AFFERENT PATHS TO BRAIN 199 



there exists in its own special segment a reflex motor path of 

 highest sensitiveness and effectiveness; (3) the various motor 

 mechanisms in the same spinal segment are of unequal acces- 

 sibility over local afferent channels; (4) groups of motor cells 

 simultaneously discharged by reflex action enervate synergic 

 muscles (those which act in the same direction in effecting move- 

 ment), and not antergic muscles (those which antagonize each 

 other). 



For long spinal reflexes it may be stated that irradiation takes 

 place more easily down than up the cord. It is easier for irra- 

 diation to cross the cord than to pass up the cord, but not as 

 easy as it is to pass down the cord. 



In the combining of reflexes it is possible to distinguish be- 

 tween simultaneous combination and successive combination. 

 The movements are orderly and are harmonious because the 

 spread of the reflexes follows a definite sequence, determined 

 partly by the varying resistance of the synapses, and partly 

 by the anatomical relations of afferent and efferent paths. 

 The final common path, furthermore, while open to the afferent 

 arcs that elicit the movement, must be closed to other afferent 

 arcs which might disturb the reflex. In addition, there is evi- 

 dence that frequently the final common paths are more widely 

 open to the arcs in question by a reenforcing or facilitating 

 influence of distant arcs which are simultaneously excited. 

 Finally, the final common paths to antagonistic muscles must 

 be closed through inhibition. 



Afferent Paths to Brain. The afferent impulses that enter 

 the cord along the posterior roots may pass to the brain along 

 any of many paths: 



1. They may pass directly up through the posterior median 

 column. Their course will then be interrupted by synapses 

 in the nucleus gracilis and the nucleus cuneatus. The path 

 is then continued across the middle line by the arcuate fibers 

 of the sensory decussation and thus along the fillet, through the 

 hinder part of the posterior limb of the internal capsule to the 

 cerebral cortex. This path may be interrupted by cells in the 

 thalamus. 



2. They may pass to the cells of Clarke's column. From 

 thence they pass up the direct cerebellar tract and through 

 the restiform body to terminate in the posterior and median 



