REINF0RCE3IENT 279 



Reinforcement and Combination of Reflexes. — When the 



scratch reflex is induced from two points situated close 

 together on the skin the motor efiect is more intense than 

 if either stimulus acted singly. The two stimuli sum in 

 their efiect upon the final common path. 



Antagonistic Reflexes — Interference.^ — Some reflexes are 

 incompatible — the scratch and the flexion reflex, for 

 example. If the flexion reflex is induced by a strong 

 stimulus while the scratch reflex is in progress, the latter 

 may be inhibited, the former taking its place. This is 

 known as interference. Whether or no interference occurs, 

 depends upon the relative strength of the stimuli causing 

 the two reflexes. The cessation of one reflex and its replace- 

 ment by another always occurs without delay and without 

 confusion. One begins immediately the other stops. 

 There is no intermediate period during which a composite, 

 purposeless reflex occurs. 



The Functions of the Cord 



The grey matter of the cord forms the lowest member 

 of the hierarchy of the central nervous system. Each 

 segment governs the nervous reactions performed by that 

 segment ; in addition, the anterior horn cells govern the 

 nutrition of the muscles which they supply. The segments 

 of the cord are bound together functionally by tracts. 

 On this account no reflex is confined to any one segment 

 of the body. When an anterior root-fibre is stimulated 

 the resulting movement is purposeless and inco-ordinate. 

 The motor impulses which form a co-ordinated movement 

 emerge by several roots. In this way certain segments of the 

 cord are bound closely together — those for the upper limb 

 form one group, those for the lower hmb another. These 

 sections of the cord have control over certain complex 

 acts — not only skeletal movements but visceral functions — ■ 

 micturition, defaecation and parturition, all of which can, 

 at any rate in lower animals, be performed when the cord 

 is severed from the higher centres. 



