448 THE NERVOUS SYSTEM. | a 
in the posterior part of the opposite lateral column of the cord. The term 
decussation of the pyramids (decussio pyramidum) is applied to the intercrossing 
of the corresponding bundles of the crossed 
pyramidal tracts of opposite sides. 
The direct pyramidal tract is, therefore, 
the only part of the pyramid which has a place 
in the anterior column of the cord. The much 
larger part of this column, termed the anterior 
basis-bundle, as it is traced up into the 
medulla is seen to be thrust aside by the 
decussating bundles of the crossed py vamidal 
rouvary. tract. It thus comes to occupy a deep position 
| in the substance of the medulla behind and 
uypociossa, £O the outer side of the pyramid. 
re Lateral Area of the Medulla.—This is 
I [_f_vramip the district on the surface of the medulla 
LIGULA 
which is included between the two rows of 
nerve-roots, viz. the hypoglossal roots in 
front, and the root-bundles of the spinal 
accessory, the vagus, and the glosso-pharyngeal 
nerves behind. It presents a very different 
appearance in its upper and lower parts. In 
its lower portion it simply appears to be a — 
continuation upwards of the lateral area of 
the cord; in its upper part a striking oval 
prominence bulges out on the surface of the 
medulla, and receives the name of the olivary 
Fic. 334.—DIAGRAM OF THE DECUSSATION OF eminence. 
THE PyRAMIDS (modified from van Gehuchten). The lower part of this district, however, 
NH, Nucleus hypogiossi; NV, Vago-glosso- is very far from being am exact counterpann 
pharyngeal nucleus; FS, Fasciculus soli- 4 e : 25 
Gree ANGE Niolodalainiiente: of the lateral column of the cord. The large 
crossed pyramidal tract is no longer present, 
seeing that it forms in the medulla the greater part of the pyramid of the 
opposite side. , Another small strand of fibres, viz. the direct cerebellar tract, 
prolonged upwards in the lateral column of the cord, gradually leaves this 
portion of the medulla. This tract les on the surface, and is frequently visible 
to the naked eye as a white streak, which inclines obliquely backwards into 
the posterior district of the medulla to join its upper part, or in other words the 
restiform body. ‘The remainder of the fibres of the lateral column of the cord, 
comprising the lateral basis-bundle and the tract of Gowers, are continued upwards 
in the lateral area of the medulla, and at the lower border of the olive the majority _ 
of these fibres disappear from the surface by dipping into the substance of the — 
medulla under cover of that projection. A certain proportion of the fibres, how- 
ever, are retained on the surface and travel upwards towards the pons in the interval, 
which exists between the hinder border of the olive and the roots of the vagus and 
elosso-pharyngeal nerves. 
The olivary eminence (oliva) is a smooth oval projection which bulges out from 
the upper part of the lateral area of the medulla. Its long axis is vertical and is 
about half an inch long. It marks the position of the subjacent inferior olivary 
nucleus, a flexuous lamina of gray matter (nucleus olivaris inferior), which is only 
separated from the surface by a very thin layer of superficial white matter. 
Posterior Area of the Medulla.—In its lower half this district is bounded 
behind by the postero-median fissure, and in its upper half by the lateral margin 
of the medullary part of the floor of the fourth ventricle of the brain. In front 
it is separated from the lateral area by the row of root-fascicles belonging to 
the spinal accessory, vagus, and glosso-pharyngeal nerves. As in the lateral 
area, we recognise a lower portion and an upper portion, which appear continuous, 
but in reality are quite distinct from each other. 
The lower part of the posterior area corresponds more or less closely with 
\\\__E CROSSED PYR.TR. 
a DIR.PYR.TR. 
