450 GENERAL BIOLOGY 



(if a bipolar cell) or more (if multipolar) additional pro- 

 cesses arising from the opposite end of the cell and extend- 

 ing in different directions. The cell with all its processes is 

 called a neurone. The process which becomes the fiber is 

 called the axone; if it gives off branches within the nerve 

 center for communication with other neurones, these 

 branches are called collaterals. The processes arising from 

 the other parts of the cell, are typically shorter and much 

 branched and are called dendrites (dendron, a tree) . 



Division of labor among the neurones has resulted in the 

 development of two complemental sorts of them, one sort 

 being afferent in function, and serving to conduct from the 

 receptors to the centers, and the other being efferent, and 

 serving to carry impulses outward from the center to incite 

 muscle or gland to action. The dendrites are at the receiv- 

 ing end of neurone, while nerve impulses are carried outward 

 from the cell body by the axone, and are distributed to 

 other cells by way of its collaterals. 



The reflex arc. A pair of these complemental neurones, 

 afferent and efferent in function, in contact by their inner 

 ends, and reaching outward the one to a receptor and the 

 other to a muscle or gland, constitute a simple reflex arc, 

 This is the unit of organization within the nervous system 

 It is a simple mechanism for the functions indicated in our 

 diagram on page 436. It was named a reflex arc because it 

 appeared to be a pathway over which the effect of a stim- 

 ulus sent to a center is reflected back in action to view. 

 Figure 2$ga shows such an arc for the body of a vertebrate. 

 The afferent cell body (u) lies in the ganglion on the dorsal 

 root of a spinal nerve ; its dendrites (not shown) are in the 

 skin ; its axone enters the cord . The efferent cell body (v) lies in 

 the gray matter of the spinal cord; its dendrites are in contact 

 with termini of the axone of the other cell, and its axone 

 proceeds outward through the anterior root of a spinal nerve 



