170 



BIOLOGY 



ess may be seen near the cell body. Other much shorter 

 processes arise also from the cell body and divide quickly 

 into branches. The long fiber is called the axon or the nerve 

 fiber, and the other branching projections are called dendrites 

 (Gr. dendron = tree). Sometimes the axons at their outer or 

 peripheral end break up into numerous branches known as 



A, a single neuron; B, a section of the ventral surface, showing the nerve 

 cord and its connection with the muscles and dermis. 



orb, arborization of an afferent nerve; sf, sensory nerve fiber; 



mf, motor fiber; so, sensory organ. 



mn, motor nerve cell; 



arborizations (Lat. arbor = tree), arb. In such a neuron im- 

 pulses enter the cell body through the dendrites and pass out 

 through the axon. 



Similar neurons make up the nervous system of all animals 

 which have been carefully studied. In shape the neurons are 

 quite varied (Fig. 85), but in all cases there is a cell body 

 with one or* more branching processes arising from it; and an 

 axon fiber of varying length extends outward from the cell. 



Vast numbers of these neurons are aggregated together to 

 make the nervous system of the earthworm. The cerebral gan- 

 glia contain them in great numbers, and the many nerves shown 

 in Figure 79 are formed chiefly of bundles of the axons of the 

 neurons, whose cell bodies are either in the ganglia or at the 



