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HUMAN PHYSIOLOGY 



effect of the reflex in the end-organ of the centrifugal nerve, 

 we distinguish between reflex movement, reflex secretion, 

 and reflex inhibition. The reflexes of the spinal cord are 

 chiefly reflex movements. 



The reflex tracts. The connections of centripetal nerves 

 with motor nerves which are necessary for the production of 

 reflex movements may be : 



(1) Direct. The end-tufts of the centripetal fibres and 

 the collaterals are in direct contact with the motor cells. 



(2) Indirect. Between the centripetal and the motor 

 neurons other neurons intervene. 



The direct and indirect tracts are illustrated in Fig. 20. In this 

 figure a represents the motor cells and roots ; b is a spinal ganglion 

 with its root. The sensory collateral r joins the motor cells directly, 

 forming a direct reflex tract. The sensory collateral c first joins the 



cell d whose axis-cylinders make 

 connections with the motor cells 

 through the collaterals e, e, e. 

 This constitutes an indirect re- 

 flex tract. 



The direct reflex tract dif- 

 fers, therefore, from the in- 

 direct in that but two kinds 

 of cells are intercalated in its 

 course, namely, the cells of 

 the spinal ganglion and the 

 motor cells of the anterior 

 horn. In the indirect tract 

 \^ one or more cells are placed 

 between these two kinds of 

 cells. 



It is apparent that the 

 union of centripetal with 

 motor nerves may take place 



in a great many ways, as is also required by the manifold 

 spreading of the reflex. The fibres of these reflex tracts, by 

 which various heights of the gray matter are connected with 



FIG. 20. 



