MODE OF ACTION OF THE MOTOR NERVES. 85 



and is here manifested by contraction. Their mode of ac- 

 tion, therefore, is centrifugal. "When these motor filaments 

 are divided, the connection between the parts animated by 

 them and the centre is interrupted, and motion in these 

 parts, in obedience to the natural stimulus, becomes impossi- 

 ble. But, while we cannot induce generation of nerve-force 

 in the centres by the direct application of any agent to 

 them, this force may be imitated by stimulation applied to 

 the nerve itself. A nerve that will respond to direct stimu- 

 lation is said to be excitable ; but this property does not ex- 

 tend throughout the entire conducting motor system. For 

 example, we shall see when we come to study the properties 

 of the encephalon, that certain fasciculi capable of conduct- 

 ing the motor stimulus from the centres to the muscles are 

 not affected by direct stimulation, and seem to be inexcit- 

 ablc. 



If a motor nerve be divided, galvanic, mechanical, or 

 other stimulation applied to the extremity connected with 

 the centres produces no effect ; but the same stimulation 

 applied to the extremity connected with the muscles is fol- 

 lowed by contraction. The phenomena indicating that a 

 nerve retains its physiological properties are always mani- 

 fested at its peripheral distribution, and do not essentially 

 vary when the nerve is stimulated at different points in its 

 course. For example, stimulation of the anterior roots near 

 the cord produces contraction in those muscles to which 

 the fibres of these roots are distributed ; but the same effect 

 follows stimulation of the nerve going to these muscles in 

 any part of its course. 



As far as their physiological action is concerned, the dif- 

 ferent nerve-fibres are entirely independent, and the rela- 

 tions which they bear to each other in the nervous fasciculi 

 and in the so-called anastomoses of nerves involve simple 

 contiguity. If we compare the nerve-force to galvanism, 

 each individual fibre seems completely insulated ; and a stim- 

 ulus conducted by it to muscles never extends to the adjacent 



