THE STRUCTURE OF MAN 
thoracic and lumbar portions of the column. 
This shortening, as above said, is more 
apparent than real, for the vertebral column 
[growing the more rapidly] extends farther 
and farther back beyond the posterior 
limit of the spinal cord. [It is worthy of 
remark that this inequality of growth, so 
marked in Man, is still more conspicuous 
among certain lower Mammals—eg. the 
Hedgehog, in which the filum terminale 
commences in the anterior thoracic region. | 
The filum terminale (ft, Fig. 76) 
runs through the lumbar and_ sacral 
regions of the vertebral column into the 
caudal; and this terminal filament, which 
grows with the growing vertebral column, 
is the vestigial homologue of the posterior 
portion of a spinal cord which, in the 
ancestors of Man, may have run evenly 
throughout the whole length of the 
vertebral column, as it now does in many 
lower Vertebrates. This process of reduc- 
tion, which sets in at the posterior end of 
the spinal cord, is profoundly significant, 
as we have already had to describe a 
sunilar process of reduction going on at 
the posterior end of the axial skeleton itself 
(ante, pp. 28 et seq.). 
i, \\* 
I should lke to suggest the consideration 
whether certain pathological conditions may not 
be traced to this source, if only indirectly? I 
refer to those frequent diseases of the spinal cord 
known as tabetic, which in by far the greater 
number of cases arise at its posterior end. May 
not the above described condition of the lumbar 
Fic 76.—LOWER PoRTION OF THE SPINAL CORD, WITH THE CAUDA EQUINA AND THE 
ENVELoPING Dura Mater. (Dorsal aspect.) One-half natural size. (After 
Schwalbe. ) 
The dura matral sheath has been opened up from behind and laid back ; on the left side 
the roots of the nerves are represented entire ; on the right, the lower of these are 
shown removed above their passage through the sheath, and the bones of the coccyx 
are delineated in their natural relative positions, in order to show the relations of the 
filum terminale and the coccygeal nerves. 
cc., coccygeal nerves ; f.s., dorsal longitudinal fissure ; /.¢., filum terminale, slightly dis- 
placed to the right side ; 7b. i and vy, first and fifth lumbar nerves ; /.d., ligamentum 
denticulatum ; sc. i and v, first and fifth sacral nerves ; sh., the dura matral sheath ; 
th. x and xii, tenth and twelfth thoracic nerves. 
