NERVOUS SYSTEM. 319 



thin lamina of medullary matter ; but this is partly obscured 

 by the cortical substance of which their interior is exclusively 

 composed. 



Hippocampi. — The two superior eminences, smaller and whiter 

 than the last, whose anterior extremities lie between the pos- 

 teiior of the corpora striata, are the hippocampi. These bodies, 

 remarkable for their prominence and whiteness, occupy the su- 

 perior spaces of the ventricles, where they lie in contact with the 

 septum ; but, in proceeding backward, they diverge, stretch first 

 outward, then downward, and lastly forward ; in fact, they de- 

 scend into the superior cornua, and there end in bulbous ex- 

 tremities, called the pedes hippocampi. This descent or elonga- 

 tion, which may be denominated the cms hippocampi, is continu- 

 ous above with the crus fornicis, but below it gets to the outward 

 part of the crus fornicis, and gives off a thin, plaited, medullary 

 border, which may be said to be an extension of the corpus 

 fimbriatum. Their medullary covering appears to be continued 

 from the corpus callosum ; their internal cortical substance, 

 which is interspersed with medullary strite, originates from the 

 very middle of the hemispheres. If sliced, they will be found 

 to consist of alternate laminae of medullary and cortical matter. 



Plexus Choroides. — Deeply lodged in the channel between the 

 corpus striatum and hippocampus, lies a red, soft, vascular sub- 

 stance, consisting of a plexus or collection of minute bloodvessels, 

 invested in an extension of pia mater, called the plexus choroides. 

 This plexus first makes its appearance from behind the fornix : 

 inferiorly, it ends abruptly in a round bulbous mass; superiorly, 

 it sends down a process into the superior cornu. The arteries 

 composing it come from the posterior arteries of the cerebrum : 

 they enter the interior of the brain along a fissure, which in one 

 place is a complete canal, existing between the posterior lobes of 

 the cerebrum, and gain admission into the ventricles around the 

 sides of the fornix. Its veins assemble and form a large branch, 

 the vena Galeni, which branch unites with a similar one coming 

 from the opposite plexus : the two conjoined make a single 

 trunk, and that proceeds upward, along the above-mentioned 

 fissure. 



The fornix is that part which receives the posterior border of 

 the septum lucidum. It is extended, after the manner of an 

 arch, between the corpora striata, below, and the heads of the 

 hippocampi, above; where it forms a junction with the corpus 

 callosum, which it meets at an acute angle. It has four pro- 

 cesses or crura. The two inferior crura spring from the corpus 

 albicantium, at the base of the brain; in their course forward 

 from which, they approximate and unite into one main crus or 



