LOBUS OLFACTORIUS POSTERIOR.
The substantia perforata anterior is an oblique quadrangular field lying behind the trigonum olfactorium, between the latter and the tractus opticus. It exhibits numer ous small openings for the passage of blood vessels, especially in its anterior part in the vicinity of the trigonum. This front part constitutes the substantia perforata anterior proper, the gyrus perforalus rhinencefihali.
The posterior part, bordering the tractus opticus, differs from the anterior chiefly in its lighter color and smoother surface. This part is known as the diagonal band of Broca, or the gyrus diagonalis rhinencephali.
Gyrus perforatus and gyrus diagonalis form the essential features of the lobus olfactorius posterior. To the latter belongs an additional small field on the medial sur face of the hemisphere, the gyms subcallosus of Zuckerkandl. This field is easily located, since it is continuous with the diagonal band of Broca, lying behind the area parolfactoria, separated from the latter by the sulcus parolfactorius posterior, and in front of the commissura anterior and the lamina rostralis (Fig. 18).
The gyri subcallosi (the stalks of the callosum of Broca) descend close together from the rostrum of the corpus callosum. They are separated from
each other by a medial furrow, the sulcus subcallosus median us of Retzius, and form the narrow tri gonum firaecommissurale, which lies in front of the anterior commissure and belongs to the lamina firaecom missuralis. The latter is a thin lamella that covers the anterior com missure and passes over into the lamina terminalis. At the lower border of the precommissural tri gone, the two gyri subcallosi diverge at almost right angles and proceed, on each side, outward and backward along the optic tract as a white band, the diagonal band of Broca, to reach the front end of the gyrus hippocampi.
Broca's band is distinguished by its lighter color from the deeper tinted sub stantia perforata anterior ; further, the disposition and form of the vascular foramina are characteristic. These are oval or elliptical, their diameters paralleling the axis of Broca's band. Although the band is always present, it is not always plainly visible, in some cases it being superficial only in certain places, while in others it is buried beneath a layer of gray substance that must be removed before the band is seen.