THE SPINAL CORD 



in Fig. 3, page 24, representing the reflex action 

 of the cord, but are also sent upward to the brain 

 by three separate tracts ; namely, the posterior col- 

 umns of the side on which they enter, the direct 

 cerebellar column, and the antero-lateral tract of 

 Cowers on the opposite side. 



The diagram also exhibits the course of vol- 

 untary impulses from the brain downward to the 

 motor neurons of the cord. These impulses, as 

 already described, descend both in the pyramidal 

 tract (P) and in the anterior median column (v). 

 From the descending fibres of these columns fine 

 collaterals come off, which enter the anterior horns 

 of the cord, and terminate in fine terminal brushes 

 about the motor nerve cells. For the sake of clear- 

 ness, these cells (m, n, o) are shown on one side 

 only. These collaterals are seen to come off at all 

 levels of the cord. The voluntary impulses descend- 

 ing in these columns and thus reaching the anterior 

 motor cells excite them to activity, and the result 

 is a voluntary contraction of the muscle to which 

 their neuraxons pass. 



The diagram also shows the existence of in- 

 trinsic cells in the cord (/, e\ with their neuraxons 

 passing in the antero-lateral column (y, z), and 

 ascending and descending, giving off collaterals which F I G . 4 . 



, , i r . i , , FIG. d. Diagram to illustrate the transmission of sensory, 



pass into the anterior gray matter of other levels 



motor, and association impulses through the spinal cord. 



A, B, C, are three levels of the cord. /, //, ///, are sensory nerve roots entering the posterior root zone at level B, bifurcating, ascending 

 and descending, and sending collaterals into the gray matter at many levels. Three such collaterals are shown at A, B, C, ending in the posterior 

 horn, and opposite median gray. The collaterals shown in diagrams I and 3 are not shown here. The collateral which crosses to the opposite side 

 terminates about the cell /, which sends its neuraxon into the column of Cowers, G, and thence upward to the medulla. At level C a collateral 

 ends about sensory cell A, which sends its neuraxon also into G. The collateral entering the base of the posterior horn terminates about the 

 column of Clarke on the same side. For the sake of distinctness the cell, </, of the column of Clarke is shown on the opposite side with its fibre 

 entering the direct cerebellar column, D, and ascending to the cerebellum, jr, descending cerebellar tract sending its collateral fibres into the median 

 gray, e, intrinsic cell of the anterior horn at level C, sending its neuraxon into the antero-lateral column, z, where it ascends, giving off col- 

 laterals, 2, at levels B and A, and proceeding upward to the medulla. i, intrinsic cell at level A, sending its neuraxon into the antero-lateral 

 column, y, where it ascends and descends, sending its collaterals, y, y, into the anterior horns of levels B, C. P, pyramidal tract giving off collaterals 

 which pass to the anterior horn of the cord and terminate in brushes about the motor cells. The motor cells, /, , o, are shown for the sake of 

 distinctness on the opposite side, v, anterior median column with collateral entering the anterior horn and terminating in brushes around the 

 motor cells. 



