550 THE NERVOUS SYSTEM. 
longitudinally-directed fibres in the white medullary centre of the occipital lobe. 
It hes on the outer side of the ventricular cavity, from which it is SgysEey by the 
fibres of the tapetum and the ependyma of the ventricle (Figs. 397, p. 535; ; and 400, 
p- 538). To the outer side of, and applied closely to, the optic radiation is another 
longitudinal tract of fibres in this part of the medullary centre of the cerebral hem1- 
sphere, viz. the inferior longitudinal association bundle; but the fibres of the latter 
fasciculus are coarser and are stained more deeply by the Pal-Weigert method, and 
thus they can, as a rule, be easily distinguished from the optic radiation. Traced 
in a backward direction, the fibres of the optic radiation disperse and pass to the 
cortex of the occipital lobe on both its mesial and outer aspects. This is a matter 
of interest, seeing that the visual centre is placed in this cortical district, and more 
particularly on the mesial aspect in the immediate neighbourhood of the calcarine 
fissure (Flechsig and Henschen). When the optic radiation is followed in a forward 
Choroid plexus in lateral 
/ Ventricle 
Vz 
_ Corpus callosum 
~—Thalamus (pulvinar) 
Occipital corticifugal tract to 
_———_ superior quadrigeminal body 
= -Superior quadrigeminal body 
Optic radiation -— Corpus geniculatum externum 
Corpus geniculatum internum 
Caudate nucleus ~Sylvian gray matter 
Inferior brachium 
Lateral fillet 
Optic radiation 
Inferior longitudinal bundle 
| ~ Superior cerebellar peduncle 
~ Cerebellum 
Descending horn of lateral ventricle 
) Pons 
Fimbria if 
Gyrus dentatus 7 
Dentate fissure 7 
Fic. 409.—CorRONAL SECTION 'THROUGH THE LEFT SIDE OF THE CEREBRUM, MESENCEPHALON, AND Pons, 
IN THE REGION OF THE PULVINAR OF THE THALAMUS AND THE CORPORA GENICULATA (Chimpanzee ; 
Weigert-Pal specimen). 
direction it is seen to enter the retrolenticular part of the posterior limb of the 
internal capsule, from whence its fibres pass to the pulvinar of the optic thalamus, 
to the corpus geniculatum externum and the superior quadrigeminal body. 
As we have noted, the optic radiation is composed partly of corticifugal and 
partly of corticipetal fibres (p. 511). The former arise from cells in the occipital 
cortex and end in the pulvinar and the superior quadrigeminal body ; the cortici- 
petal fibres arise in the pulvinar and in the corpus geniculatum externum and 
end in the occipital cortex (Ferrier and Turner). 
The system of fibres which belong to the mesial fillet and the superior cerebellar 
peduncle have been already more or less fully dealt with (pp. 494 and 497). The fillet 
system represents the continuation upwards of the posterior columns of the cord. 
The first nuclear internodes in the system are met with in the medulla in the shape of 
the cuneate and gracile nuclei. It is here that the fillet first takes definite shape, and, 
as 1f passes upwards through the tegmental part of the medulla and pons, it receives 
many additions to its strength in the form of fibres from the nuclei of termination 
of the afferent cranial nerves. Finally, passing through the tegmentum of the 
mesencephalon, it reaches the subthalamic region and enters the ventral aspect of 
