THE NERVOUS SYSTEM. 



569 



connect with groups of intrinsic cells in the median or posterior portion 

 of the gray matter of the cord. 



(b) The posterior roots enter the spinal cord to the inner or me- 

 dian side of the posterior cornu. The fibres, as soon as they reach the 

 cord, divide in a fork-like fashion, one branch passing down a short dis- 

 tance (only about three centimetres), the other branch passing up for a 

 longer or shorter distance. This upper branch sometimes reaches nearly 

 the whole extent of the cord, but generally it extends over only one or 

 two segments of the cord. These divisions of the posterior root fibres 

 give off in' their course numerous collaterals. The nerve-fibres of the 

 posterior roots are divided into two sets, an internal or median, an ex- 



Fig. 353. Section of the spinal cord showing the grouping 

 fibres entering in posterior and 



rof nerve-cells and the course of nerve- 

 anterior roots. 



ternal or lateral. The lateral set consists mostly of small fibres, and it 

 enters the cord opposite the tip of the posterior horn. The fibres pass 

 in part to the marginal column of Lissauer, where they ascend and de- 

 scend ; in part they penetrate the posterior horn, and come in relation 

 with its cells. The median set sends some fibres which pass to Clarke's 

 column of cells, others pass by way of the posterior commissure to the 

 median cells of the other side. Some others pass- through the median 

 gray matter to the anterior horn cells of the same side. Thus the pos- 

 terior root-fibres are connected with all the cell groups of the posterior 

 horn, of the anterior horn of the same side, and the cells of the median 

 gray of the opposite side. Besides this, they are connected through col- 



