848 



THE NER VE SYSTEM 



The chief peculiarities of the dura of the cord, as compared with that investing 

 the brain, are the following: The dura of the cord is not closely adherent to the 

 bones of the vertebral canal, and is not, as is the cranial dura, the internal peri- 

 osteum of the vertebrae. The vertebra? have an independent periosteum. It does 



F1LUM 

 TERMINALS 



COCCYGEAL 

 LIGAMENT 



FIG. 617. The filum terminate (schematic). (Poirier and Charpy.) 



not send partitions into the fissures of the cord, as the cranial dura sends partitions 

 into certain fissures of the brain. Its fibrous lamina? do not separate to form venous 

 sinuses, as in the cranium. It contains no arachnoid villi (Pacchionian- 6orf />.>). 



Structure. The dura consists of white fibrous and elastic tissue arranged in bands or 

 lamellae, which, for the most part, are parallel with one another and have a longitudinal arrange- 

 ment. Each surface is covered by a layer of endothelial cells. It is sparingly supplied with 

 vessels, and some few nerves have been traced into it. 



The Arachnoid (Arachnoidea Spinalis) (Figs. 596, 615). 



The arachnoid is exposed by slitting the dura and reflecting that membrane 

 to either side. It is a thin, delicate, tubular membrane which invests the surface 

 of the cord, and is connected to the pia by slender filaments of connective tissue. 

 Above, it is continuous with the cranial arachnoid; on each side it is continued 

 on the various nerves, so as to form a sheath for them as they pass outward to 

 the intervertebral foramina. The outer surface of the arachnoid is in contact 

 with the inner surface of the dura, and the two are, here and there, joined together 

 by isolated connective-tissue trabeculre. These trabeculse are especially numerous 

 on the dorsal surface of the cord. For the most part, however, these membranes 



