| 
) 
; 
INTERNAL STRUCTURE OF THE MEDULLA. 457 
are called deep arcuate fibres (fibre arcuate interne), and they are derived from the 
cells of the gracile and cuneate nuclei. From the deep aspects of these nuclei these 
fibres stream forwards and inwards towards the median raphe, forming a series of 
concentric curves in the substance of the medulla. They cross the mesial plane 
and decussate with the corresponding fibres of the opposite side, upon the dorsal 
aspect of the pyramids. Having thus gained the opposite side of the medulla they 
immediately turn upwards and form a conspicuous strand of longitudinal fibres, 
which ascends close to the mesial plane and is separated from its fellow of the 
opposite side by ie medial raphe alone. This strand is termed the fillet or 
lemniscus. 
As we proceed up the medulla the deep arcuate fibres which first come into sight 
appear as coarse bundles which curve forwards in a narrow group around the central gray 
matter. Soon other finer bundles appear, which describe wider curves on the outer side of 
the coarser group until a very large part of each lateral half of the medulla is seen to be 
traversed by these arcuate fasciculi. As they approach the mesial plane they come in 
contact with the remains of the anterior basis-bundle, which at this level, as already 
mentioned, lies upon the dorsal aspect of the pyramid, flattened up against the raphe. 
The deep arcuate fibres pierce the anterior basis-bundle obliquely, and in the interval 
between it and the corresponding strand of the opposite side they decussate in the middle 
line with the deep arcuate fibres of the opposite side. They then change their direction 
and turn upwards, and the fillet, as already stated, takes form and gradually increases 
in volume as it ascends. This great and important tract is thus laid down between the 
pyramid and the an- 
terior basis-bundle ; Gracile nueleus—— ——{4== 
and the consequence 
of this is that the Spinal root of trigeminal 
latter tract is pushed ae / 
still further back- _ Ee 
wards, and when the A ee 
fillet is fully estab- Fasciculus solitarius GDN i 
lished it comes to lie —-Pitect a 
immediately beneath Substantia | 
the eray matter of gelatinosa Rolandi 
the floor of the fourth Deep arcuate fibres 
ventricle (Fig. 341). 
When the fillet 
Cuneate nucleus 
Central canal 
Hypoglossal 
nucleus 
Posterior 
> longitudinal 
“. fasciculus 
Hypoglossal 
Hy poglossal nerve Z| nerve 
is fullyformed three anterior superficial Os is iA SS = AC ae 
longitudinal strands arcuate fibres (Mi MC. \ Vee AS fee NM ot 
i if LE, Z 1 oy 
are observed traver- lf cS : : 
: Inferior olivary Til Oss e 5 
sing the medulla, nucleus (ij gS ; = 
close to the mesial 
= Mesial accessory 
plane. From before olivary nucleus “of > 
backwards these 
are: (1) the pyramid, 
(2) the fillet, and 
(5) the posterior 
longitudinal bundle. 
Ty “al Fic. 340.—TRANSVERSE SECTION THROUGH THE HUMAN MEDULLA IN THE 
ae pyfamre LOWER OLIVARY REGION. 
forms a massive 
tract quite distinct from the fillet, which les behind it. The fillet and the 
posterior longitudinal bundle are, in the first instance, not marked off from each 
other. They appear as a broad flattened band applied to the raphe. One edge 
of this band is directed backwards and reaches the gray matter on the floor of 
the fourth ventricle, while the other edge looks forwards, and is in contact with 
the pyramid. In the upper part of the medulla the fillet and the posterior longi- 
tudinal fasciculus begin to draw asunder from each other. The intermediate 
longitudinal fibres become reduced in number and the two strands grow denser— 
the one on the dorsal aspect of pyramid, and the other immediately beneath the 
gray matter of the floor of the fourth ventricle (Fig. 541). 
Superficial 
anterior 
arcuate fibres 
