THE ROOTS OF THE PERIPHERAL NERVES, ETC. 



153 



from the upper lumbar region down. Fig 1 . 91 shows cross- 

 sections made at different levels in the spinal cord. 



Aside from the proportionate differences in the gray and 

 white substances, you will notice that the most lateral portion of 

 the anterior horns in the lower cervical and upper dorsal regions 

 become more and more developed (Z^ and D 2 } until they form 

 an independent process, which is called the lateral horn or inter- 

 medio-luteral tract. In the lower dorsal region the lateral horns 

 again disappear. They are shown at o in Fig. 90. 



. 90. 



Semi-diagrammatic cross-section of spinal cord. a. anterior, b, posterior longitudinal 

 fissuivs : c, anterior column ; cl, lateral column ; e, posterior column ; /, f uniculus gra- 

 cilis ; ;/. funiculus cuneatus ; fy anterior, i, posterior roots ; ft, central canal ; I, sulcus 

 intermedius posticus; tn, anterior horn; n, posterior horn; o, intermedio-lateral tract; 

 p, processus rcticularis ; q, anterior white commissure ; r, posterior or gray commissure ; 

 s, column of Clarke, or vesicular column. (After Erb.) 



Throughout the whole cervical and the upper part of the 

 dorsal regions, in the angle between the intermedio-lateral tract 

 and the posterior horn, the gray matter is not sharply defined, 

 but projects out into the white substance in the form of a 

 net-work of gray bands and fasciculi. This net-work is called the 

 processus reticularis. 



At the terminal cone the gray matter is covered only by a 

 thin layer of white substance (Co, Fig. 91). 



