LATERAL VENTRICLE. 535 
jacent collection of gray matter termed the amygdaloid nucleus. The tenia semi- 
circularis and the attenuated tail of the caudate nucleus are both prolonged into the 
descending horn and are carried forwards, in its roof, to the amygdaloid nucleus. 
On the floor of the descending horn the following structures are seen: (1) 
BULBUS CORNU BULBUS CORNU 
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Fic. 397.—CoroNAL SECTION THROUGH THE POSTERIOR HORNS OF THE LATERAL VENTRICLES. 
hippocampus major, or the cornu ammonis; (2) the choroid plexus; (3) the fimbria ; 
and (4) the eminentia collateralis. 
The hippocampus major (hippocampus) is for the most part covered by the 
choroid plexus. It is a prominent elevation on the floor of the descending horn of 
the lateral ventricle and is strongly curved, in conformity with the course taken 
by the horn in which it lies. It therefore presents an internal concave margin and 
an external convex border. Narrow behind, it enlarges as it is traced forwards, 
and it ends below the amygdaloid tubercle in a thickened extremity, which 
presents some faint grooves or notches on its surface. In consequence of this, the 
anterior end of the hippocampus major receives the name of the pes hippocampi. 
The hippocampus major is the internal elevation which corresponds to the dentate 
fissure on the exterior of the hemisphere (Fig. 599). 
The fimbria (fimbria hippocampi) is the narrow band of white matter which is 
attached by its outer margin along the inner concave border of the hippocampus 
major. The white matter composing it 1s continuous with the thin white layer 
(the alveus) which is spread over the surface of the hippocampus major, and it 
presents two free surfaces and a sharp free inner border. The fimbria has already 
been examined in connexion with the hippocampal fissure and the gyrus dentatus 
(p. 527), and the relations which it presents to the fornix and the uncus have been 
pointed out. 
When the pia mater in the region of the hippocampal fissure is removed from 
the surface of the brain, the choroid plexus in the interior of the descending horn 
of the lateral ventricle is usually withdrawn with it, and a fissure appears between 
the fimbria and the roof of the ventricular horn. This is the choroid fissure. It 
appears at a very early date in the development of the cerebral hemisphere, and 
takes an arcuate course upwards and forwards round the hinder end of the optic 
thalamus. In the region of the body of the lateral ventricle it extends as far 
forwards as the foramen of Monro, and is formed by the involution of an epithelial 
part of the wall of the ventricle over the choroid plexus (p. 533). In the region of 
the descending horn, when the choroid plexus with the involuted epithelial layer 
which covers it is withdrawn, the choroid fissure is converted into an artificial gap 
which leads directly into this part of the ventricular cavity. 
The choroid plexus is a convoluted system of blood-vessels in connexion with a 
fold of pia mater, which is prolonged into the descending horn of the lateral 
