THE EARTHWORM 



171 



outer ends of the nerves, in the prostomial lobe, etc. The 



ventral cord also is a mass of neurons, and since it is simpler 



than the brain it 



may be more easily 



understood and will 



illustrate better the 



relation of neurons 



to the rest of the 



body. 



The ventral cord. 



A cross section of 



the cord shows it 



to be covered on 



the outside by a 



thin layer of epi- 

 thelium, the perito- 

 neum, inside of 



which is a muscular 



sheet containing a 



few blood vessels; 



Fig. 86. Near the dorsal surface of the cord are three clear rods 



running lengthwise, 

 called giant fibers, 

 g, containing nerve 

 fibers. The cord 

 itself is really two 

 cords fused to- 

 gether. Embedded 

 in the cord may be 

 seen many large 

 cells, which are the 

 bodies of the neu- 

 rons making up the 



cord; and extending out to form the nerve fibers which arise 



from the cord are the axons of these neurons. 



FIG. 85. NEURONS OF VARIOUS TYPES 



FROM HIGHER ANIMALS 



A, a complex of neurons from the cerebrum; B and C, 

 neurons from the cerebellum; D, a single neuron from the 

 cerebrum. 



n 



FIG. 86. MAGNIFIED SECTION OF THE VENTRAL 

 CORD OF THE EARTHWORM 



g, giant fibers; 

 n, neurons; 



nf, nerve fibers; 

 v, blood vessels. 



