- COMMISSURAL FIBRES. 547 
the form of a horse-shoe, the concavity of which looks backwards. ‘The middle free 
portion is placed immediately in front of the anterior pillars of the fornix as they 
curve downwards, and also in intimate relation to the anterior end of the third 
ventricle. Posteriorly, the small portion of the anterior commissure which appears 
in the ventricle between the two pillars of the fornix is clothed with the ventricular 
ependyma ; anteriorly, the commissure is connected with the lamina cinerea as it 
stretches from the optic chiasma upwards towards the rostrum of the corpus 
callosum. 
The lateral part of the anterior commissure penetrates the cerebral hemisphere, 
and, gaining the lower part of the internal capsule, divides into two portions, viz. 
a small lower olfactory part and a much larger temporal part. 
The olfactory portion of the anterior commissure is an exceedingly small fasci- 
culus. It passes downwards and forwards, and finally enters the olfactory tract. 
It is composed (1) of true commissural fibres, which bind one olfactory bulb to the 
other; and (2) of other fibres, which connect the olfactory bulb of one side with the 
temporal lobe of the other side. 
Ventricle V. Corpus callosuin 
Ae y/ Lateral ventricle 
Corpus callosum 
aN 
Lateral ventricle 
Wy 
Internal 
capsule 
ae Putamen 
Internal fi 
capsule . 
= Uf, 
Temporal Lf ay 
| Globus 
x ] pallidus 
/ 1 
by /— Claustrun 
Ms Fronto- 
part (aya | an parietal 
Z pear operculum 
wy — Insula 
go) Temporal 
operculum 
ex 
Olfactory = 
part x ye 
Anterior commissure 
~~ Anterior 
commissure 
Fria. 407.—Two CoronaL SECTIONS THROUGH THE CEREBRAL HEMISPHERES OF AN ORANG, IN THE PLANE 
OF THE ANTERIOR COMMISSURE. 
A, Section through the left hemisphere in a plane a short, distance behind B, which is a section through the 
right hemisphere. 
The temporal portion is formed of almost the whole of the fibres of the com- 
missure. It is carried transversely outwards, under the lenticular nucleus, until it 
gains the interval between the globus pallidus and the putamen. At this point it 
changes its direction and sweeps backwards. In coronal sections through the 
brain, behind this bend, the temporal portion of the anterior commissure appears as 
an oval bundle of fibres cut transversely and placed in close contact with the under 
surface of the lenticular nucleus (Fig. 403, p. 540). Finally, it turns sharply down- 
wards on the outer aspect of the amygdaloid nucleus, and its fibres are lost in the white 
medullary centre of the temporal lobe. The precise part of the cerebral cortex with 
which these fibres stand in connexion is not known. When the lateral part of the 
anterior commissure is displayed by dissection, it is seen to be twisted like a rope. 
The psalterium, or the hippocampal commissure, is composed of fibres which con- 
nect the cornu ammonis of one side with the corresponding structure of the opposite 
side. It is described on page 531. 
Association Fibres.—The association fibres bind together different portions of 
