EXTERNAL FORM. 



493 



of its membranes, the spinal cord is seen to be marked by longi- 

 tudinal fissures. Of these, two, which are the most obvious, run along 



Fig. oiG. — Lower Part of the Spinal Cord with Fig. 34G. 



THE Cauda Equina and Sheath, seen from be- 

 hind (Allen Thomson), h 



The sheath has been opened from behind and 

 stretched towards the sides ; on the left side all the 

 roots of the nerves are entire ; on the right side both 

 roots of the first and second lumbar nerves are entire, 

 while the rest have been divided close to the place of 

 their passage through the sheath. The bones of the 

 coccyx are sketched in their natural relative position 

 to show the place of the filum terminale and the lowest 

 nerves. 



a, i^laced on the posterior median fissure at the 

 middle of the lumbar enlargement of the cord ; h, b, 

 the terminal filament, drawn slightly aside by a hook 

 at its middle, and descendmg within the dura-niatral 

 sheath ; b', b', its prolongation beyond the sheath 

 and upon the back of the coccygeal Ijones ; c, the 

 dura-matral sheath ; d, double foramina for the 

 separate passage of the anterior and posterior roots of 

 each of the nerves ; e, pointed ends of several ijro- 

 cesses of the ligamentum denticulatum ; Dx, and 

 Dxil, the tenth and twelfth dorsal nerves ; Ll, and 

 Lv, the first and fifth lumbar nerves ; Si, and Sv, 

 tlie first and fifth sacral nerves ; Ci, the coccygeal 

 nerve. 



*v 



■^LV 



SI 



i 



% 



the middle line, one in front and the other 

 behind, and are named the anterior and pos- 

 terior median fissures. j_,<v 



The (inferior median fissure (fig. 847, 1) f^m 



is more distinct than the po.sterior, and pene- 

 trates about one-third of the thickness of 

 the cord, its depth increasing towards the 

 lower end. It contains a fold of the pia 

 mater, and also many blood-vessels, which 

 are thus conducted to the centre of the cord. ,^, ™ 



At the bottom of this fissure is seen the ^ '^'^ ''' 



transverse connecting portion of white sub- 

 stance named the anterior ivhite commissure. 



The posterior median fissure (fig. 347, 2) is 

 less marked in the greater part of its extent 

 than the anterior, but l)ecomes more evident 

 towards the upper part of the cord. Its posi- 

 tion is marked, especially in the lumbar en- sxj 

 largement and in the cervical region, by a "^^ 

 superficial furrow. It is not an actual fissure, 

 for, although the lateral halves of the i)osterior 

 part of the cord are quite separate, there is 

 no distinct inflection of the pia mater be- 

 tween them, but merely a septum of connec- 

 tive tissue and blood-vessels which passes in nearly to the centre of 

 the cord, as far as the posterior grey commissure. 

 Besides these two median fissures, two lateral furrows or fissures have 



