456 THE NERVOUS SYSTEM. 
which traverse it in different directions and convert it into a formatio reticularis. 
By this means the rounded head of the posterior horn becomes cut off from the 
central gray matter, and from this point upwards it remains as an isolated gray 
column intimately associated with the spinal root of the trigeminal nerve. 
The gracile and cuneate nuclei take shape before the decussation of the pyramids 
is fully completed (Fig. 556). The gracile nucleus appears in the form of a small 
irregular mass of gray matter in the interior of the funiculus gracilis, which 
eradually infiltrates the entire strand. At first it is not directly connected 
with the gray matter which surrounds the central canal; but as it is traced 
upwards it increases in bulk, absorbs more of the strand in which it lies, and such 
a connexion becomes established (Figs. 337 and 338). 
The cuneate nucleus, from the first, is a direct offshoot from that part of the base 
of the posterior horn of gray matter which is preserved as a portion of the central 
geay mass. In transverse section it is seen to invade the funiculus cuneatus upon 
its deep aspect, and it gradually grows backwards into its substance. It presents a 
very different appearance from the gracile nucleus, because throughout its whole 
length the gray nucleus and the fibres of the strand are separated from each other 
by a sharp line of demarcation. A second and much smaller mass of gray matter 
appears in the funiculus cuneatus, superficial to the main nucleus, soon after the 
region of the decussation of the pyramids is left. This is termed the accessory or 
the external cuneate nucleus (Fig. 338). 
Gradually the fibres of the gracile and cuneate strands become absorbed in these 
nuclei. As the gray masses gain in size a corresponding diminution in the number 
of fibres composing the corresponding tracts is observed until, at the level of the 
clava and cuneate tubercles, it is seen that these eminences are composed almost 
entirely of the gray 
nuclei, covered by a 
thin skin of the few 
Cancale remaining fibres of 
the two strands in- 
“2\ substantia volved. It is ex- 
y| gelatinosa tremely doubtful if 
3 any fibres belonging 
za} Decussationof +9 the funiculus 
gracilis and funi- 
Rema a culus cuneatus get 
_accessory beyond these nuclei. 
Nac see ae They apparently all 
Sv/ — hypoglossal end in fine terminal 
opie ramifications around 
Pyramid the cells of the 
nuclei. In the case 
Fic, 339.—TRANSVERSE SECTION THROUGH THE CLOSED PART OF A F@TAL of the funiculus 
MEDULLA, IMMEDIATELY ABOVE THE DECUSSATION OF THE PYRAMIDS. cuneatus the bundles 
Treated by Pal-Weigert method. of fibres, as they pass 
from the surface into 
the subjacent gray nucleus, are very distinctly seen in transverse sections through 
the medulla. 
When the medulla oblongata opens up into the fourth ventricle the gracile and 
cuneate nuclei are pushed outwards by the expanding ventricular floor, and the 
gracile nucleus soon comes to an end; but the cuneate nucleus extends upwards for 
a short distance further, and only terminates when the restiform body begins to 
take definite shape on its outer aspect. 
Decussation of the Fillet (decussatio lemniscorum).—Immediately above the 
level of the decussation of the pyramids another decussation of fibres in the median 
plane, and upon the dorsal aspect of the pyramids, takes place in the substance of 
the medulla. This is termed the decussation of the fillet, or the sensory decussation, 
in contradistinction to the term “motor decussation,” which is sometimes applied 
to the decussation of the pyramids. The fibres which take part in this decussation 
Funiculus gracilis Gracile nucleus 
Funiculus 
cuneatus 
Spinal 
root of fifth 
nerve 
Formatio_{ 
reticularis © 
Direct cere- 
bellar tract 
Lower end 
of olivary 
eminence 
en 
