Pe as on =) ae 
530 THE NERVOUS SYSTEM. 
It is termed the genu. ‘The recurved lower part of the genu is separated from the 
part of the corpus callosum, which les above, by an interval. It rapidly thins as 
it passes backwards and receives the name of the rostrum. The fine terminal edge 
of the rostrum becomes connected with the lamina cinerea. 
The gyrus supracallosus, wit h its contained medial and lateral longitudinal strive, when traced 
backwards, is seen to turn round the splenium and become continuous with the gyrus dentatus. 
In front the mesial striae, with the associated gray matter, are carried round the genu and then 
backwards on the under surface of the rostrum. As they turn round the genu they diverge from 
each other, and are termed the gyri geniculi. Reaching the hinder edge of the rostrum, each 
eyrus geniculus runs into the gyrus subcallosus—a narrow cortical strip on the mesial surface of 
the hemisphere, which runs downwards immediately in front of the lamina cinerea to the locus 
perforatus anticus. The gyrus subcallosus is often called the peduncle of the corpus callosum, and 
the fibres of the stria which it contains proceed backwards and outwards along the posterior limit 
of the anterior perforated spot to the anterior extremity of the temporal lobe. Dr. Elliot Smith 
has shown that these parts have an important morphological significance. 
The wnder surface of the corpus callosum on either side of the mesial plane is 
for the most part free, and, lined by ependyma, it forms the roof of the anterior 
horn and body of the lateral ventricle. In the mesial plane, however, it is attached 
to subjacent parts, viz. to the septum lucidum in front and to the body of the — 
fornix behind. 
The transverse fibres of the corpus callosum, as they enter the white medullary 
centre of the cerebral hemisphere, radiate from each other so as to reach every part 
of the cerebral cortex. This radiation is termed the radiatio corporis callosi and 
the fibres which compose it intersect the fibres which form the corona radiata, or, in 
other words, the fibres which extend between the internal capsule and the cerebral 
cortex (Figs. 375, p. 505; and 4035, p. 540). The more anterior of the fibres which 
compose the genu of the corpus callosum sweep forwards in a series of curves into 
the prefrontal region of the hemisphere. A large part of the splenium, forming a 
solid bundle termed the forceps major, bends suddenly and abruptly backwards 
into the occipital lobe (Fig. 598, p. 536). Fibres from the body and upper part 
of the splenium, curving round the lateral ventricle, form a very definite stratum, 
called the tapetum. This is a thin layer in the medullary centre of the hemisphere, 
which constitutes the immediate roof and outer wall of the posterior horn and the 
outer wall of the hinder part of the descending horn of the lateral ventricle. In 
coronal sections through the occipital and hinder temporal regions the tapetum 
stands out very distinctly (Figs. 391, p. 527; 397, p. 535; and 400, p. 538). 
Fornix.—The fornix is an arched bilateral structure composed, for the most 
part, of longitudinally-directed fibres. In its intermediate part its two lateral 
halves are joined together in the mesial plane, and form what is called the body of 
the fornix; but in front and behind they are quite separate, and constitute the 
anterior and posterior pillars of the fornix. | 
The body of the fornix is triangular in form. In front, where it 1s continuous 
with the anterior pillars, it is narrow; whilst behind it broadens out, becomes 
flattened, and is finally prolonged into the posterior pillars. The upper surface of 
the body of the fornix is in contact with the under surface of the hinder part of 
the body of the corpus callosum, and posteriorly is adherent to it. In front of this, 
and in the mesial plane, it is attached to the 
septum lucidum. Beyond these attachments, on 
hae each side, the upper surface of the body of the 
commissure ~ aE ie Temes fornix forms a part of the floor of the lateral 
~ ventricle and is clothed by the hning ependyma. 
It presents a sharp lateral edge or margin, from 
under which the choroid plexus projects into the 
Aho aes pee vw cavity of the lateral ventricle and with which 
+, 393.—DIAGRAMMATIC PROFILE VIEW : : . j 
a aa ee the epithelial layer which covers that plexus is 
continuous. The lower surface of the body of 
the fornix rests upon the velum interpositum, which separates it from the roof of 
the third ventricle and the upper surface of the optic thalami. It is not at all 
an uncommon occurrence to find the two lateral portions of the body of the fornix 
of unequal size (Fig. 396, p. 534). 
Foramen of 
Monro 
