172 BIOLOGY 



The relation of these neurons to the body may be seen from 

 Figure 84. Most of the cells which appear so prominently 

 in the cord have connections as shown at mn. Each has a 

 complex of dendrites which branch in, the substance of the 

 cord, and a single axon which passes out through the nerve 

 to be finally distributed to the muscles. These neurons send 

 impulses to the muscles and are called motor cells. Some send 

 their axons to muscles on the same side, as shown in the figure, 

 and others send theirs across the cord to the muscles on the 

 other side of the body. These axons are known as efferent 

 (Lat. ex = out + ferre to bear) nerve fibers. Some of the 

 axons do not pass out of the cord, but simply connect dif- 

 ferent parts of the cord itself. 



The neurons which carry impulses from without toward the 

 center are called afferent (Lat. ad = to + ferre = to bear) 

 neurons. These never have their neuron bodies within the 

 cord but somewhere outside it. Many of them take their 

 origin in special cells called sense cells (Fig. 84 so), which are 

 sensitive to certain external stimuli. The impulses excited in 

 the cell pass over the axon to the ventral cord. Where the 

 axon enters the cord it breaks up into numerous branches, or 

 arborizations, arb, which spread out in the cord itself. The 

 impulses entering by the axons may pass from the arboriza- 

 tions to the dendrites of the motor cells and excite them to 

 action. Hence a stimulus applied to the skin may produce a 

 movement. 



The sense organs. The cells at the end of the afferent 

 nerves constitute the sense organs, and they are so constructed 

 as to be influenced by different external forces. The earthworm 

 has no eyes, although some of its sense organs appear to be 

 slightly affected by a bright light. They have no ears and no 

 sense of sound, though they are very sensitive to a slight jar. 

 They have a sense of taste, located in the mouth, and also a 

 sense of smell. None of the sense organs is visible to the naked 

 eye, but they may be seen by microscopic study. The end of the 



