REFLEX ACTION 273 



3. A conductor mechanism, composed of the afferent 

 neurone, the motor or efferent neurone, any neurone or 

 neurones which connect them centrally, and the inter- 

 neuronic synapses. 



From every segment of the cord there emerges on each 

 side two nerve-roots, which soon unite to form a spinal 

 root. Of these roots, one, the posterior, normally conveys 

 impulses towards the cord, the other, the anterior, away 

 from it (Bell's law). 



Posterior root fibres, when they enter the cord, ramify 

 and connect with other cells as follows (Fig. 45, p. 286) ; — 



1. They arborise around posterior horn cells as soon as 

 they enter. 



2. They pass to the opposite side of the cord. 



3. They arborise around anterior horn cells at the same 

 level. 



4. They arborise around cells of Clarke's column. 



5. They form a tract running up and down the cord 

 for a short distance and terminating in the substantia 

 gelatinosa, a mass of grey matter which caps the posterior 

 horns. 



6. They enter the white matter to form the posterior 

 columns. Here each fibre divides into an ascending and 

 descending branch. The latter group pass a short distance 

 down the cord and end by arborising around posterior 

 horn-cells. The ascending branches pass upwards, ter- 

 minating at various levels, the largest of them reaching 

 the medulla, where they arborise around cells of the nucleus 

 gracilis and nucleus cuneatus. 



The anterior root fibres, with the exception of those 

 destined to supply the visceral system, all arise from 

 nerve-cells in the anterior horns. 



It will thus be seen that the path of conduction from 

 the receptor to the effector organ must involve at least 

 two neurones, with the synpase between them. In point 

 of fact, in most reflexes more than two are involved, since 

 one or more neurones are intercalated between the posterior 

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