PROJECTION FIBRES. 501 
the thalamus. This may be looked upon as being the second internode laid across 
_ the path of the fillet, and many of its fibres end in arborisations around the thalamic 
cells. Still, it is probable that a proportion find their way directly through the 
thalamus, and, gaining the hinder limb of the internal capsule, they are carried up 
- through the corona radiata to end in the Rolandic area, and more particularly in 
the posterior central convolution. 
The fibres of the superior cerebellar peduncle encounter two nuclear internodes as 
they pass towards the cerebral cortex, viz. the red tegmental nucleus and the optic 
thalamus (p. 494). As in the case of the fillet, however, it is believed that many 
of the fibres gain the internal capsule, and, without break or interruption, proceed up 
through the corona radiata, to end in the cortex of the Rolandic area. 
The fibres of the auditory radiation arise as the axons of cells situated in the 
internal geniculate body. They enter the retrolenticular part of the posterior limb 
of the internal capsule and proceed under the lenticular nucleus towards the 
| temporal lobe. Here they end in the area of cortex which constitutes the auditory 
centre. This corresponds to the middle portion of the superior temporal convolu- 
tion, and also to the rudimentary transverse gyri of Heschl, which are present on 
the insular surface of the temporal operculum. 
The thalamo-cortical system includes the fibres which arise within the thalamus 
and which proceed to all parts of the cortex. They are sufficiently described 
at p. 904. 
The remarkable researches of Flechsig have added greatly to our knowledge of the different 
_ tracts of fibres in the cerebral hemisphere. By studying the periods at which these tracts 
- myelinate he has been able to note the manner in which the different areas of the cortex are 
bound together and also linked on to subjacent centres. He has arrived at a completely new and 
highly important conception regarding the functional value of different districts of the cortex, 
founded upon their anatomical connexions. He recognises four sense areas in the cortex, viz. the 
 someesthetic area, the visual area, the auditory area, and the olfactory area. 
| The somasthetic area is the field of general sensibility and is the most extensive of all. It 
includes the two central convolutions, the posterior portions of the three frontal convolutions, the 
paracentral convolution, and the adjoining part of the callosal convolution, 
The visual area is placed on the inner aspect of the occipital lobe, and more particularly in 
the immediate neighbourhood of the calcarine fissure. 
The auditory area corresponds to the middle third of the superior temporal convolution 
and to the transverse gyri of Heschl. 
The olfactory area includes the locus perforatus anticus, the trigonum olfactorium, the 
anterior part of the callosal convolution, and the uncus. 
These sense areas are peculiarly rich in their supply of projection fibres, and each is provided 
with an extensive system of both corticifugal and corticipetal fibres. Thus the somesthetic area 
HETIC 
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Fic. 410.—Dr1acrams to show Flechsig’s sensory and association areas on the surface of the cerebral ' 
hemisphere. 
is the field where the motor pyramidal tract takes origin and within which the tracts of general 
sensibility end. The visual area has the corticipetal and corticifugal fibres of the optic radiation. 
The auditory area has the corticipetal auditory radiation and also the corticifugal temporo- 
pontine tract. In man the olfactory area is feebly developed, and Flechsig has not yet been able 
to establish, with certainty, its corticipetal and corticifugal projection tracts. 
The sense areas differ greatly from each other in the extent of cerebral surface which they 
cover. The size in each case is in strict conformity with the peripheral area with which each is 
in connexion. It can easily be understood, therefore, how the somesthetic area, representing as 
it does on the cortex all the parts of the body outside the special organs of sense, from which 
sensory nerves proceed, should be so large. Further, it is manifest why the visual sensorial area, 
3D a 
