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INTERNAL STRUCTURE OF THE MEDULLA. 453 
rendered evident in transverse sections by the presence of a distinct median 
line, called the raphe, which occupies the mesial plane. The raphe is formed by the 
close intersection of fibres running in different directions and crossing from one side 
to the other. 
Each half of the medulla is composed of: (a) strands of white matter; (>) gray 
matter; and (c) the formatio reticularis. 
The white matter, as in the cord, is to a large extent disposed on the surface, and 
the gray matter in the interior; but in the upper open part of the medulla the gray 
matter comes to the surface on the dorsal aspect, and is spread out over that area 
which forms the medullary part of the floor of the fourth ventricle. In the cord 
the white matter, in the shape of massive longitudinal strands of fibres, forms a 
thick coating round the central 
gray matter. In the medulla 
the only massive longitudinal  Pumiculus cuneatus 
strands which are seen on the 
Funiculus gracilis 
Gracile nucleus 
Substantia gelatinosa 
surface are the gracile and Rolandi ~ 
cuneate strands (until they be- eine Poo lraus 
come absorbed by the subjacent HR UL LIEN 
nuclei), the inferior cerebellar tract — 
peduncles or restiform bodies and — Crossed pyramidal _¥ 
the pyramidal tracts. Elsewhere 
the coating of white matter is 
thin, and in certain places is 
composed chiefly of the super- 
ficial arcuate fibres. New longi- 
tudinal strands, however, take Detached anterior horn of gray matter 
shape within the medulla, and 5 
two of the most important are _ Gracile nucleus 
placed on either side of the  -Cuneste nuclets ag 
median raphe. 
The gray matter of the cord, 
as it 1s continued upwards into 
the medulla, becomes greatly 
modified. A considerable part 
of it is broken up in the formatio 
reticularis, whilst the only por- 
tions which remain as compact 
masses in direct continuity with 
the gray matter of the cord are: 
(1) the thick layer which sur- 
rounds the central canal, and 
which, in the open part of the 
medulla, becomes spread out on 
the floor of the fourth ventricle : Fic. 336.—Two SECTIONS THROUGH THE JUNCTION BETWEEN 
3 
and (2) the substantia gelatinosa cag UR cere aan eeceay ti 
Rolandi. New masses of gray 
matter, which are not represented in the cord, and which in some cases appear in 
isolated clumps, are also added. The chief of these are the gracile and cuneate 
nuclei, the olivary nuclei, and the arcuate or pyramidal nuclei. 
The formatio reticularis is only feebly represented in the cord, but in the 
medulla it forms a very considerable part of its bulk. It is composed of gray 
matter coarsely broken up by fibres, which traverse it in different directions. 
In the following detailed account of the internal structure of the medulla, it 
Central canal 
Decussation of pyramids 
Cuneate nucleus 
Spinal root of fifth 
nerve 
Substantia gelatinosa 
Rolandi 
Direct cerebellar tract 
Crossed pyramidal 
tract 
_ Detached anterior 
horn of gray matter 
Decussation_ of 
pyramids 
Anterior basis-bundle 
B 
must be understood that the appearances described are such as are seen when 
successive transverse sections through the bulb are examined. 
Decussation of the Pyramids and the Changes produced thereby.—As we 
pass under the microscope a series of successive transverse sections through the 
upper end of the cord and the lower end of the medulla, the most striking change 
which meets the eye is the decussation of the pyramids. The crossed pyramidal 
