1040 



HUMAN ANATOMY. 



FIG. 894. 



sensory neurones lies outside the spinal cord ; it is, however, with the intramedullary 

 portion of these neurones, as constituents of paths within the cord, that we are here 

 concerned. 



On entering the spinal cord along the postero-lateral groove, the dorsal root- 

 fibres for the most part penetrate the tract of Burdach, close to the inner side of the 

 posterior horn. Some of the more external root-fibres, however, do not enter Bur- 

 dach' s tract, but form a small adjoining field, the tract of Lissauer, that lies im- 

 mediately dorsal to the apex of the posterior horn. Soon after gaining the posterior 

 column, with few exceptions, each dorsal root-fibre undergoes a >- or | like divi- 

 sii >n into an ascending and a descending limb, which assume a longitudinal course 

 and pass upward and downward in the cord for a variable distance, the descending 

 limb being usually the shorter. During their course from both, but particularly from 

 the descending limb and from the proximal part of the ascending fibre, collateral 



branches are given off which bend sharply 

 inward and pass horizontally into the gray 

 matter to end chiefly in relation with the 

 neurones of the posterior horn, from \vhich 

 cells secondary paths arise. Not only the 

 collaterals, but also the main stem-fibres of 

 the descending and shorter ascending limbs 

 end in the manner just described. In addi- 

 tion to the short collaterals destined for the 

 cells of the dorsal horn, others, the ventral 

 reflex collaterals, pursue a sigmoid course, 

 traversing the substantia gelatinosa Rolandi 

 and the remaining parts of the posterior 

 horn and the intermediate gray matter, to 

 end in arborizations around the radicular 

 cells of the anterior horn, and thus complete 

 important reflex arcs, by which impulses 

 transmitted through the dorsal roots directly 

 impress the motor neurones. The latter are 

 usually of the same side, but some collaterals 

 cross by way of the anterior commissure to 

 terminate in relation with the anterior horn 

 cells of the opposite side. It is probable 

 that a considerable number of such anterior 

 horn reflex collaterals are given off from the 



fibres that ascend in the long tracts of the posterior column to the medulla oblongata. 

 With possibly the exception of certain fibres which pass directly to the cerebellum 

 (Hoche), all the sensory root-fibres (axones of neurones of the I order) end around 

 the neurones situated either within the gray matter of the spinal cord or within the 

 nuclei of the medulla ; thence the impressions are conveyed by the axones of these 

 neurones of the II order to higher centers, to be taken up, in turn, by neurones of 

 th- IIIor even higher order, in the sequence of the chain required to complete the 

 path for the conduction and distribution of the impulse. 



Tin- most important groups of the collaterals and stem-fibres of the posterior 

 roots are: 



1. The long ascending tracts passing chiefly to the nuclei of the medulla. 



2. The fibres passing t<> the cells of tin- column of Clarke. 



3. Tin- collaterals passing to the anterior horn cells. 



4. The fit ncs entering the posterior horn from the tract of Burdach and of 

 ier to ,-nd a!..,m the neurones of the II order situated within the gray matter 



of the posterior horn and the intermediate gray matter. 



'Hie direct ascending posterior tract includes the dorsal root-fibres that 



Uninterruptedly upward within the posterior column as far as the nuclei of the 



medulla. On entering the cord they lie at first within the tract of Burdach, but in 



their ascent are gradually displaced medianlv and dorsally by the continued addition 



of other root -fibres from the succeeding higher nerves. In consequence, in cross 



Diagram showing division of posterior root-fibres 

 Into ascending and descending branches: long fibre 

 sends collateral! to anu-rior root cells; other fibres 

 end at different levels around cells in gray matter of 

 posterior horn ; S. G., spinal ganglion. 



