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FORNIX AND SEPTUM LUCIDUM. dol 
The anterior pillars of the fornix (column fornicis) are two rounded strands 
which emerge from the anterior part of the body of the fornix, and then diverge 
very slightly from each other as they curve downwards in front of the foramina of 
Monro. Sinking into the gray matter on the lateral wall of the third ventricle, 
each anterior pillar proceeds downwards to the base of the brain, and in the inter- 
peduncular space protrudes, to take part in the formation of the corpus mammillare. 
When the corpus mammillare is dissected it appears to be largely formed of a 
twisted loop of the anterior pillar of the fornix, in which the pillar bends upon 
itself, and is then continued upwards and backwards into the optic thalamus. 
This appearance, however, is misleading. The fibres of the anterior pillar end in 
the gray nucleus of the corpus mamumillare, and the strand which passes from this 
to the thalamus is the bundle of Vicq d’Azyr (p. 507). 
The posterior pillars of the fornix (crura fornicis) are flattened bands which 
diverge widely from each other. At first they are adherent to the under surface 
of the corpus callosum, but soon they sweep downwards round the posterior ends of 
the optic thalami and enter the descending horns of the lateral ventricles. Here 
each pillar comes into relation with the corresponding hippocampus major, and a 
portion of its fibres are spread out on the surface of this prominence, forming the 
alveus, whilst the remainder constitute the fimbria or tenia fornicis—a narrow but 
very distinct band of white matter, which is attached by its outer margin along the 
inner border of the hippocampus major and ends in front by joining the uncus 
(p. 527). 
A certain number of transverse fibres enter into the formation of the fornix. 
The diverging posterior pillars enclose between them a small triangular space on 
the under surface of the hinder part of the corpus callosum. This area is crossed 
by transverse fibres, which form a thin lamina called the psalterium or lyra. Some- 
times the psalterium is not completely fused to the under surface of the corpus 
callosum, and in these cases a narrow space is left between them, which receives the 
name of Verga’s ventricle. 
The fornix is intimately connected with the olfactory apparatus. Its fibres, for the most part, 
arise from the pyramidal cells in the cornu ammonis or hippocampus major and ascend in the 
fimbria and posterior pillar. In the region of the psaltertum numerous fibres cross the mesial 
plane, enter the opposite posterior pillar, and in it proceed to the opposite cornu ammonis. 
These fibres constitute a commissure between the two cornua ammonis. The remainder of the 
fibres proceed forwards in the body of the fornix, and by means of the anterior pillar the 
majority of the fibres are carried downwards, behind the anterior commissure, to the corpus 
mammillare. Some, however, curve backwards into the stria medullaris (p. 506); whilst others, 
forming the olfactory bundle of the cornu ammonis, pass in front of the anterior commissure and 
enter the septum lucidum, through which they reach the subcallosal gyrus and the locus per- 
foratus anticus. Finally, this bundle divides into two parts, of which one joins the inner root of 
the olfactory tract, whilst the other goes to the uncus. 
The greater number of the longitudinal fibres of the fornix must, therefore, be regarded as 
establishing a connexion between the cornu ammonis and the optic thalamus. The nucleus of 
the corpus mammillare is an internode interposed in the path of this connecting tract. The 
bundle of Vicq d’Azyr, formed by the relay of fibres which takes origin in this internode, forms 
the second link in the chain. 
The striz longitudinales on the upper surface of the corpus callosum are to be regarded, from 
a morphological point of view, as forming an outlying part of the fornix system. 
Septum Lucidum (septum pellucidum).—This is a thin vertical partition which 
intervenes between the anterior cornua and foreparts: of the bodies of the two 
lateral ventricles. It is triangular in shape, and posteriorly it is prolonged back- 
wards for a variable distance between the body of the corpus callosum and the 
fornix, to both of which it is attached by its upper and lower edges. In front it 
occupies the gap behind the genu of the corpus callosum, whilst below, in the 
narrow interval between the posterior edge of the rostrum of the corpus callosum 
and the fornix, it is prolonged downwards towards the base of the brain in the 
gyrus subcallosus. The septum lucidum is composed of two thin lamin in appo- 
sition with each other in the mesial plane (Fig. 396, p. 534). 
Fifth Ventricle (cavum septi pellucidi).—This is the name which is applied to 
the mesial cleft between the two lamine of the septum lucidum. It varies greatly 
in size in different brains and contains a little fluid. It is completely isolated and 
