THE TELENCEPHALON. 



1165 



collateralis), that extends for a variable distance along the outer part of the floor 

 of the inferior horn. This elevation is uncertain as to prominence and length, but 

 even when well developed does not reach the lower extremity of the ventricle. 

 It results from the invagination of the wall of the early hemisphere by the anterior 

 part of the collateral fissure. 



A second longitudinal elevation, constant and much more conspicuous than the 

 collateral eminence and separated from the latter by a groove, forms the inner part of 

 the floor and the adjoining mesial wall of the inferior horn of the lateral ventricle. 

 This elevation, known as the hippocampus, is the most prominent feature of the 

 horn and curves downward and inward to the extreme lower limit of this part of the 

 ventricle. It is due to the early invagination of the hemisphere by the hippocampal 

 fissure. The lower end of the hippocampus is distinctly broader and somewhat 

 flattened and marked by a number of oblique shallow furrows and intervening low 

 radiating ridges (digitationes hippocampi). These confer on the upper surface and 

 especially on the outer rounded border of the elevation, a corrugated and notched 

 appearance, (Fig. 1004) which suggests a fancied resemblance to a paw, the lower 

 end of the projection being 



known as the pes hippo- FIG. 1004. 



campi. The upper surface 

 and the anterior and lateral 

 border of the pes are free 

 and well defined, but its 

 deeper surface and inner 

 border, to a large extent, are 

 blended with the surround- 

 ing parts of the hemisphere. 

 The minute structure of the 

 hippocampus is described 

 with that of the cerebral 

 cortex ( page 1 1 8 1 ) . 



The dorso-mesial aspect 

 of the hippocampus is over- 

 laid by a white flattened 

 band, the fimbria (fimbria 

 hippocampi), which, although 

 bearing a special name, is 

 the direct prolongation of the 

 posterior crus of the fornix, 

 continued from the lateral 

 angle of the corpus fornicis 

 into the inferior horn. Its 

 mesial 



Pes hippocampi 



Cyrus hippocampi 

 Fimbria 



ps posterior 



concave 



margin is 



Trijjonum Calcar 

 collaterale avis 



Inferior horn of left lateral ventricle, viewed from above. 



smooth, rounded and free, 



whilst its sinuous lateral border is thin and sharp and gives attachment through- 

 out its entire length to the delicate ependymal layer which completes the mesial 

 wall and thus closes in the descending horn (Fig. 1005). Above narrow and then 

 broader, on reaching the pes the fimbria becomes abruptly reduced to a narrow 

 strand, which may be followed along the inner margin of the pes to the uncus 

 where it ends. Traced upward the fimbria passes without interruption into the 

 posterior limb of the fornix, of which, as already noted, it is the direct downward 

 prolongation. Beginning in the uncus, the fimbria continually receives accessions 

 of fibres from the underlying hippocampus, with which it is closely united along 

 its deep surface, and therefore increases in bulk as it ascends towards the body 

 of the fornix. 



When the structures within the inferior horn of the lateral ventricle are viewed 

 in their undisturbed relations (Fig. 1002), little of the hippocampus and nothing of 

 the fimbria are seen, as these parts are hidden by the overlying mass of vascular tissue 

 constituting the choroid plexus, which is not confined to the body of the ventricle,' 

 where its connections have been already described, but follows the descending 



