634 
MR. W. H. FLOWER ON THE CEREBRAL COMMISSURES 
Its fibres principally connect, across the middle line, the parts of the cerebral hemi- 
spheres forming the inner wall of the middle horn of the ventricle, especially the folded 
part constituting the hippocampus major. As its free edge forms the hinder boundary 
of the region called the “psalterium” in human anatomy, the fibres composing it may 
be distinguished as the “ psalterial fibres” of the corpus callosum. At a little distance 
behind and rather lower than the point of the rostrum of the corpus callosum, is the 
very distinct oval outline of the section of the white “anterior commissure” (F), and 
between this and the under surface of the corpus callosum, and prolonged into the con- 
cavity of the genu, is a portion of the inner wall of the hemisphere (G) closing the 
lateral ventricle towards the middle line, and with the corresponding portion of the 
opposite side forming the median septum which divides the two cavities from each 
other, as will be better seen in the transverse section. This important region Professor 
Huxley has distinguished as the “septal area”*. 
To return to the upper arched border of the ventricular aperture. The middle part, 
which when united to the corresponding portion of the other hemisphere constitutes 
the “ body of the fornix” (K), is composed of a considerable number of white fibres 
closely adherent posteriorly to the under surface of the body of the corpus callosum, and 
running in a longitudinal direction. Tracing these fibres forwards, a small round white 
cord (L) is seen to pass down from them behind the anterior commissure, constituting 
the part commonly spoken of as the “ anterior pillar ” of the fornix, but which, to avoid 
confusion, had better be designated as the “column” of the fornix ( Columna fornicis, 
Reichert). The further course of this into the corpus albicans and optic thalamus need 
not be detailed here. But a large portion of the fibres (I) running forwards from the 
body of the fornix do not pass down into these cords, being continued above the anterior 
commissure, and then curve downwards in front of that structure to join the inner wall 
of the anterior lobe of the hemisphere. For these fibres the name of “ precommissural 
fibres ” has been suggested by Professor Huxley. The presence of the precommissural 
fibres, as well as that of much grey matter, gives to the lower part of the septal area a 
much greater thickness than the upper part (to which the name of “ septum lucidum ” 
is applied) possesses. But the two divisions of the area are perfectly continuous in 
structure, the upper thin part also containing fibres prolonged from the fornix, radiating 
forwards and upwards to the under surface of the corpus callosumf . 
Posteriorly the fibres of the fornix, following the border of the aperture they encircle, 
change their longitudinal direction, and gradually turn outwards, downwards, and finally 
forwards, and even slightly inwards. Although in their anterior and middle portions the 
fibres of the fornix run at right angles with the fibres of the corpus callosum, this change 
of direction in their posterior part brings them parallel to, and allows them to blend with, 
the transverse fibres of that body. The prominent sharp free margin of the ventricular 
aperture formed by the “ posterior pillars ” of the fornix is called “ corpus fimbriatum ” 
* Lectures at the Royal College of Surgeons, Medical Times and Gazette, March 5th, 1864. 
t See Solly ‘ On the Human Brain,’ 2nd Edit. 1847, p. 261. 
