334 NERVOUS SYSTEM. 



terminating below in a cul-de-sac, or closed extremity. It does 

 not adhere to the surface of the spinal canal, but lies loosely 

 attached to it, with the exception of the first cervical vertebra, 

 to which it is closely fastened. Between it, and the ligaments 

 and periosteum on this surface of the bones of the spine, is a 

 long, loose, and spare cellular substance, generally somewhat 

 watery, and containing in the lumbar and sacral regions, a 

 reddish adipose matter. 



This membrane is so much larger than the medulla, that it 

 invests it very loosely and always presents a collapsed appear- 

 ance. Where the nerves penetrate, it furnishes to each one, a 

 sheath as far as the intervertebral foramen. Having reached 

 the intervertebral foramen, the sheath then enlarges so as to 

 enclose the ganglion, adheres by cellular substance to the con- 

 tiguous periosteum, and is then insensibly lost in the tunics of 

 the nervous trunk. Those sheaths are longer for the cauda 

 equina than elsewhere, and, of course, observe the same suc- 

 cessive obliquity with the nerves to which they belong. 



The internal surface of the dura mater is smooth and shining, 

 which is probably owing to the tunica arachnoidea being re- 

 flected over it. 



This membrane has a fibrous texture, and with inconsidera- 

 ble exceptions, is like that of the brain. 



Of the Tunica Arachnoidea of the Medulla Spinalis. 



This membrane is next to the dura mater, and is easily dis- 

 tinguished by its extreme delicacy, thinness, and almost perfect 

 transparency. It is destitute of red blood vessels. It forms 

 a complete envelope for the medulla spinalis, and adheres to 

 the Pia Mater very loosely by means of long, slender, and scat- 

 tered filaments of cellular substance. If the dura mater be 

 slit up its whole length before and behind, and a blow-pipe 

 be introduced at one end of the medulla, between the pia 

 mater and the arachnoidea; inflation will cause the latter to 

 rise, and to present itself as a long capacious tube, detaching 

 on each side processes which surround the roots of the nerves. 

 These processes having reached the points where the nerves 

 penetrate the dura mater, are then reflected upon its internal 



