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Abstract

The human cortical white matter has a volume nearly as large as the grey matter. Most of it is composed of bers connecting the cortex to itself. These bres can be divided into the
following systems: 1) short bres which follow the gyri and sulci of the grey matter (the U-
bre-system), 2) longer bres which make short-cuts between more distant gyri, 3) long bres
which run in fascicles in the depth of the white matter and connect the different cortical lobes
with each other, and 4) bres of the Corpus callosum. In this study we make an estimate of the
quantitative composition of some of these sytems. This gives insights into the organization of
cortico-cortical connectivity in the human brain which is otherwise difcult to approach.
In a rst approach, we dissected the long-range fascicles in the depth of the white matter.
Measuring their cross sectional areas and multiplying these by the assumed density of bres
one can estimate the number of bres in these fascicles. It turned out to be of the orders of
and P.QSR
P.Q<T
in the individual fascicles and to add up to the order of
P.Q7U
for the fascicles of one
hemisphere. Thus, the number of bres connecting the lobes of one hemisphere to each other
is similar to the number of bres in the Corpus callosum [1]. Both of these long bre systems
comprise only a few percent of the total number of cortico-cortical bres [2].
In a second approach, we made estimates on the number of short cortico-cortical bres
(up to a few centimeters) in the white matter (based on volume measurements of the U-ber
system) and on the number of horizontal axon collaterals which connect the cortex in itself up
to a few millimeters within the grey matter. It turns out that there is an inverse relation between
number and range of bres connecting the cortex in itself. The bres in the U-bre system
outnumber the cortico-cortical bres in the rest of the white matter by a factor of 10, and are
themselves about 10 times less than the horizontal collaterals in the grey matter.