THE SPINE. 273 



the internal periosteum of the vertebral canal. Be- 

 tween the dura and the vertebrae is a space called 

 the epidural space, which is filled with areolar tissue, 

 fat, and a plexus of spinal veins. 



2. The Pia. This is a thin connective-tissue 

 layer that lies close to the surface of the cord, dips 

 into the anterior fissure, and also sends fibers or 

 trabeculae into the cord substance. Many small 

 blood-vessels accompany this layer. 



3. The Arachnoid. This is a delicate membrane 

 between the other two, but much nearer the dura. 

 Its external surface is clothed with a single layer 

 of flat epithelial cells which secrete a serous fluid. 

 The arachnoid is therefore a serous membrane. 



Fissures. i. Posterior Median Fissure. This is a 

 median dorsal fissure that extends the whole length 

 of the cord. It is extremely narrow but deep, as it 

 'penetrates to the central gray matter, being inti- 

 mately connected with the two sides in its course. 

 The single septum is derived from the neuroglia tis- 

 sue, and not from the pia, which sends no prolonga- 

 tion of any kind into it. 



2. Anterior Median Fissure. This also extends 

 the whole length of the cord. It is shallower but 

 wider than the posterior fissure and does not quite 

 reach the central gray matter. The pia forms a 

 fold into this fissure with which is associated many 

 blood-vessels. The two median fissures or clefts 

 divide the cord into a right and a left half, each 

 practically identical with the other. 



3. Poster o-lateral Groove. This is a shallow de- 

 pression on each side of the posterior fissure and 



