MEMBRANES OF THE SPINAL MARROW. 333 



lower fasciculi, both on the anterior and posterior chords of the 

 medulla, but more uniformly as regards the latter. Modifica- 

 tions of this arrangement, which it is unnecessary to specify, 

 are met with in different subjects. ^ 



The Dorsal, or Thoracic Pairs, are much inferior in size to 

 any other nerves, except the inferior sacral. Anastomosing 

 filaments do not generally prevail, yet they are found occasion- 

 ally, as in the neck, upon the two or three upper pairs. The 

 first one has the broad pyramidal or triangular root of a cer- 

 vical nerve, and resembles it also in volume. The second is 

 the smallest of any; they then go on increasing in size to the 

 lowest, but not in uniform gradation. They are successively 

 more oblique, and consequently longer from their bases to their 

 passage through the dura mater. 



The Lumbar and the Sacral Pairs arise closely upon each 

 other, indeed in absolute contact successively, from the lower 

 end of the medulla spinalis, and form a cluster of filaments re- 

 sembling the tail of a horse, hence it is called Cauda Equina. 

 As their place of origin is within the precincts of the first lum- 

 bar vertebra and the two or three last dorsal, they all observe 

 a very oblique course in their descent to the vertebral forami- 

 na, and the lower ones are almost vertical. Notwithstanding 

 they are in contact, and adhere by a loose cellular substance, 

 yet there are no anastomosing filaments between the adjacent 

 roots. From the sacral ganglions presenting the -peculiarity 

 of being situated in the spinal cavity of the sacrum, instead of 

 in the foramina, the single nerve formed from the ganglion and 

 the anterior fasciculus, has to proceed a distance more or less 

 considerable in the spinal cavity before it can escape from it. 



SECT. II. MEMBRANES OF THE SPINAL MARROW. 



Of the Dura Mater of the Medulla Spinalis. 



This membrane, forming the exterior envelope of the spinal 

 marrow, extends from one end of the spinal canal to the other, 

 being continuous above with the dura mater of the brain, and 



