STRUCTURE OF THE NERVOUS SYSTEM 283 



class consist of three distinct parts, known as the cell-body, 

 the dendrites, and the axon (Fig. 126). 



The cell-body has in itself the form of a complete cell 

 and was at one time so described. It consists of a rounded 

 mass of protoplasm, containing a well-defined nucleus. 

 The protoplasm is similar to that of other cells, but is 

 characterized by the presence of many small granules and 

 has a slightly grayish color. 



The dendrites are short extensions from the cell-body. 

 They branch somewhat as the roots of a tree and form in 

 many instances a complex network of tiny rootlets. Their 

 protoplasm, like that of the cell-body, is more or less 

 granular. The dendrites increase greatly the surface of 

 the cell-body, to which they are related in function. 



The axon, or nerve fiber, is a long, slender extension 

 from the cell-body, which connects with some organ or 

 tissue. It was at one time described as a distinct nervous 

 element, but later study has shown it to be an outgrowth 

 from the cell-body. The mon-axonic neurons are so 

 called from their having but a single axon. 



Di-axonic Neurons. Neurons belonging to this 

 class have each a well-defined cell-body and two 



axons, but no 



ody T K%&*^ parts just like 



the dendrites of 



FIG. 127. -Diagram of a di-axonic mon . axon i c neu- 

 neuron. The diagram shows only the con- _,, .. 



ducting portion of the axon, or axis cylinder. 



body is smooth 



and rounded, and its axons extend from it in opposite di- 

 rections (Fig. 127). 



Structure of the Axon. The axon, or nerve fiber, has 

 practically the same structure in both classes of neurons, 

 being composed in most cases of three distinct parts. In 



