278 ME. J. L. CLAEKE ON THE INTIMATE STEUCTUEE OE THE BE AIN. 
(17) It has been shown that the upper roots of the spinal-accessory nerve arise from 
a special nucleus in the medulla oblongata, behind the canal, and that the lower roots 
have their origin in the lateral grey substance and anterior cornu of the spinal cord. It 
is an interesting fact that the hypoglossal nerve has a similar kind of double origin. 
Its lower roots arise from the upper remains of the anterior cornu, as I formerly showed 
in the Sheep*, and as may now be seen in Man (fig. 28 x, Plate X.). Here the central 
nucleus t', which gives origin to the upper roots, is only just marked out and contains 
only a few small cells ; while the remains of the anterior cornu (f) contain two little 
groups of large multipolar cells in connexion with the lower roots x. 
(L8) In fig. 25, Plate X., J is an exact representation of the central nucleus of the 
upper roots of the hypoglossal nerve in Man, a little below the level of the calamus 
scriptorius, and on a level with the upper roots of the spinal-accessory nerve (r). It con- 
sists of a group of multipolar cells, which are larger, more varied in shape than those 
of the spinal-accessory nucleus, and resemble in every respect those of the anterior 
comu of the spinal cord. These cells send their processes in different directions, and 
many of them are elongated longitudinally in connexion with fibres running in the 
same direction. There is no actual line of separation between this group and the oval 
group (H, fig. 25) belonging to the spinal-accessory nucleus, for each passes gradually, as 
it were, into the other by means of more sparingly scattered cells and nuclei which occupy 
the intervening space, exactly as is represented in the figure. In this intervening space 
some fine fibres may be seen passing from one nucleus to the other as well as from 
within outward ; but at a higher level in the medulla I discovered a more intimate and 
important communication between the two nuclei, by means of nerve-processes and 
fibres, which will be described further on. At the anterior part of the hypoglossal 
nucleus, just where the nerves enter it, there is a little separate group composed of 
smaller cells than the others, and from which several large bands of commissural fibres 
proceed to the raphe, where they decussate with those of the opposite side (see J', 
fig. 25). This little group is most conspicuous at a higher level of the medulla, above 
the calamus scriptorius, where the other cells of the nucleus are also imperfectly divided 
into separate groups (see fig. 32 J', Plate XI.). There is also a remarkable fan-shaped 
set of commissure fibres (J", fig. 25, Plate X.), which, arising from different parts of 
the hypoglossal nucleus, converge inward and forward to the raphe (F), where, like the 
others, they decussate their fellows of the opposite side. More externally, the roots of 
the nerves (x) are crossed, as already described, by numerous arciform fibres (M, A A) 
which proceed from the spinal-accessory nucleus and the slender column (n) adjoining. 
In animals these fibres are exceedingly numerous. 
(19) In the Sheep, as I formerly stated, a few of the roots of the hypoglossal nerves 
(x) join these commissural fibres and cross the raphe; and in Man a few of the higher 
roots take the same course ; but almost all of them enter the nucleus without decus- 
sating across the raphe. They traverse it from before backward and in other directions, 
* See “ Medulla Oblongata,” Philosophical Transactions, 1858, fig. 13 z, Plate XII. 
