Beginning at the trigonum collaterale (Figs. 45 and 49) and extending along the outer border of the floor to the end of the inferior horn there is sometimes a low ridge caused by the collat eral fissure. It is the eminentia collateralis, and is present only when the anterior part of the collateral fissure, as well as the middle part, is a complete fissure. The short eminence at the entrance to the inferior horn, called the trigonum collaterale, is constant in its presence; it is produced by the middle division of the collateral fissure. In front of this eminence and medial to it is a prominent ridge, the hippocampus, which enlarges downward to a lobulated extremity, the pes hippocampi, divided by shallow grooves into several digits (digitationes hippocampi, Fig. 45). The ridge is due to the large development of olfactory cortex along the chorioidal fissure. The hippocampal sulcus is a very superficial groove between the hippocampal gyrus and the dentate fascia; it is parallel with the hippocampus, but it has nothing to do with its formation. The hippocampus was formerly considered an invagination due to the hippocampal fissure (G. Elliot Smith).
The ventricular surface of the hippocampus is formed by a lamina of white matter, the alveus, but the deeper part is cortical matter composed almost entirely of pyramidal cell bodies. The crus of the fornix (fimbria hippocampi) rests in the concavity of the hippocampus, where most of its fibers originate, though a small bundle of them originates beyond it in the uncus.
The chorioid epithelium (lamina chorioidea epithelialis) (Figs. 37, 46 and 85), representing the hemisphere wall, forms the floor of the chorioidal fissure and the whole medial wall of the inferior horn. It covers the cushion-like projection (the pul
vinar) of the thalamus, which forms a small part of .both roof and inner wall. Behind, it is attached to the crus of the fornix, from which it extends forward to the stria terminalis. The epithelium covers the chorioidal fissure except at the lower part, where there is a small cleft which forms a communication be tween the horn and the anterior subarachnoid space. Through the chorioidal fissure a fold of pia mater projects toward the ventricle, and pushing the epithelium before it into the horn, forms the chorioid plexus of the inferior horn (Figs. 46 and 47). The chorioid plexus of the inferior horn is continuous with that in the body of the ventricle, both lying in the chorioidal fissure.
In connection with the medial wall of the inferior horn, mention should be made of the hippocampal formation. It is de veloped along the convexity of the inferior part of the chorioidal fissure from which it is separated by the crus fornicis and its filamentous extension, the fimbria hippocampi. The hippo campal formation comprises the dentate fascia, the hippocampus and the hippocampal gyrus. Its resemblance to the "sea horse" can be seen only in section. Behind the crus fornicis and below the fimbria lies the dentate fascia, already described (p. 82) which in frontal section constitutes the nose of the horse; the hippocampus forms the top of the head and the arched neck; the hippocampal gyrus represents the front of the neck and chest. The hippocampal formation is like the letter S. In man it includes almost the entire olfactory cortex (Fig. 85).