MOTOR AND SENSORY NERVES. 591 



nerves, after division of these roots, have the remarkable power of preserving the ana- 

 tomical integrity of the fibres to which they are attached. Now, it has been shown by 

 Schiff that, after division of the posterior roots beyond the ganglia, the anterior roots 

 contain altered fibres, which he believes come from the posterior roots and give to these 

 roots their sensibility. 



Dr. Brown-Sequard offers a different explanation of the pain developed upon irrita- 

 tion of the anterior roots. He believes this to be due entirely to cramp or convulsive 

 contractions of the muscles. This may be accepted, perhaps, as a partial explanation, 

 for there can be no doubt of the fact that violent muscular action, produced indepen- 

 dently of volition, is more or less painful ; but it does not explain the great sensibility 

 sometimes observed when the muscular contraction is comparatively feeble. There can 

 be hardly any doubt that the explanation offered by Magendie, and sustained by the 

 ingenious histological observations cited above, is in the main correct. 



Mode of Action of the Motor Nerves. Having established the anatomical distinction 

 between the motor and sensory nerves, it becomes necessary to study the differences in 

 the mode of action of these two kinds of nervous conductors. In the first place, it is 

 evident, taking the nerves and their roots as we find them in the organism in a normal 

 condition, that certain fibres act from the centres to the periphery, conducting motor 

 stimulus, while others act from the periphery to the centres, conducting sensory impres- 

 sions. 



As regards the motor nerves, the force, whatever it may be, generated in the centres, 

 is conducted from the centres to the peripheral distribution of the nerves in the muscles, 

 and is here manifested by contraction. Their mode of action, 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 impossible. But, while we cannot always 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 

 stimulation is said to be excitable ; but this property does not extend 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 conducting the motor 

 stimulus from the centres to the muscles are not affected by direct stimulation and seem 

 to be inexcitable. 



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 followed by contraction. The phenomena 

 indicating that a nerve retains its physiological properties are always manifested at its 

 peripheral distribution, and these 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 dis- 

 tributed ; 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 different nerve-fibres are entirely 

 independent, and the relations 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 stimulus 

 conducted by it to muscles never extends to the adjacent fibres. That it is the axis- 

 cylinder which conducts and the medullary tube which insulates, it is impossible to say 

 with positiveness ; but, as we have already seen, it is more than probable that the central 

 band is the only conducting element. 



We have incidentally noted the fact that direct stimulation applied to the centres, 

 even when the connection between these and the muscles is perfect, is generally inca- 



