THE SPINAL CORD 89 



others, more remote, segments. Their collateral and terminal branches re- 

 enter and ramify within the gray substance. Those which remain throughout 

 in the same lateral half of the cord are called association fibers; while others, 

 known as commissural fibers, cross the median plane chiefly in the white com- 

 missure (Fig. 68). Some of the commissural fibers are short and confined to a 

 single level of the cord (Fig. 66). 



Cell-columns. The nerve-cells are not uniformly distributed throughout 

 the gray matter, for many of them are arranged in longitudinal cell-columns. 

 In transverse sections each of these columns appears as a distinct group of 

 cells, somewhat separated from other similar groups within the gray matter 

 (Fig. 65). The large motor cells of the anterior column, which give origin to 

 the ventral root fibers, form several subgroups. One of these, known as the 

 anteromedian cell-column, occupies the medial part of the anterior column through- 

 out almost its entire length, being absent only in the fifth lumbar and first 

 sacral segments. Behind it is the posteromedian cell-column, which is, however, 

 present only in the thoracic and first lumbar segments and for a short stretch 

 in the cervical region. The axons from these two medial groups of cells prob- 

 ably supply the musculature of the trunk. In the cervical and lumbar enlarge- 

 ments there are laterally placed groups of cells the axons of which supply the 

 muscles of the limbs. These are: (1) the anterolateral cell-column, present in 

 the fourth to the eighth cervical and in the second lumbar to the second 

 sacral segments; (2) the posterolateral cell-column in the last five cervical, 

 last four lumbar, and first three sacral segments; (3) the retro posterolateral 

 cell-column in the eighth cervical, first thoracic, and first three sacral seg- 

 ments, and (4) the central cell-column in the second lumbar to the second sacral 

 segments. 



The intermediolateral cell-column is found in the lateral column in the tho- 

 racic region of the cord and is prolonged downward into the upper lumbar seg- 

 ments. It is composed of small cells, the axons of which run through the ven- 

 tral roots, spinal nerves, and white rami communicantes into the sympathetic 

 nervous system (Fig. 37). They have to do with the innervation of smooth 

 and cardiac muscle and glandular tissue. The longitudinal extent of this 

 column corresponds quite accurately to that of the spinal origin of the white 

 rami. A group of cells, having a similar function, is also found in the third 

 and fourth sacral segments. 



The cells of the posterior gray column are smaller, as a rule, than those of the 

 ventral column: and except for the nucleus dorsalis they are not arranged in 



