FORM AXD POSITION OF THE COED. 



505 



The lower or lumbar enlargement is situated nearly opposite the last dorsal 



vertebra ; its antero-posterior diameter is nearly equal to the transverse. 



Below this enlargement, the cord tapers in 



the form of a cone, from the apex of which Fig. 342. 



the small filiform prolongation is continued 



downwards for some distance within the 



sheath. 



Fig. 342. LOWER PART OP THE SPINAL CORD WITH 

 THE CAUDA EO.UINA AND SHEATH, SEEN FROM BE- 

 HIND. 



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 aud 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, placed on the posterior median fissure at the 

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

 the terminal filament, drawn slightly aside by a hook 

 at its middle, aud descending within the dura-matral 

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

 and upon the back of the coccygeal bones ; 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 pro- 

 cesses of the ligamentum denticulatum ; Dx, and 

 DXII, the tenth and twelfth dorsal nerves ; Li, and 

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

 the first and fifth sacral nerves; Ci, the coccygeal 

 nerve. 



The cervical and lumbar enlargements have an 

 evident relation to the large size of the nerves 

 which supply the upper and lower limbs, and which 

 are connected with those regions of the cord, in 

 accordance with the general fact observed in the 

 animal kingdom, that, near the origin of large nerves, 

 the central nervous substance is accumulated in 

 larger proportion. At the commencement of its 

 development in the embryo the spinal cord is des- 

 titute of these enlargements, which, in their first 

 appearance and subsequent progress, correspond 

 with the growth of the limbs. 



Sometimes the cord presents one or two bulbs or 

 swellings towards its lower end. 



According to Foville, the lumbar enlargement is 

 chiefly due to an increase in bulk of the anterior 

 region of the cord. (Trait6 compl. de 1'Anat., &c., 

 du Syst. Nerv. Cerebro-Spinal. Paris, 1844. Part 

 I., p. 138.) 



"DXD 



Li 



St 



The terminal filament (filum terminale, cen- 

 tral ligament) descends in the middle line 

 amongst the nerves composing the cauda 



equina, and, becoming blended with the lower end of the sheath opposite to 

 the first or second sacral vertebra, passes on to be fixed to the lower end of 



L L 



