282 MICROSCOPIC ANATOMY OF THE OltGAXS. 



(I)) cells whose axones pass over to the opposite side through 

 the white commissure (commissural or heteromeric cells) (Figs. 

 209 and 212). These cells are present throughout the whole 

 gray substance, and are smaller than the motor cells. Their 

 axones usually give off numerous collaterals before entering 

 the white matter, while those of the motor cells possess few col- 

 laterals. The axones of the cells of the columns usually pass 

 into the ventral and lateral columns. Here they undergo fork- 

 like divisions, one branch ascending and the other descending 

 in the cord. Other axones do not divide, but pass either up or 

 down in the cord. Still other cells divide in the gray matter, 

 one branch remaining on the same side, and the other passing 

 over in the white commissure to the opposite side (hecatero- 

 meric) (Fig. 214). 



Some of the fibres of the columns run to the brain and 

 cerebellum to form the long paths, while others have only a 

 short course and make up the short paths in the white matter. 



3. Spider cells (Binnenzellen) are cells whose much- 

 branched axones do not leave the gray substance, but end 

 by arborizations in its interior. They occur mainly in the 

 dorsal horns. 



The gray substance possesses nerve fibres as well as cells. 

 These are in part processes of the cells, and in part originate 

 elsewhere and end here, as, for example, collaterals from the 

 axones of spinal ganglion cells. Netiroglia, to be spoken of 

 later, is also abundant in the gray matter. 



The white matter consists of medulla ted nerve fibres and 

 ueuroglia. The fibres may originate from three different 

 sources, namely: from the column cells lying in the cord, from 

 the cells of the cerebral cortex (centrifugal cells), and from 

 spinal ganglion cells (centripetal). 



By reason of histological, embryological, and experimental 

 investigations we have a fairly exact knowledge of the course 

 of some of these nerve bundles. In a cross-section of the cord 

 we can map out certain fields in the white matter which contain 

 fibres having a definite course. In the ventral column (funicu- 

 lus ventralis) along the ventro-median fissure there is situated 



