THE SPINAL CORD. 337 



Sixth Ventricle (Fig. 101, B). The central canal of the spinal 

 cord (canalis centralis spinalis) is the representative of the cavity 

 of the neural tube. It is just visible to the naked eye; but it 

 extends throughout the cord and expands above into the fourth 

 ventricle. In the filum terminale internum it is also dilated, 

 forming the ventriculus terminalis (Krausei). It is lined with 

 columnar ciliated cells which stand on a thick lamina of substantia 

 gelatinosa. 



Fissures of the Spinal Cord (Fig. 101). The spinal cord 

 is incompletely divided into symmetrical lateral halves by the 

 anterior and the posterior median fissure. 



The anterior median fissure (fissura mediana anterior) is 

 the broader and shallower of the two (Fig. 101). It extends in 

 length from the inferior end of the ventral surface of the pons 

 (foramen caecum of Vicq d'Azyr) down the anterior median line 

 of the medulla and cord. As to depth, it equals one-third of the 

 cord's axis. Its floor is formed by the white anterior commissure. 

 Both layers of pia mater dip down into it and inclose the anterior 

 spinal artery and its branches. The anterior median fissure is 

 interrupted at the junction of the cord and medulla by the decus- 

 sation of the pyramids. In the lumbar enlargement it gradually 

 disappears. 



The posterior median fissure (fissura mediana posterior) 

 is narrow and deep (Fig. 101). It extends, longitudinally, down 

 the posterior median line of the cord from the middle of the 

 posterior surface of the medulla. It divides the cord, dorso- 

 ventrally, beyond its middle. The floor of the fissure is formed 

 by the posterior commissure, which, with the gray and white 

 anterior commissures, separates the posterior from the anterior 

 median fissure. The posterior median fissure is not an open 

 fissure; it is occupied by a lamina of connective tissue, the posterior 

 septum, which is attached to the deep layer of the pia mater. In 

 the posterior septum ramify branches of the two posterior spinal 

 arteries and tributaries of the external spinal veins. 



PosteriorLateral Sulcus. (5. lateralis posterior). Each lateral 

 half of the spinal cord is partially divided, near the junction of 

 the posterior fourth with the anterior three-fourths of its semi- 



