270 ME. J. L. CLAEKE ON THE INTIMATE STEUCTUEE OE THE BEAIN. 
their bases. In the lower or anterior part of each is seen the group of large cells ( t ') 
which form the hypoglossal nucleus ; and in the upper or posterior part is seen another 
and oval group of smaller cells ( t "), forming part of the nucleus of the spinal-accessory 
nerve. Behind this group of cells, the part t is finer in texture, consisting of granules, 
nuclei, small cells, and fine fibres. The posterior pyramid (b) is rather larger than in 
fig. 19, but it now begins to be bevelled off at its base, just below b ; and between it and 
the spinal-accessory nucleus (£) there is a small triangular and paler mass (b”), which is in 
fact the inner side of the base of the pyramid, where it was joined to its fellow at the 
bottom of the median fissure, as seen at b", fig. 19, enclosed in the dotted line on the 
right side. The outer restiform nucleus (d) has increased considerably in dimensions, 
while the inner nucleus (o) has somewhat diminished (compare figs. 7 & 8, Plate VIII. 
of human medulla on the right side). The decussating fibres (n, y') proceeding from 
the side of the spinal-accessory nucleus to the opposite olive and anterior pyramid are 
less numerous. In fig. 21, Plate IX. the outer restiform nucleus (d) is quite as large as 
in the preceding figure ; while the inner nucleus (o) is much smaller, and is divided into 
several separate masses, which are partially blended with the outer nucleus (d). The 
posterior pyramid (b) has considerably diminished, and the part b" at its base has swollen 
into a larger eminence, which is now directly continuous with the posterior part (t) of 
the vagal nucleus, and seems to arise out of it (compare fig. 20), while its cells have 
increased somewhat in size. At the point between t and b" there is an oval mass of 
epithelial cells and fibres (seen at f on the left side) which is continuous with the layer 
that covers the ventricle ; it exists also in Man, but is much smaller than in the Ape. 
The hypoglossal nucleus (t 1 ) has increased considerably in dimensions, pushing, as it were, 
outward, the oval group of cells ( t "), which now becomes the nucleus of the vagus , instead 
of the spinal-accessory nerve. The bundles of fibres ( n , y') have ceased to proceed from 
the side of the spinal-accessory (now the vagal) nucleus to the opposite olive and anterior 
pyramid, the point whence they issued presenting now only the cut end of a slender 
column of longitudinal fibres ( n ) enclosed in a crescent between the horns of the nucleus. 
In fig. 22, Plate X. the outer restiform nucleus (d) and the scattered masses of the inner 
nucleus (o) have somewhat diminished and become still more intermixed. At the same 
time it will be observed that the posterior pyramid ( b ) is still further reduced, while the 
part b " (which in fig. 20 belongs to the base of the pyramid) has extended in a corre- 
sponding proportion ; so that now the posterior part ( t ) of the vagal nucleus, and the 
part b" of the posterior pyramid form together a triangular mass (i) uniform in struc- 
ture, and overlying the slender longitudinal column n and the oval group of cells (t") 
which gives origin to the upper roots of the vagus nerve. The hypoglossal nucleus ( t ') 
reaches the surface of the ventricle, and completely separates i from its fellow of the 
opposite side. In fig. 23, Plate X. at jp" is seen a large bundle of arciform fibres coming 
from the outer part of the restiform body, and sweeping forward through the extremity 
of the caput cornu (e e) to the olivary body (W). The hypoglossal nucleus has con- 
siderably diminished, and retreated further backward and outward at the inner side of 
