impulses should be communicated to these molecules, in 

 order to throw them into vibration. 



Velocity of the Nervous Force. The velocity with which 

 the nervous force is propagated is very great, and may be 

 compared to that of light and electricity. I would ob- 

 serve, however, that without being able to deny, that its 

 velocity is as great as that of the two latter agents, we are 

 in want of experiments to support this supposition. The 

 distances to which we are accustomed to observe the ner- 

 vous force propagated, are very short, and we ought not 

 to be very much surprised at the velocity of its propaga- 

 tion, s , :.. 



When we observe a muscle at the moment of its con- 

 traction, we soon perceive that its longitudinal fibres be- 

 come shorter, and augment in diameter. Such is the 



