front matter 1

Sp

ino

ce

ph

al

ic

tr

a

c

ts

?

Sp

ino

ce

ph

al

ic

tr

a

c

ts

?

Ventral white

commissure

Ventral white

commissure

VVentral gray commissureentral gray commissure

VVentral funiculusentral funiculus

Ventral corticospinal tract

Central canal

surrounded by ependyma

Central canal

surrounded by ependyma

VVentral grayentral gray

Intermediate grayIntermediate gray

Dorsal root bifurcation zone

Fasciculus cuneatus

Fasciculus gracilis

Dorsal root collateralization zone

Lateral funiculusLateral funiculus

Tectospinal tract?

Medial longitudinal fasciculus?

Vestibulospinal tract?

Dorsal

gray

Dorsal

gray

Myelinated terminals of

dorsal root collaterals

Dorsal intermediate septum

Dorsal median septum

Examples of

staining densities

representing

degrees of

myelination

Ventral

median fissure

Dorsal funiculus

Lissauer's tract

I

ntr

aspin

al

tr

a

c

ts

I

ntr

aspin

al

tr

a

c

ts

S

p

i

n o c e r e b e l l a r t r a c t s

S

p

i

n o c e r e b e l l a r t r a c t s

myelination

gradient

myelination

gradient

myelinationmyelinationgradientgradient

Reticulated areaReticulated area

Subgelatinosal pleSubgelatinosal plexusxus

dense reactive glia

sparse reactive glia

myelinated

many myelinating fibers

some myelinating fibers

Lateral

corticospinal

tract

Lateral

corticospinal

tract

Rubrospinal tract?Rubrospinal tract?

Lateral

reticulospinal

tract?

Lateral

reticulospinal

tract?

See the matched cell body stained section in Plates 94A and B

Note the lighter staining of the ventral commissure in this sec-

tion. This commissure is myelinated in the GW37 fetus and

4-day-old infant. It is possible that the unmyelinated ventral cor-

ticospinal tract (present in each of the specimens at the cervical

level) is growing decussating collaterals in the 4-week-old infant

that dilute the number of myelinated fi bers in the commissure.

The fi ne dashed lines in the lateral and ventral funiculi segregate regions of differing densities of the myelin stain, not the borders of fi ber tracts.

These lines may correspond to the borders of some fi ber tracts, such as the spinocerebellar tracts. Other lines run across fi ber tracts, such as the

area surrounding the ventral horn that includes the medial and central parts of the intraspinal tracts and the internal fi bers of the spinocephalic

tracts. Most tracts in the ventral and lateral funiculi overlap and do not have distinct borders. The lateral corticospinal tract overlaps with the

rubrospinal tract. The spinocephalic tracts (outer curved arrow) overlap with the vestibulospinal tract and include fi bers in the ventrolateral

reticulospinal tract (not labeled), the spinotectal tract (not labeled), and the spino-olivary tract (not labeled). The intraspinal tracts (inner curved

arrow) overlap with the medial longitudinal fasciculus, the tectospinal tract, and the lateral reticulospinal tract. Straight arrows in the lateral

and ventral funiculi indicate myelination gradients (see comments on these gradients in Plate 95).

PLATE 93B