. 
676 
The horizontal portion of the fornix, as ex- 
posed by this dissection, has the form of a 
triangle, the apex of which is directed forwards, 
and nds to the posterior angle of the 
septum lucidum. Its base is situate behind, 
and is enclosed by the posterior folded portion 
of the corpus callosum. The apex is prolonged 
into two rounded cords of fibrous matter, which 
pass downwards and outwards, in a somewhat 
curved course, with their convexity directed for- 
wards. These are the anterior pillars of the 
fornix. As they descend, they diverge from 
each other. We can follow them down to the 
base of the brain, where they form two small 
tubercles, the corpora mamillaria, from which 
fibres are continued upwards and outwards 
into the substance of the optic thalamus. 
The posterior pillars of the fornix are ex- 
pansions of fibrous matter which are continuous 
with the angles of the base of its horizontal 
esata These bands are continued into the 
ateral ventricle, and expand partly over the 
posterior horn, and partly over the hippocampus 
major in the inferior horn. The portion of the 
fornix which is thus continued into the inferior 
horn presents a fine concave edge directed in- 
wards, which is the corpus fimbriatum. 
It would thus appear that the fornix consists 
of a horizontal triangular portion (corpus for- 
nicis ) resting on four pillars, which take some- 
what of a curved course, and form numerous 
connections with deep-seated and important 
portions of the brain. The anterior pillars are 
closely connected with the optic thalamus, 
with the tuber cinereum, with the white matter 
which forms the floor of the ventricle. The 
posterior pillars are in intimate union with the 
posterior and middle lobes of the brain. 
The fibres of the fornix are distinctly longi- 
tudinal. So that, supposing it to be commis- 
sural in its office, it may be stated to connect 
the anterior lobe of the brain and the optic tha- 
Jamus with the posterior and middle lobes. 
The fornix is divisible into two equal and 
symmetrical portions, one belonging to each 
cerebral hemisphere. These portions are united, 
as has been a my stated, by the deep-seated 
transverse fibres of the corpus callosum, and 
by the terminal fibres of its posterior reflexion, 
which form, on the inferior surface of the fornix, 
a peculiar appearance called the lyra. The 
transverse white fibres stand out in relief, cross- 
ing at right angles the proper fibres of the 
fornix. In many subjects, however, this ap- 
pearance is but faintly indicated. 
The horizontal portion of the fornix rests 
upon a triangular process of pia mater, which 
is introduced into the interior of the brain, at 
the fissure beneath the posterior reflexion of 
the corpus callosum. This process is the velum 
interpositum already described at page 635. 
The anterior pillars of the fornix bound in 
front a space in which the velum interpositum 
and choroid plexuses unite, and through which 
the lateral ventricles communicate with each 
other. This is the foramen commune anterius, 
described by the first Monro.* If a probe be 
* But previously recognised and described by 
Vieussens. 
NERVOUS SYSTEM. (Nervous Cenrnes. Tar Enceruaton.) 
a 
laid transversely in this orifice, it will have’ 
above it the anterior extremity of the for 
in front the anterior pillars, and behind it th 
point of junction of the three processes of pi 
mater. fi 
Of the third ventricle —If the fornix be di- 
vided transversely at about its middle, and the 
segments reflected, and if the velum inter 
positum be removed, a fissure, the third vei 
tricle, is exposed, situate between the opti 
thalami. This fissure extends forwards betwet 
the anterior pillars of the fornix, where it | 
limited by a band of white matter visible wit 
out dissection in that interval. That band 
the anterior commissure, which lies just in fro 
of, and as a tangent to the convex border ¢ 
the anterior pillars of the fornix. a 
At its posterior extremity the third ventric 
becomes very much contracted in all its dimer 
sions, and is continuous with a canal whiel 
leads to the fourth ventricle (iter eta 
it). The 
quartum ventriculum, Aqueductus Sylv 
orifice of this canal is apparent at the 
extremity of the third ventricle, and is bound 
superiorly by a transverse cord of white matt 
the posterior commissure, which extends for 
short distance into the cerebral matter on eithe 
side. The base of the pineal gland rests upoi 
this commissure. 
In this stage of the dissection, a joes rie’ 
of the third ventricle is gained. This cavit 
evidently results from the apposition of th 
lateral halves of the brain proper, the pai 
which more immediately correspond being t 
inner surfaces of the optic thalami. The dept! 
of the ventricle corresponds, in a great degre 
to that of these bodies; but it manifestly i 
creases towards the anterior extremity. Its floc 
is formed by a layer of grey matter continue 
from one side to the other, of the same natu 
as that which has been already described a 
covering the thalami. The de part of the 
ventricle is an infundibuliform depression, fron 
which the tubular process, seen at the base of tl 
brain (fig. 382, h is continued down to tl 
pituitary body. Just beyond this part is 
anterior extremity of the ventricle, situate 
tween the anterior pillars of the fornix a 
behind the anterior commissure; the de 
which is much less than that of the in 
bulum. 1 
The floor of the third ventricle correspo 
to several parts of interest which have b 
enumerated along the middle of the bas 
the brain. Corresponding to the posteri 
tremity of the ventricle is the interval bety 
the crura cerebri, the pons Tarini, or 1 
peduncular space. Next in order, in the dire 
from behind forwards, are the m 
laria, which are succeeded by the cine? 
and commissure of the optic nerves. The 
terior extremity of the ventricle correspot 
that portion of the tuber cinereum which 
tends between the optic commissure anc 
anterior reflection of the corpus callosum. — 
The roof of the third ventricle is formed 
the velum interpositum, already describe 
giving support to the horizontal portion of 
rnlx. 7 
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