- Project Runeberg -  Bidrag til Myzostomernes Anatomi og Histologi /
74

(1885) [MARC] [MARC] Author: Fridtjof Nansen
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74

The ganglionic cells arc, most of them, unipolar. I have observed, in the central nervous system, only a few cells of
undoubt-edlv multipolar nature, and they have been confined, principally, to the posterior part of the cerebral ganglia mentioned above. 1
consider that, as a rule, each of the ganglionic cells — whether unipolar or multipolar — has only one real nerve-prolongation. Of these
prolongations there are two types: one passing, directly, into a peripheric nerve to form a nervous tube (tube nerveux) corresponding
to the cylinder-axis« of vertebrates, and this type is, I think, the most frequent one. The other type becomes loosened
out into the fibrillar reticulation of the central mass, out of which, again, nervous tubes spring. Regarding these two types,
vide Pl. IX, fig. 3—6. From the prolongations passing directly into the nerves, small branchlets issue, whose purpose is, I
think, to bring the prolongations into communication with other parts of the nervous system. The cells of the ventral
nerve-cord var)- very much in size; their nuclei vary, however, less than their protoplasm. I have, sometimes, observed nerve-cells
situated in the fibrillar mass of the longitudinal commissures; these cells have, however, been always small, vide pl. IX, fig. 4
and 6. The cells of the cerebral ganglia are of more uniform size, and are smaller than those of the ventral cord. The
ganglionic cells are, as previously mentioned, provided with connective-tissue membranes in which nuclei often occur. I have
not observed ail)- true cellular membrane belonging to the cells. The protoplasm of the ganglionic cells always appears, in
sections, to have a distinct »spongious aspect. The true structure of this protoplasm is difficult to determine; we find a
structural support extended (»spongioplasm ), and this, I am inclined to regard, more, as isolating the hyaloplasm into
fibres, than Leydig appears to be. The spongioplasm extends, also, into the prolongations and there, I believe, partly, isolates
the hyaloplasm into small tubes, or fibrils, and the prolongations acquire, thus, their fibrillar appearance, and what appears, in
sections, as fibrils, is consequently, the spongioplasm. Some of these slender tubes diverge, and form the branchlets issuing
from the prolongations; most of them, however, pass into the nerve, and form the nervous tube which springs from the cell.
In the cells of the second type, whose prolongations become lost in the fibrillar reticulation, the slender tubes diverge from
each other, and form the branchlets in which the prolongations terminate.

In the peripheric, nerves, I have, often, observed ganglionic cells, more or less, distant from the ventral cord;
they, however, occur in greatest number close to the origin of the nerves. In most of them. I have observed only one
prolongation, but, occasionally, I have, also observed two prolongations. In the ventral nerve-cord at the origin of the large
nerves, I have, also, observed unipolar cells sending their prolongations, directly, into these nerves, without passing through
the fibrillar mass of the commissures. The seobservations are quite opposed to Yignals negation of the existence of such cells,
situated in the ventral ganglia of Hirudo, as well as of ganglionic cells occurring in the peripheric nerves.

Direct division of nuclei in the ganglionic cells. I have, several times, observed nuclei of the ganglionic
cells in a dividing state, vide Pl. IX, fig, 8, and PL MI, fig. 11. I have never observed an}- trace of > karyokinese«, and
believe that, here, we have a direct division of nuclei. I have not, distinctly, observed the division of the cells, but I have seen
indications of it, and I have, therefore, no doubt that this direct division of the nuclei is succeded by division of the cells.
I believe we have, here, thus, instances of direct division of cells in the nervous system.

The fibrillar mass. The fibrillar appearance of the commissures, as well as that of peripheric nerves, is occasioned
by the spongioplasm which encloses, and isolates, the hyaloplasm into tubes. I think, therefore — as Leydig states —
that the French designation »tubes nerveuse« is a more correct one than die Fibrillen« of the German scientists.
In the peripheric nerves, these tubes vary, considerably, in diameter; some of them measure only o.ooi—0.002 mm.
whilst others measure up to 0.006 mm. in diameter. Most of these tubes spring, directly, from the ganglionic cells, as stated
above, but some of them, however, spring from the fibrillar central mass of the commissures, and these ones are, in my
opinion, the more slender tubes, especially. As mentioned in connection with the description of the cells, the hyaloplasm of
the tubes is, by a structural support of spongioplasm, divided into several extremely slender tubes;- this spongioplasm, and
these tubes are, however, in both, transversal and longitudinal sections, extremely difficult to observe. I believe it must be
this spongioplasm that has given rise to the Primitiv-fibrillen« of Hermann. Whether the spongioplasm is, originally, a part
of the nervous substance, or is derived, originally, from the connective-tissue, I am unable to decide. It is, at all events, ad"
herent to the septa of the inner neurilemma-sheath, and it is scarcely possible to distinguish it from the smaller septa of the
sheath. In transversal sections of the nerves, granulations can, usually, be seen; the granules are, however, not trans-sected
fibrils, but are tumefactions in the spongioplasm, produced by the union of several septa at these points.

The ventral nerve-cord. The fibrillar mass of the longitudinal commissures is composed of nervous-tubes,
passing, transversally, into the nerves, and, longitudinally, along the whole length of the commissures; and, also, of a fibrillar
reticulation extending throughout the commissures, but especially situated in their centre. This fibrillar reticulation is constructed, I
believe, partly of the fibrils or branchlets, issuing from the celi-prolongations passing directly into the nerves, and parti)-,
from the cell-prolongations loosening themselves up into this reticulation, one of whose purposes is, I think, to promote
communication between the various tubes and the different parts of the nervous system. I am not disposed to assume that this
reticulation has such a spongious nature as Leydig supposes. I think that there are more-isolated fibrils, or slender tubes,
constructed in much the same way as the tubes of the nerves, with a cord of hyaloplasm enclosed in spongioplasm; these

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