636 
ME. W. H. FLOWER ON THE CEREBRAL COMMISSURES 
Through this a portion of the pia mater (afterwards developed into the choroid plexus) 
enters. The fissure is at first perpendicular in direction. In front of it (at G) the two 
hemispheres are united across the middle line, immediately behind it (A) they are con- 
nected with the parts formed by the second cerebral vesicle, the subsequent optic thalamus 
and crus cerebri. The last-named point (the crus or “ hirnstiel”) forms a pivot around 
which the whole hemisphere curves itself as development proceeds. The fissure under- 
goes a corresponding change of form and direction. The anterior edge becomes its 
upper convex border. The upper end gradually becomes depressed until it is finally 
the lowest part, and the characteristic form of the ventricular aperture is already recog- 
nized at this early age (Plate XXXYI. fig. 3, III). The point of union between the 
hemispheres is still confined to the part immediately in front of the anterior end of the 
fissure, the “ septal area.” About this time the wall of the hemisphere commences to 
undergo a folding upon itself, producing certain definite grooves or sulci on the outer 
surface, and corresponding elevations upon the interior. At a very early period an 
arched sulcus (bogenfurche) appears parallel to the upper border of the fissure, marking 
off an arched convolution or gyrus between it and the fissure, the “ marginal arch” 
( randbogen , Schmidt). It is the hinder part of this groove which afterwards forms the 
“hippocampal sulcus.” Into the further development of the convolutions and sulci it 
is unnecessary to enter. A more important subject in connexion with the present com- 
munication is the mode of formation of the corpus callosum, the fornix, and adjacent 
parts. Kolliker* has given so good an abridgement of Schmidt’s views, that I have 
thought it best to follow pretty closely his words. 
The convolutions of the hemispheres are distinctly seen from the third month to 
consist of two layers, an external with perpendicular fibres, which at a later period con- 
stitutes the grey or cortical substance of the convolutions, and an inner layer with fibres 
running horizontally. The fibres of the inner layer, constituting the medullary substance 
of the hemispheres, are found already in the third month, before the corpus callosum 
exists, to converge towards two points ; first, towards the crus ( hirnstiel , A), where they 
form the so-called stabJcranz ; and secondly, towards a point situated immediately above 
the place of union of the two hemispheres. This last arrangement of fibres is the first 
indication of the radiation of the corpus callosum ( balkenstrahlung ). It is at this 
spot (B) that in the fourth month the horizontal fibres break through the cortical 
substance and unite with the corresponding fibres of the opposite hemisphere. 
This is the commencement of the corpus callosum, which in its earliest form (see 
Plate XXXVI. fig. 3, IV) is a very small nearly cylindrical commissure, situated in the 
“marginal arch ” immediately above the most anterior part of the ventricular aperture. 
In order to indicate more closely the relation of the marginal arch to the corpus callo- 
sum, it is to be noticed that the former separates into two parts, a lower division imme- 
diately bordering the ventricular aperture, consisting only of horizontal or antero-pos- 
terior fibres, without the cortical layer, and an upper division possessing both layers. 
* Entwicklungsgeschichte des Menschen und der hoheren Thiere, p. 237, Leipzig 1861. 
