222 ELEMENTARY BIOLOGY 



These interlacing dendrites allow the nerve action to pass over 

 from one cell (neuron) to another (Fig. 84). It is possible that 

 in this region the protoplasm of one neuron is in touch with 

 the protoplasm of the next one, so that a disturbance passes 

 from one cell to the next just as it might pass from one part 

 of a single cell to another part. 



The disturbed spinal-cord cell sets up nerve action in an 

 efferent muscle nerve, with the result that the arm or hand is 

 drawn back. At the same time, it sets up nerve action in a 

 fiber leading to a brain cell, with the result that you become 

 aware of the pain. But the movement has taken place before 

 you realize what has happened. The nerve disturbance from 

 the finger to the spinal cord along an afferent fiber is reflected 

 out through an efferent fiber, which stimulates the muscle to 

 action (Fig. 85). 



259. Afferent and efferent neurons. If a certain part of the 

 sciatic nerve (which is the main nerve trunk running down 

 the leg) were cut, destroying the afferent fibers (a lt Fig. 86), 

 one might walk on carpet tacks or hot iron and not know it 

 (unless he happened to be watching his feet), and accordingly 

 one would not jump to escape the damage. Under these cir- 

 cumstances a person would still be able to move his legs or to 

 jump if he wanted to. On the other hand, if another portion 

 of this nerve were cut, the portion carrying efferent fibers 

 (e^ Fig. 86), one would remain just as sensitive as ever to 

 carpet tacks or hot iron or tickling, but he could not move his 

 legs, no matter how hard he tried. And they certainly would 

 not move of themselves, for the reflex arc would be broken in 

 the part connecting the spinal cord with the muscles. 



260. Reflexes without consciousness. If the brain of a frog 

 is removed or injured, or if the spinal cord is cut near the 

 brain (a^ e^ Fig. 86), the animal will still be able to perform a 

 large number of reflex actions similar to the one described. 

 Thus, a frog with its brain destroyed will scratch with its leg 

 at a spot on the skin that has a drop of acid placed upon it. 



