298 ANIMAL BIOLOGY 



The r brain of the animal has nothing to do with this act, as 

 it no longer has any connection with the parts involved 

 in the reaction. The impulse set up in certain sense cells 

 of the foot travels toward the cord, passing through the 

 dorsal root into the gray matter. Thence it passes out of 

 the cord along the ventral or motor root to the muscles 

 of the leg causing them to contract, thus drawing the leg 



FIG. 212. Diagram of a cross section of a spinal cord showing paths 

 taken by nerve impulses in a simple reflex act. A, afferent or sensory 

 nerve fiber entering the dorsal side of the cord; DG, dorsal ganglion; DR, 

 dorsal root; G, gray matter of cord; M, muscle; SO, sense organ; V, branch 

 of ventral or motor root of spinal nerve; VR, ventral root; W, white matter 

 of cord. 



away. If a drop of acid is put on the side of the same frog 

 the hind foot on that side is brought forward to wipe away 

 the irritating substance. These are but a few of the 

 reflex acts which may be performed by the spinal cord. 

 Such acts, although involuntary, are adaptive in that they 

 are directed so as to perform some useful function. They 

 are carried out in ourselves much as in the frog. If the sole 

 of the foot is tickled the foot is quickly withdrawn even 

 before we are aware of our action. We can check or pre- 

 vent many of our spinal reflexes by means of impulses sent 



