702 
and as its anterior pillars pass upwards in this 
situation, they receive fibres from the neigh- 
bouring convolutions. These pillars remain 
separate from the mamillary bodies to the fo- 
ramen of Monro, where they adhere to each 
other and form the apex of the body of the 
fornix. Traced backwards, the fibres of the 
fornix pass into the posterior and inferior horns 
of the lateral ventricle. In the former they 
connect themselves with the hippocampus 
minor by expanding over it, and in the latter 
they spread over the hippocampus major, 
forming the posterior pillar of the fornix, or 
tenia hippocampi. 
The relation of the anterior pillars of the 
fornix to the foramen commune anterius has 
been already sufficiently described. Superiorly 
the fornix adheres to the corpus callosum or to 
the septum lucidum, and the anterior commis- 
sure crosses in front of its anterior pillars, and 
almost touches them. 
4. The fourth longitudinal commissure is the 
tenia semicircularis. It may be traced from 
the corpus mamillare outwards and back- 
wards in the groove between the optic thala- 
mus and corpus striatum into the inferior 
horn of the lateral ventricle, when its fibres 
mingle with those of the middle lobe. It is 
evidently part of the same system of fibres with 
the fornix. 
Take away the corpus callosum, the grey 
matter of the internal convolution, the-veutri- 
cular prominence of the optic thalamus, and all 
these commissures fall together and become 
united as one and the same series of longitu- 
dinal fibres. 
It is very remarkable how few fibres pass 
between the great mass of the cerebrum and 
the cerebellum. The processus cerebelli ad 
testes are the only fibres which can be regarded 
as forming commissures between these two seg- 
ments of the encephalon, and they are of the 
nature of longitudinal commissures. 
The transverse commissures are the corpus 
callosum, the anterior commissure, the posterior 
commissure, the soft commissure. 
1. The corpus callosum, so called according 
to some from the density of its tissue, is the 
reat commissure of the lateral halves of the 
rain proper—commissura cerebri maxima of 
Soemmering. The fasciculated character of 
this structure is as obvious as that of any nerve 
in the body, and the direction of the fibres is 
clearly from one hemisphere to the other, 
From the description already given of the 
corpus callosum, it is evident that its fibres 
sink into the white substance of each hemisphere 
above the level of the corpora striata, as well 
as into that of the anterior and of the posterior 
lobes. By its principal or horizontal portion 
it connects the white matter of the lesser cen- 
trum ovale ofeach side, and by the fibres which 
form the anterior and erior reflexions it 
connects the anterior and posterior lobes. It 
needs only a very superficial dissection to ascer- 
tain thus much. 
To determine the precise fibres with which 
those of the corpus callosum are continuous, 
NERVOUS SYSTEM. (Nervous Centres. Tue Excepuatoy.) 
and the relation which bear to the lateral 
ventricles, demands a much more minute dis- 
section. This must be done, according to the — 
directions of Foville, who has’ given a most 
elaborate description of this commissure, by 
carefully separating it in the transverse direc. 
tion from the internal convolution, on a har 
dened brain. Pursuing the dissection in this 
direction, it may be detached from the sub- 
stance of the hemispheres as far outwards a 
the external border of the co callosum an 
optic thalamus. Along this edge the fibre 
curve downwards and inwards, and appear} 
become continuous with some of those whic 
radiate from those bodies. The anterior 
posterior fibres enclose the anterior and pos- 
terior horns of the lateral ventricles in rat 
ating forwards and backwards from the corpor 
striata and optic thalami to those parts of thes 
cavities, phe 
This view of the connections of the corpu 
callosum would indicate it to be a commissu 
between the thalami and Striata, 
between the crura cerebri, as Tiedemann su: 
osed, rather than between the hemispher 
othing is more difficult than the dissecti 
of the fibres of the corpus callosum bey: 
the internal convolution: and it cannot | 
regarded as in any d certain that the cot 
nections of its fibres are limited to those’ 
described. 
The developement of the corpus calle 
in the fetus, se to that of the hemisp 
or convolutious, is favourable to the 
its connections maintained by Tiedemann 
Foville. Comparative anatomy is, howe 
more in accordance with the opinions 
Gall and Reil, that it is a commissure 
tween the convolutions of opposite sides. 
exists only in those animals in which con 
lutions are amply developed. In Fish, Rept 
and Birds it is absent, and the Mammalia 
least perfect brains, as the Rodents and I 
supialia. ‘% 
1e corpus callosum is a stratum of 
siderable thickness. Its fibres are situ 
different planes, which interlace 
other so much as to render it impossibl 
separate any layer for any distance, an 
difficulty is much increased as the fibr 
nearer the white mass of the hemisphere 
2. The anterior commissure may be re 
as truly a bond of connection betwe 
hemispheres, as well as between the ¢ 
striata. It is a cylindrical cord of 1 
matter, very definite in its course am 
nections, and easily traced throughout its 
extent. Its situation in front of the an 
~~ of the fornix has been already de 
f followed on either side from this « 
portion, it may be traced through th 
matter of the anterior and inferior por 
each corpus striatum into the fibrous 
of each middle lobe of the brain. Its 
is curved witli convexity directed fore 
As it passes outwards on each side it beet 
flattened, and after it has traversed each e¢ 
Striatum, it expands considerably and its 
fu 
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