Georo-e L. Streeter 



3 



as belonging to the dura, and as forming its outer layer. The cleft be- 

 tween the two, the epidural cavity, is bridged over by loose strands of 

 tissue supporting a plexus of blood-vessels. 



More or less adherent to the inner surface of the dura is the arach- 

 noidea. Whether or not this, in the fresh state, is completely adherent 

 to and possibly a part of the dura, could not be decided, as all the mate- 

 rial used in this study had been through a prolonged hardening in formol. 

 In the preparations at irregular intervals, they were still adherent, but in 

 the greater part there was a separation of the two membranes, having 

 more the appearance of an artificial tearing apart, or slirinkage forma- 



Arachnoidea 



Epidural plexus 



Epidural space 



'achnoid space 



Mucl. marg' minor 

 Dorsal rool 

 Venlral rool 



Fig. 1. Cross-section through the 4th cervical vertebra of the ostrich, show- 

 ing the spinal cord and its membranes. One side is drawn at a point some- 

 what higher than the other. Enlargement x 6. 



tion, than a natural cleft. In the space thus formed, there was no 

 trace of serum, blood-cells, or other tissue. In cross-sections the arach- 

 noidea shows itself as a delicate, thickly nucleated membrane, connected 

 from its inner surface with the pia by a network of fine strands which 

 form a meshwork of lymph spaces for the cerebro-spinal fluid, siibaracli- 

 noideal cavity. 



The rootlets forming the ventral and dorsal nerve roots take their 

 course through this loose tissue caudad or cephalad to the nearest inter- 

 vertebral foramen, where they pass outward, piercing the dural sheath. 

 In their course they carry along with them a connective tissue contri- 



