PROJECTION FIBRES. 549 
The fasciculus occipito-frontalis is a bundle of fibres which runs in a sagittal direction 
in intimate relation to the lateral ventricle (Fig. 403, p. 540). It has been pointed out 
(Forel, Onufrowicz, and others) that, in cases where the corpus callosum fails to develop, 
the tapetum remains apparently unaffected, and Déjerine has endeavoured to prove that 
the fibres of this layer really belong to the fasciculus occipito-frontalis. According to 
Déjerine, the fasciculus occipito- frontalis lies on the inner aspect of the corona radiata in 
intimate relation to the caudate nucleus, and posteriorly it spreads out over the upper 
and outer aspect of the lateral ventricle, immediately outside the ependyma, where 
it constitutes the tapetum (see p. 546). There isa considerable amount. of literature 
dealing with this subject, and the most probable explanation of the difficulty would 
appear to be that the tapetum is composed of fibres derived from both the corpus 
callosum and the fasciculus occipito-frontalis of Déjerine. 
Projection Fibres.—The projection fibres are those which connect the cerebral 
cortex with nuclear masses placed at a lower level. The great bulk of these fibres 
are found in the corona radiata. This has already been seen to be formed by the 
continuation upwards of the internal capsule (p. 542). In the corona radiata the 
fibres which, lower down, are gathered together in the compact mass which con- 
stitutes the internal capsule, radiate in every direction, intersect the radiation of 
the corpus callosum, and finally reach every region of the cortex. Although the 
fibres of the corona radiata represent the chief bulk of the projection fibres, it should 
also be borne in mind that a certain number gain the cortex by a different route, 
notably through and under the lenticular nucleus and by the path offered by the 
external capsule. 
The projection fibres of the cerebral hemisphere may be classified into (1) 
corticipetal, and (2) corticifugal groups; and under these headings the following 
great strands may be arranged : — 
Corticipetal Projection Strands. | Corticifugal Projection Strands. 
1. Thalamo-cortical. 1. The pyramidal or great motor tract. 
2. The fillet system of fibres. 2. The fronto-pontine strand. 
3. The superior cerebellar peduncle. 3. The temporo-pontine strand. 
4. Corticipetal fibres of the optic radiation. 4. The corticifugal fibres of the optic radia- 
5. The auditory radiation. tion. 
The great motor or pyramidal tract is composed of fibres which arise from pyramidal 
cells in that portion of the cortex which is spread over the Rolandic area, or, in 
other words, in the district immediately in front of and immediately behind the fissure 
of Rolando. The fibres descend through the corona radiata into the posterior limb 
of the internal capsule. From this point the further course of the pyramidal tract 
has been traced, viz. through the central part of the crusta of the crus cerebri, the 
ventral part of the pons, and the pyramid of the medulla oblongata. At the level of 
the foramen magnum it decussates in the manner already described and enters the 
spinal cord as the crossed and direct pyramidal tracts. The fibres composing these 
end in connexion with the ventral or motor column of cells, from which the fibres 
of the anterior roots of the spinal nerves arise. 
The fronto-pontine strand is composed of fibres which arise as the axons of the 
cells in the cortex which covers the portion of the frontal lobe, which lies in front of 
the precentral furrows. It descends in the anterior limb of the internal capsule, 
enters the mesial part of the crusta of the crus cerebri, through which it gains the 
ventral part of the pons. In this its fibres end, by forming arborisations around the 
cells of the nucleus pontis. 
The temporo-pontine tract consists of fibres which spring from the cells of that 
part of the cortex which covers the middle portions of the two upper temporal con- 
volutions. It probably represents a corticifugal tract belonging to the auditory 
system, seeing that it springs to some extent from the auditory cortical area. The 
temporo-pontine tract passes inwards under the nucleus lenticularis, enters the 
retrolenticular part of the hinder limb of the internal capsule, and thus gains the 
outer part of the crusta of the crus cerebri. From this it descends into the ventral 
part of the pons, in which it ends in the nucleus pontis. 
The optic radiation forms a very definite and easily demonstrated tract of 
