446 



TEXT-BOOK OF PHYSIOLOGY. 



an anterior root-fiber. These nerve-cells, with their dendrites, 

 axons, and terminal branches, form efferent neurons of the first 

 order.' The intimate histologic and physiologic relationship existing 

 between the nerve-cell and the axon is revealed by the degenerative 

 changes which arise in the latter when separated from the former. 

 TtigjceU- apparently determines the nutrition of the axon and may 

 be" regarded as trophic in function^Some of the cells of the ante- 

 rior horn send their axons into the white matter of the same side, 



PR 



FIG. 205. DIAGRAM ILLUSTRATING THE CHIEF CELLULAR ELEMENTS OF THE SPINAL 

 CORD, AND THE PROBABLE RELATIONS BETWEEN THE CELLS AND THE FIBERS 

 AND THE PRINCIPAL TRACTS; THE LEFT HALF OF THE FIGURE EXHIBITS THE 

 COMMUNICATIONS OF THE SEVERAL VARIETIES OF NERVE-CELLS. A, P. Ventral 

 or anterior and dorsal or posterior horns. PR. Posterior root bundles. DP. 

 Direct pyramidal tract. CP. Crossed pyramidal tract. DC. Direct cerebellar 

 tract. GB. Gowers's tract, a. Motor cells passing directly into fibers of ventral 

 roots, b. Various cells of the antero-lateral column. Some give off collateral 

 branches of remarkable size. c. Commissural (heteromeral) cells, d. Cells to 

 dorsal column (tautomeral). e. Golgi cells of dorsal horn. The right half of the 

 diagram shows the communications established by means of the collateral fibers. 

 (Piersol, after Lenhossek.) 



after which they divide into two branches, one passing up, the other 

 down, the cord, to re-enter the gray matter at different levels. They 

 are probably associative in function. Other cells send their axons 

 into that portion of the white matter on the same and opposite sides 

 known as Gowers's antero-lateral tract. (Fig. 205.) 



In the posterior horn nerve-cells are also present, though they 

 are not so numerous as in the anterior horn. At the base of the horn 



