THE SPINAL DURA 



847 



The Spinal Dura (Dura Spinalis) (Figs. 615, 617). 



The spinal dura represents only the menipgeal or supporting layer of the cranial 

 dura. The endocranial or endosteal layer ceases at the foramen magnum dor- 

 sally, but reaches as low as the third cervical vertebra ventrad; below these 

 levels its place is taken by the periosteum. The dura forms a loose sheath which 

 surrounds the cord and the cauda equina, and is loosely connected with the 

 vertebral periosteum and the ligaments by a quantity of lax areolar tissue and a 

 plexus of veins, the meningorachidian veins (plexus venosi vertebrates interni). The 

 space containing the fat and veins is called the epidural space (eavum epidurale). 

 The situation of the veins between the dura and the periosteum of the vertebra 

 corresponds therefore to that of the cranial 

 sinuses between the endocranial and sup- 

 porting layers. The dura is attached to 

 the circumference of the foramen mag- ^Jfil/ 



mini and to the axis and third cervical 

 vertebra ; it is also fixed to the posterior 



ANTERIOR 

 NERVE ROOt 



Fn;. 015. The spinal cord and its membranes. 



FIG. 616. The dentate ligament. The dura 

 h:is been opened and turned back. The ventral 

 surface is seen. (Hirschfeld.) 



common ligament, especially near the lower end of the spinal canal, by fibrous 

 slips; it extends below as far as the second or third piece of the sacrum, where 

 its cavity terminates, and, ensheathing the filum terminate, constitutes the filum 

 durae spinalis (Fig. 598), and descends to the dorsum of the coccyx, to blend with 

 the periosteum. This part of the dura is called the coccygeal ligament (Fig. 617). 

 The dura is much more capacious than is necessary for its contents, and its size 

 is greater in the cervical and lumbar regions than in the thoracic. Its inner 

 surface is smooth. On each side may be seen the double openings which transmit 

 the two roots of the corresponding spinal nerve, the fibrous layer of the dura 

 being continued in the form of a tubular prolongation on them as they pass through 

 these apertures. These prolongations of the dura are short in the upper part of 

 the vertebral column, but gradually become longer below, forming a number of 

 tubes of fibrous membrane, which enclose the sacral nerves, and are contained 

 in the vertebral canal. 



