SPINAL CORD. 117 



trally there is a narrow layer of white substance extending from one side 

 of the cord to the other; this is the ventral white commissure. 



In the adult cord (Fig. 140) the central canal is usually reduced to 

 a cavity 0.5 to i.o mm. broad; sometimes it is obliterated. The canal is 

 surrounded by the ependyma which appears as a single layer of neuroglia 

 cells. Around the ependyma is the central gray substance, containing 

 special neuroglia cells to be described later. In addition to the ventral 

 gray commissure of the younger stage, there is now a dorsal commissure, 

 by which the vertical portions of the gray H are united dorsal to the central 



FIG. 141. NEUROGLIA CELLS AND FIBERS FROM THE SPINAL CORD OF AX ELEPHANT. 



(Hardesty, from Ferguson's Histology.) 

 The letters indicate the neuroglia cells. L, a leucocyte. Benda's stain. X 940. 



canal. Besides the dorsal and ventral columns, a lateral column may now 

 be recognized as a bulging of the ventral column on a line with the central 

 canal. Lateral columns are most evident in the upper thoracic part of the 

 cord. On the lateral side of the dorsal column there is a network of 

 strands of gray substance called the reticular formation (formatio retic- 

 ularis). Near the dorsal commissure in the dorsal column there is an 

 important group of nerve cell bodies named the dorsal nucleus [column of 

 Clark]. ('Nucleus' is a term applied to many such groups of cell bodies 

 in the brain.) The dorsal nucleus extends through the thoracic cord 

 and is well denned in the anterior lumbar portion; it is wholly absent 



