944 CENTRAL NERVOUS SYSTEM 



contraction of the active muscles, and the resistance of the synapses lying 

 in the path to the antagonistic muscle groups must be equal. 



Rules for the Spread of Spinal Reflexes 



Sherrington 37 has laid down the following rules for the spread of 

 short spinal reflexes, as the components of a reflex figure which in- 

 volve only a few spinal segments may be designated: 



1. The degree of reflex spinal intimacy between afferent and efferent 

 spinal roots varies directly as their segmental proximity. 



2. For each afferent root there exists in immediate proximity to its 

 own place of entrance in the cord, e. g., in its own segment a reflex 

 motor path of as low a threshold and of as high potency as any open 

 to it anywhere. 



3. Motor mechanisms lying in the same region of the cord are un- 

 equally accessible to the local afferent channels, if judged by pressor ef- 

 fects, i. e., the production of contraction. This rule breaks down, how- 

 ever, when it is considered that an afferent path may produce inhibition 

 in many of the motor mechanisms to which it has access. 



4. The groups of motor nerve cells contemporaneously discharged by 

 spinal reflex action innervate synergic and not anergic muscles. As 

 a consequence the muscular contractions are harmonious with one another 

 (Reciprocal Inhibition). 



5. The spinal reflex movement elicitable in and from any one spinal 

 region will exhibit much uniformity despite considerable variety of 

 the locus of incidence of the exciting stimulus. 



The spread of long spinal reflexes, which are parts of a reflex figure 

 involving many segments of the cord, is too inconstant in a single re- 

 flex figure and varies too greatly in different figures to permit any defi- 

 nite rules to be laid down. 



