RESPONSIVE LIFE OF ORGANISMS 



451 



to end in a muscle. This is a simple and direct nerve ])ath, 

 sufficing for the reception of a stimulus and the development 

 of an appropriate automatic response. 



^ 1 7r^ 



Fig. 259 Diagram of reflex arcs in the spinal cord, a, a simple reflex arc shown 

 in half the spinal cord in cross section. 6, a direct connection of one aff'erent 

 with a number of efferent neurones, c, indirect connection through a neurone 

 intercalated in the arc. u, afferent, v, efferent fibers, x, intercalated tract cell 

 in the cord, (a, after Howell, b and c after Kolliker) 



But there are no nerve paths quite so simple and indepen- 

 dent as indicated by this diagram. There are always by- 

 paths which a stimulus may take to reach other efferent cells 

 and to produce multiple response. These bypaths may be 

 provided by various modes of branching within the nerve 

 centers; by cgllaterals as indicated in figure 259/^, or by 

 intercalated nerve cells that lie wholly within the nerve 

 centers as indicated in figure 2^gc. By reason of such con- 

 nections a stimulus on a single receptor, may according to 

 its strength, give rise to impulses in many motor nerves. 



The nervous system is a unit. No reflex arc exists by 

 itself alone. All the circuits of the body are connected, in 

 the first instance by collaterals and dendrites, and in the 

 next, by the arborisations of additional nerve cells inter- 

 calated within the nerve centers. Even in so lowly an 

 animal as the earthworm, in whose segments the several 

 ganglia appear to exercise a degree of independence and 



