OE THE MAESUPIALIA AND MONOTEEMATA. 
635 
(M). A little way external and parallel to this, on the surface of the hemisphere, is a 
deep sulcus, corresponding in direction and extent with the hinder third of the ventricular 
aperture. This is the “ dentate ” or “ hippocampal ” sulcus (Q). It terminates above 
under the posterior end of the corpus callosum. If the cortical grey matter of the hemi- 
sphere is traced from the external border of the hemisphere towards the ventricular 
aperture, it will be found to dip down into this sulcus, and rising again to the surface to 
terminate abruptly just external to the corpus fimbriatum. The free border in which it 
terminates, lying between the “ hippocampal sulcus ” and the “ corpus fimbriatum,” is 
called the “ fascia dentata ” (P), its surface being generally somewhat notched or indented 
at' intervals. The cerebral wall folded inwards at the sulcus just described, forms a cor- 
responding projection in the cavity of the ventricle called the “hippocampus major.” 
The relation of some of the parts above mentioned will be better understood by a 
reference to Plate XXXVI. fig. 2. It is drawn from a vertical transverse section of the 
human brain, at the point indicated by the line drawn across Plate XXXVI. fig. 1, viz., 
through the middle of the anterior commissure. B is the corpus callosum, passing from 
hemisphere to hemisphere, across the bottom of the great longitudinal fissure*. As its 
fibres pass outwards from the middle line, they curve slightly upwards before separating 
to radiate throughout the medullary substance of the hemispheres. Immediately under- 
neath the corpus callosum lie the cavities of the hemispheres or “ lateral ventricles,” com- 
pletely separated from each other in this section by a septum (G), attached above to the 
under surface of the corpus callosum, and below resting on the small transverse “anterior 
commissure” (F). This part, the “septal area” of the former section, may be demonstrated 
to consist throughout of two lateral portions, applied closely together in the middle line 
below, but in the upper part slightly separated, the interval constituting the fifth ventricle, 
or ventricle of the septum lucidum. The lower part of the septum, much thicker than 
the septum lucidum, contains the precommissural fibres of the fornix with much grey 
matter interposed. It seems never to have received any special name, or to have been 
sufficiently distinguished from the septum lucidum, although it is the most constant, and 
therefore important division of the septal area, as will be shown hereafter. The grey 
masses (B, R) forming the outer boundaries of the ventricles are the “ corpora striata.” 
The anterior commissure is seen as a small cylindrical bundle of white fibres (F) passing 
between the corpora striata. 
The true nature of these parts cannot be perfectly understood without a glance at their 
development. This is a subject confessedly still involved in some obscurity. I follow, 
however, the observations of F. Schmidt, who has given a detailed and apparently truthful 
account of the process^. Without entering into previous changes, it may be stated that 
each hemisphere consists, in a very early condition, of a hollow thin-walled body, with a 
fissure (O) in its inner surface, leading to the cavity within (Plate XXXVI. fig. 3, 1). 
* “ — the cross portion of white substance which lies between the hemispheres at the bottom of the longi- 
tudinal fissure,” Qttain and Shaepey’s ‘ Anatomy,’ 5th edit. vol. ii. p. 464. 
f Zeitschrift fur Wissenschaftliche Zoologie, vol. xi. (1861) p. 43. 
4 S 2 
