96 MANUAL OF ANIMAL BIOLOGY 



and ventral branch for the innervation of the body wall. Each 

 of the posterior nerves in a segment gives off a septal nerve 

 which runs dorsally along the septum to innervate the alimentary 

 canal. 



The structural features of the nerve tissue as revealed by micro- 

 scopic study may now be described. Considering first the ventral 

 nerve cord in the region of a ganglion, we find on the outside a 

 sheath of non-nervous tissue, which consists, externally, of a layer 

 of epithelial cells similar to those which cover various other organs 

 in the coelom. Then comes an inner sheath layer consisting 

 largely of connective tissue, but also containing numerous muscle 

 fibers and blood vessels. (W. f. 139.) 



Embedded dorsally in the cord, below the inner sheath layer, 

 are three tubular structures, the giant fibers, which extend prac- 

 tically the entire length of the cord. The median giant fiber is 

 somewhat larger than the other two which lie laterally. All three 

 fibers are connected with each other and with processes from the 

 nerve cells at regular intervals along the cord. It is clear that 

 they have important nervous functions and, therefore, are better 

 designated as neurochords. 



In Lumbricus, although the nerve cord has the appearance of a 

 single structure, certain features of the internal arrangement are 

 quite markedly bilateral. As a matter of fact, in certain other 

 species, two separate ventral cords are found which are connected 

 segmentally by transverse fibers. The bilobed central region of 

 the cord is largely occupied by innumerable fine nerve fibers 

 which form a dense interlacing mass. The nerve cells, or neurons, 

 from which the fibers arise, constitute the basic structural and 

 functional units. They are situated in the ventral and lateral 

 portions of the cord, below the fiber mass. The neurons are much 

 more numerous in the ganglia of the cord than in the intergangli- 

 onic regions. 



A typical neuron may be described as a pear-shaped cell with a 

 prominent nucleus. At one end the cytoplasm is extended to 

 form a long process, the axon, and, at the other end, one or more 

 dendrites which are somewhat smaller. Functionally it is clear 

 that a neuron receives communications by way of the dendrites, 

 and that the response is sent from the neuron to the peripheral 

 regions over the axon. 



We may distinguish three types of neurons. There is, first, the 



