NERVOUS SYSTEM. VERTEBRATA. 501 



abruptly by a few large and distinct rootlets ; the ventral 

 rootlets take their origin from a more extended area, and 

 are numerous and small, converging and uniting to form 

 several fasciculi before they finally pass through the dura 

 mater. The arachnoid membrane can be seen at the cut 

 edge of the dura, separated from it by a narrow subdural 

 space. The much larger space between the arachnoid and 

 pia mater is occupied by laminar and trabecular subarachnoid 

 tissue. On either side a stout ligament (I. denticulatum) 

 runs longitudinally within this tissue between the dorsal 

 and ventral nerve-roots. In each interspace between the 

 nerves it bends outwards towards the arachnoid and unites 

 with it, and with the dura mater. 



0. C. 1369. Hunterian. 



D. 789. The posterior end of the spinal cord of the same 

 Elephant, showing the crural enlargement and the origin 

 of the cauda equina. The dura has been removed from the 

 ventral surface. Where the cord gives place to the cauda 

 equina the dura lies at some distance from it, and encloses 

 within its general cavity a considerable length of the nerve- 

 roots that form the cauda. The median spinal artery is 

 shown within the pia mater in the ventral mid-line. 



O. C. 1370. Hunterian. 



D. 790. Part of the spinal cord of an Elephant from the lumbo- 

 sacral region. The dura mater is removed from both 



O 



dorsal and ventral surfaces, showing the origins of the 

 nerves. The specimen is suspended by the ligamentum 

 denticulatum. A small portion of the pia mater is reflected 

 from the dorsal surface of the cord, showing the lamina that 

 projects from its inner surface into the dorsal longitudinal 

 fissure. The corresponding lamina in the ventral fissure is 

 seen upon the upper sectional surface. A portion of the 

 dorsal columns of the cord has been removed at the lower 

 end or the specimen, showing the fibrils of the dorsal roots 

 passing a little way into the substance of the cord. A 

 bristle has been placed beneath them on the left side. 



O.C. 1371. Hunterian. 



