232 THE NERVOUS SYSTEM 



able law to which reference has already been made, that a nerve 

 fiber thus conducting a message in either direction is not inter- 

 fered 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 uninterrupt- 

 edly to its destination. 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 corresponding mus- 

 cles 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 associated 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 ex- 

 planation 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 movements 

 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 center 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 manifested not usually by any pain at 

 the point of infliction, but on the ulnar side of the hand where 



