THE REFLEX ARC 97 



Neurofibrils. — \n \\\m^ cells it is difficult to demonstrate anything 

 but a more or less homofjeneous cyto])lasm, hut in fixed and stained 

 preparations, especially siher impregnated tissues, very fine long 

 thread-like structures api)ear to extend throughout the cell body 

 and its })rocesses. At first they were thought to be the primary 

 means by which impulses were transported, but there seems to be 

 little proof as to their function, though they appear as characteristic 

 featiuTs in nerve cells. 



Golgi Apparatus .—^y special techniques, using osmic acid and 

 silver, a blackened network appears in the cytoplasm usually con- 

 centrated about the nucleus. This a])])aratus of (lolgi disappears 

 in cells subjected to injin-y and cannot be demonstrated in li\ing 

 unstained cells. 



Chondrlosomes (Mitochuudria).— Tiny rods and granules appear 

 scattered through the cytoplasm and may be demonstrated in living 

 cells with Janus green B. Certain methods also preserve them in 

 fixed and stained jireparations. Little is known of their functional 

 significance. 



Cytoplasmic Processes. — It is customary- to recognize two types of 

 processes extending from the cyton, namely, axon and dendrites. 

 There is no Nissl substance in the axon. It is a slender fiber-like 

 extension of uniform diameter throughout and has a smooth clean 

 surface. Usually the axon arises from a particular place in the 

 cyton marked by a conical extension called the axon hill. Slender 

 collateral branches may arise from the axon along its course and are 

 at right angles to its surface. In many neurons the axon ends dis- 

 tally in a brush of finer branching processes known as the terminal 

 arborization or telodendria. The dendrites are thick, irregularly 

 branching, cytoplasmic processes extending out from the cyton and 

 contain the Nissl substance and other cytoplasmic elements gener- 

 ally present in the cyton. 



THE REFLEX ARC. 



The simplest physiological organization of neurons is called a 

 reflex arc. It involves at least two neurons, as in the following 

 example. A stimulus on the skin is translated into a nerve impulse 

 in the peripheral terminus of a dendritic process of neuron A. 

 (Fig. 58.) The nerve impulse travels centrally to the cyton of this 

 neuron, which is located in a spinal ganglion. From this cyton the 

 nerve impulse passes over its axon into the gray matter of the coid 

 to terminate in the ventral horn. Here it connects with the den- 

 7 



