196 



COMPARATIVE ANATOMY 



appearance of a lymph-space in the primitive meningeal membrane, 

 dividing it into an outer dura mater spinalis and an inner primitive 

 pia mater. There is thus &peridural or epidural space external to 

 the dura, and a suldural space between it and the primitive pia 

 (B). 



Pi-!r,uttve pin mater 

 f 



FIG. 146. THE INVESTING MEMBRANES OF THE SPINAL CORD IN THE CHIEF 

 VERTEBRATE GROUPS. A, Fishes ; B, Amphibia and Sauropsida ; C, Mammalia. 

 Semidiagrammatic. (After Sterzi.) Riicltenmark, spinal cord. 



In Mammals, the pia is considerably thicker, and the lymph 

 spaces which it encloses are more marked : it becomes separated 

 into two layers, the inner of which gives rise to the vascular 

 secondary or definitive pia, and the outer to the arachnoid. 



