Fig. 126—*Diagram of a mon-axonic neuron* (greatly enlarged except as to length). The central thread in the axon is the axis cylinder. The neurons are in all respects cells. They differ widely, however, from all the other cells of the body and are, in some respects, the most remarkable of all cells. They are characterized by minute extensions, or prolongations, which in some instances extend to great distances. Though the neurons in certain parts of the body differ greatly in form and size from those in other parts of the body, most of them may be included in one or the other of two classes, known as _mon-axonic_ neurons and _di-axonic_ neurons. *Mon-axonic Neurons.*—Neurons of this 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 parts just like the dendrites of mon-axonic neurons. The cell-body is smooth and rounded, and its axons extend from it in opposite directions (Fig. 127). [Fig. 127] Fig. 127—*Diagram of a di-axonic neuron.* The diagram shows only the conducting portion of the axon, or axis cylinder.