THE MAMMALIAN BRAIN 
8 l 
cortex from one gyrus to another, while other bundles converge 
medially to enter into the formation of the corpus callosum, and 
still others take a more ventral course which will be further 
demonstrated later. With great care dissect down to the level 
of the surface of the corpus callosum, changing the direction of the 
sectioning as is needed to follow the curve of its upper surface 
and to trace it laterally into each cerebral hemisphere. If suffi¬ 
cient care is used it will be possible to disclose the delicate lateral 
and medial striae which extend in an antero-posterior direction 
along its free surface. By scraping or tearing lightly with the 
forceps, the transverse direction of the bundles of fibers of which 
the corpus callosum is composed may be demonstrated. 
With great caution continue the horizontal sectioning until by 
cutting through the lateral regions of the corpus callosum, in 
each hemisphere, the central part of each lateral ventricle is laid 
open. Finally cut through and remove the body of the corpus 
callosum leaving only the anterior bundles of transverse fibers 
which form its genu, and the posterior bundles which form the 
splenium, and exposing the cut upper edge of the septum pellu- 
cidum, the relation of which as a thin vertical partition between 
the two lateral ventricles will now be evident. Locate again the 
external surface of the insula and in the subsequent dissection do 
not slice away any of its cortex. 
Continue the dissection by cutting narrow wedge-shaped 
portions of the cerebral hemisphere away as need be to remove the 
roof of the anterior cornu, and of the extensive descending cornu, 
and thus lay bare the whole floor of the ventricle. Study the 
whole cavity and its relationships with great care. Note that 
the central part lies near the midline and was roofed over by the 
corpus callosum, while the septum pellucidum forms its medial 
wall, and a rounded eminence, the corpus striatum, lies in its floor 
and lateral wall. Note that the lateral wall is very thick and 
that the convoluted cortical surface of the insula forms the external 
boundary of this lateral wall. The anterior cornu of the cavity 
curves around the corpus striatum and extends into the frontal 
and olfactory lobes. The descending cornu curves ventrally into 
the temporal lobe, where it ends in that portion of the hippo¬ 
campal gyrus known as the uncus. The extensive rounded 
