ox THE BRAIN. 
521 
being. The corpus striatum is visible again at the base of the 
brain, where it evidently becomes one of the origins of the ol¬ 
factory nerve, as well as the commencement of the mammillary 
process. In the rodentia and edentata the corpora striata are 
much larger than they are in the carnivora, the ruminantia, or 
the solipeda. In the bird they are larger still, and the olfactory 
nerve plainly arises from their anterior extremity. The corpus 
striatum varies in form in the different species of birds, but in 
almost all of them it constitutes nearly the whole of the hemi¬ 
sphere. 
The Hippocampi. —The convex body which occupies the inner 
and posterior portion of the floor is the hippocampus —narrower 
and more prominent than the corpus striatum, and consisting of 
cortical or striated matter, covered by a thicker lamina of medul¬ 
lary substance. As this body proceeds superiorly, outwardly, 
and presently downwards, and narrows as it goes, it assumes the 
name of the crus hippocampi, and it descends into a blind pouch 
or hollow—the superior cornu—where it ends in a distinct bulb, 
the pes hippocampi. In none of our domesticated animals have 
I observed the ventricle in the pes hippocampi which exists in 
the human being. The medullary covering is derived from the 
corpus callosum ; the striated portion is obtained from the very 
centre of the hemispheres. 
The Corpus Fimhriatum. —Along the side of the hippocam¬ 
pus, of considerable width anteriorly, and'narrowing as w^e trace 
it posteriorly, is the corpus Jimhriatiim, of which I have already 
spoken, skirted by the plexus choroides, and partly covering it. 
It is the margin of the fornix prolonged. It enters wdth the 
hippocampus into the posterior cornu, and terminates acutely 
near the pes hippocampi. 
The Plexus Choroides.^ Occupying the channel between the 
hippocampus and the corpus striatum, we find the plexus cho¬ 
roides, a considerable collection of minute bloodvessels, both 
arterial and venous, enveloped in folds of the pia mater. It en¬ 
ters, or the arteries of which it is composed enter, the ventricle 
under the arch of the fornix, then taking a direction anteriorly, it 
speedily terminates in a kind of bulb ; but, being reflected back¬ 
wards, a portion skirts the whole edge of the corpus fimbriatum, 
and dips deeply into the superior cornu, where it also terminates, 
but rather acutely. After it has discharged some important 
office within the ventricle, the veins escape by the same open¬ 
ing ; and those from the opposite ventricles uniting form the 
vena Galeni. There can be little doubt, I apprehend, with re¬ 
gard to the function of this plexus. It secretes the fluid con¬ 
tained in the ventricles. It is occasionally subject to disease; a 
