60 



PSYCHOBIOLOGY 



approximately opposite sides : in the process of development the axon and 

 the dendrite move together, and fuse for a short distance of their length, 

 giving rise to a T-shaped process [Fig. 44] ; hence these cells are some- 

 times called T-cells, and (incorrectly) 'unipolar' cells. (There are true 

 unipolar cells found elsewhere.) 



The axon of the cell is sent into the spinal cord through the posterior 

 root. Each posterior root, on entering the cord, divides into two bundles.. 

 The smaller bundle passes to the outer side of the tip of the posterior 

 horn (' Lissauer's' tract) where each fiber bifurcates, one branch running 

 up the cord and the other down. These branches run only a short dis- 

 tance, sending off lateral ' branches which penetrate the gray matter and 

 arborize around cells there. 



Fig. 45. Free nerve endings in the epithelial lining of the esophagus of a rabbit. 

 Highly magnified. (Barker, Nervous System, after Retzius.) 



The larger bundle of the posterior root fibers passes to the inner side of 

 the horn and enters the posterior column, the fibers bifurcating, and 

 one branch passes up and the other down as described for the fibers of the 

 smaller bundle. The ascending branches of some of these fibers run up 

 to the medulla. The ascending branches of the other fibers run up a short 

 distance and then into ' Clark's column ' at the bases of the horns, arbor- 

 izing there around cell-bodies whose axons run up to the cerebellum. Cer- 

 tain of the posterior root fibers arborize around cells in the anterior horn, 

 i. e.. motor cells. 



