STHUCTUEE OF THE SPINAL COED. 



69 



elements, which lie so close to the origin of the motor fibers, -would be well 

 adapted to bring about directly many short reflexes (see Fig. 34). 



Not all of the cells in the spinal cord nor all of the fibers stand in 

 direct relation to nerve-roots. There are very many cells which send their 

 neurite, or axis-cylinder, out of the gray matter, either on the same or 

 opposite side. Usually the neurite divides then into an ascending and a 

 descending branch. Then, after a longer or shorter course, again, bending 



Fig. 34. — ^From the spinal eord of a newborn mouse. (After Lenhossek.) 



back into the gray matter, both branches end. On the way the main 

 branches have sent numerous collaterals into the gray. Such cells are 

 adapted to connect among themselves different levels of the spinal cord. 

 Such neurons are called cellulce commissurales. Many neurites from such 

 commissural cells cross over ventrally to the opposite side, forming a decus- 

 sation quite ventral to the gray: the decussatio ventralis of the spinal cord 

 (Fig. 33, Dec. vent.). At just the same place there are still other decussating 



