REFLEX ACTION 



587 



conduct the impulses up or down in the spinal cord. This arrange- 

 ment is clearly indicated in Figs. 291, 292 

 and 293. The first two illustrate the conduc- 

 tion paths required for a simple reflex in which 

 a single posterior extremity is involved. In 



FIG. 291. FIG. 292. 



FIG. 291. SCHEMA TO ILLUSTRATE SIMPLE REFLEX CONDUCTION IN THE SPINAL CORD. 

 A, The sensory impulse is immediately transferred to the motor path E. 

 FIG. 292. SCHEMA TO SHOW SIMPLE REFLEX CONDUCTION IN THE SPINAL CORD. 

 A, The sensory impulse is transferred in the anterior horn to the motor neuron E. 



this case, the impulse arriving by way of the 

 mediately transferred to the motor neuron in 

 the anterior horn of the gray matter, and 

 from here to the corresponding effector. 

 Figure 293 shows how intermediary neurons 

 enable the impulse to attain higher or lower 

 levels of the spinal cord, where connections 

 are formed with the motor cells and effectors 

 situated at a more remote distance from the 

 primary circuit. The spreading of a reflex to 

 adjoining arcs may be demonstrated in various 

 ways. If the leg of a frog to which a stimulus 

 has been applied, is firmly held in place so 

 that the motor effects cannot fully develop 

 on this side, the primary action eventually 

 spreads to the muscles of the opposite limb as 

 well as those of the trunk and forelimbs. It 

 is also possible to elicit this phenomenon by 

 stimulating the central end of the divided 

 sciatic nerve of one side. As this section ren- 

 ders the muscles of the same side functionally 

 useless, the afferent impulses generated at the 

 seat of the stimulation, find their way into 

 the motor paths of the opposite leg as well 

 as into those of the trunk and forelimbs. 



In general, it may be said that reflexes may 



sensory neuron, is im- 



FIG. 293. SCHEMA TO 

 ILLUSTRATE REFLEX 

 SPREADING IN THE SPINAL 

 CORD. 



A, The sensory impulse 

 is transferred to an inter- 

 mediary neuron j which 

 conveys it to higher and 

 lower motor paths E. 



be made to spread (a) 



