THE NERVOUS SYSTEM 1319 



and pointed. In the thoracic region both comua are narrow, 

 though the posterior is more so than the anterior. In the lumbar 

 region both comua are broad, though the anterior is more so 

 than the posterior. These differences render sections of the spinal 

 cord in the cervical, thoracic, and lumbar regions easily recog- 

 nisable. As stated, the lateral comu is also a characteristic ol 

 the cord in the thoracic region, more particularly in its upper 

 part. 



Central Canal. — ^This minute canal is situated about the centre 

 of the grey commissure, and extends throughout the entire length 

 of the spinal cord. Superiorly it is continued into the lower half 

 of the medulla oblongata, and it opens into the lower part of the 

 fourth ventricle at the calamus scriptorius. Interiorly, near the 

 apex of the conus medullaris, it becomes enlarged, and assumes 

 the shape of an inverted T, thus ±. This enlargement is known as 

 the ventrlculus terminalis of Krause. From this point it is prolonged 

 for some distance into the filum terminale, and it ends in a closed 

 extremity. In the cervical and thoracic regions the central canal 



Fig. 552. — Transverse Sections of the Spinal Cord in Different 



Regions. 



A, Cervical Region ; B, Mid-thoracic Region ; C, Lumbar Region ; 

 D, Conus Medullaris. 



is nearer the anterior surface of the cord than the posterior, but 

 in the lumbar region it occupies the centre. In the conus medullaris 

 it is near the posterior surface. 



The canal is lined with ciliated columnar epithelium, the columnar 

 cells being known as epend^Tnal cells. 



The central canal represents the lumen of the neural tube of epiblast from 

 which the spinal cord is developed. 



White Matter of the Spinal Cord. — ^The white matter forms the 

 outer part of the cord, and is arranged in three columns, anterior, 

 lateral, and posterior. The anterior column is situated between 

 the anterior median fissure and the anterior comu of grey matter, 

 and extends as far as the most external fasciculi of the anterior 

 nerve-roots. It therefore includes the superficial coating of the 

 anterior comu, where it is traversed, over an area of some breadth, 

 by the scattered fasciculi of the anterior ner\^e-roots. The lateral 

 column is situated between the anterior and posterior comua of 

 grey matter, in the concavity of the crescent. Its superficial limits 

 are the most extemal fasciculi of the anterior nerve-roots and the 

 fasciculi of the posterior nerve-roots at the postero-lateral sulcus. 



