A REFLEX ACTION 



149 



over (4) an efferent or motor nerve to (5) a muscle cell. The 

 stimulus thus conveyed, starts contraction in the muscle. It 

 is probable that other centers of activity are stimulated and 

 that nerve cells and their processes the nerve fibers transmit 

 impulses from one ganglion to another along the entire course of 

 the central nervous system. This is probable because of the 

 presence in the ventral nerve chain of different kinds of nerve 

 fibers some of which originate in the ventral ganglia and send 

 processes in both directions along the ventral cord without 



71. C 



FIG. 61. Portion of a transverse section of the ventral part of the body of 

 Lumbricus, to show the nerve connections, w.c., Ventral ganglion giving off a 

 lateral nerve /..; p.e., peritoneal endothelium ; l.m., longitudinal muscles; hy, 

 hypodermis; in the nerve l.n. are sensory fibers proceeding inward from the 

 sensory cells (in black) of the hypodermis, and terminating in branching extremi- 

 ties. (From Sedgwick and Wilson, after Lenhossek). 



leaving the cord at any point. Such cells and fibers, known 

 as coordinating cells, extend from somite to somite and give rise 

 to coordinating impulses which cause the entire worm to act as 

 a single unit. 



The ventral cord and ganglia have other cells than those 

 mentioned. These are grouped together as (i) afferent nerve 

 cells which receive impulses, (2) efferent nerve cells which trans- 

 mit impulses to motor fibers, (3) coordinating cells which bring 



