680 
Anterior view of the medulla oblongata, shewing the 
decussation 0 he pyramids, and of the upper part 
of the spinal cord. (After Mayo. ) 
Pp» anterior pyramids. 
0, olivary bodies. 
7, restiform bodies. 
d, decussating fibres. 
al, antero-lateral column of the spinal cord. 
¢c, anterior fissure of the cord, the floor of which 
forms the anterior commissure. 
sure. Superiorly the pyramids pass into the 
mesocephale above the inferior fibres of the 
pons Varolii, and interlace with other fibres of 
the same system which occupy a more elevated 
plane. In its passage into the mesocephale, 
each pyramid experiences a marked constric- 
tion, which alters its form from a prism to a 
cylinder. The fibres, however, soon diverge 
and expand. As they ascend through the me- 
socephale they are crossed by the transverse 
fibres of the pons, and some grey matter occu- 
pies the interstices between them, with which it is 
probable that other fibres are connected, and are 
added to those of the pyramids, as they emerge 
from the mesocephale at its anterior extremity. 
The pyramids gradually diminish in size 
towards the inferior extremity of the medulla 
oblongata. And here three sets of fibres may 
be distinctly noticed. The first, or decussating 
Sibres, are the most numerous; they pass 
downwards and backwards into the antero- 
lateral column of the spinal cord on the oppo- 
site side, so that the right pyramid sends fibres 
into the left half of the cord, and the left pyra- 
mid into the right half of the cord. These decus- 
sating fibres consist of from three to five bundles 
from each pyramid, which in their descent cross 
and interlace with each other /figs.384, 385, d). 
They differ in distinctness as well as in number 
in various subjects. The point at which the 
decussation takes place is about ten lines below 
the margin of the pons Varolii, and the inter- 
ruption to the fissure, occasioned by the cross- 
ing of the fibres, occupies a space of from two 
to four lines. To expose these fibres clearly it 
is necessary to remove the pia mater carefully 
from the anterior surface of the medulla ob- 
NERVOUS SYSTEM. (Nervous Centres. Tue ENcEruaton.) 
Fig. 384. 
longata to some distance below the decussation, — 
and it is, in general, of advantage to the pre-. 
paration to place it in alcohol immediately after 
the removal of the pia mater. wk 
A second set of fibres, very few in numbe 
are continued from the pyramids directly d 
to the anterior surface of the cord on the 
side, and appear to be continuous with 
of the superficial fibres of the antere 
column. These fibres may be regard 
the direct channel of communication of 
half of the medulla oblongata with the co 
sponding half of the spinal cord (fig. 385, ” 
The third series of fibres vary consic 
in point of developement in different indiy 
dane They pass een the pyramids at 
the postero-lateral columns of the medulla o 
longata, the restiform columns. They fe 
series of curves with their concavities direete 
upwards (fig.383,A),crossing beneath the: 
rior extremity of the olivary body, and 
times extending over a considerable portion | 
its surface. I have on several occasions — 
these fibres so largely developed as to ¢ 
nearly the whole surface of each olivary 
These fibres are appropriately distinguist 
the name arciform from their arched 
(processus arciformes, Santorini). 
When these fibres are so numerous as | 
cover the surface of the olivary body, we mi 
observe that those which are nearest the mang 
Anterior surface of the medulla oblongata, with a, 
pomp ps pach fie pi» Varolii, a 
obliquely from the right side, (After M 0.) 
P, pons Varolii—its left half. ; 
0, 0, olivary bodies. : 
p» part of the right anterior pyramid, eu 
near the inferior edge of the pons, and torn do 
showing the of some of its fibres ove 
the left side and backwards. ia 
d, decussating fasciculus of fibres of right 
ramid. heme rh 
d’, decussating fasciculas o pyramid. a 
n, non-decussating fibres of the right pyramil 
