232 THE NERVOUS SYSTEM 



be. It is an invariable law to which reference has already 

 been made, that a nerve fiber thus conducting a message in 

 either direction is not interfered with by the proximity of 

 other fibers, similar or dissimilar. Such message is not in 

 any way imparted to a neighboring fiber or diffused through 

 the fasciculus, but is conveyed uninterruptedly to its destina- 

 tion. It is possible that the myeline sheath has an insulating 

 effect upon the contained axis cylinder, just as an electric 

 wire may be insulated by non-conducting substances like 

 silk, but this is doubtful. 



Interesting manifestations of motor centrifugal impulses 

 are seen in certain movements associated with correspond- 

 ing muscles on different sides of the body and with sets of 

 muscles on the same side. It is almost impossible to effect 

 certain movements with a single finger or toe without causing 

 similar movements in other fingers and toes; a part of a 

 muscle cannot be made to contract separately ; it is doubtful 

 if it be possible to move one eye-ball without the other, even 

 by the most persistent practice. Other similar examples are 

 numerous. It is quite probable that in most cases these as- 

 sociated movements are solely matters of habit. But the 

 connection by commissural fibers of the cells in the centers 

 controlling and regulating the movement of these muscles 

 and sets of muscles would offer a not unreasonable explana- 

 tion of the phenomena in question, since such an arrange- 

 ment might render impossible separate and individual action 

 by the cells thus connected. Excepting, perhaps, the move- 

 ments of the eye-balls, these associated movements can be 

 greatly modified by education. 



Characteristics of Afferent Nerves. Impressions received 

 by these fibers, although they are conveyed toward the cen- 

 ter and must reach a center before there is any nervous 

 manifestation, are always referred to the periphery. A most 

 common illustration of this fact is furnished by injury to the 

 ulnar nerve as it passes the elbow such injury being mani- 

 fested not usually by any pain at the point of infliction, but on 



