George L. Streeter 303 
separans.” The ala cinerea bounded by the funiculus separans pos- 
teriorly, and arising with it from the median line out of the central 
canal, extends forward, more or less triangular in shape, wedging itself 
in between the area acustica and the trigonum hypoglossi. It ends an- 
teriorly with a pointed tip in the region of the striae medullares. Its 
surface as a whole is depressed, and forms a shallow pit, the floor of 
which is slightly convex. 
Lying median to the ala cinerea is the hypoglossal field, arising out of 
the central canal by a slender stile or wedge, gradually broadening, and 
ending in the region of the striae. In the foetus and yet more evident 
in the adult the trigonum hypoglossi is resolved longitudinally into 
two fields, of which the one lying on the median side is the narrower. 
The boundary between them consists, as a rule, of a single or double 
formation of numerous short oblique furrows and ridges, or a series of 
wrinkles of the surface of the floor, affording a “ feathered ” appearance. 
This is brought out strongly by magnification under water with a simple 
lens. This peculiar wrinkling occurs also between the lateral division 
of the hypoglossal field and the ala cinerea, giving the appearance of 
a “bird feather.” Retzius therefore designates this lateral field as the 
“Area plumiformis.” The narrow field, median to the area plumifor- 
mis, he calls the “Area medialis trigoni hypoglosst.” Near the middle 
of this field there often is a slight enlargement and elevation—the 
“ Eminentia medialis trigont.” 
The area acustica is an irregularly triangular or quadrilateral raised 
surface with its convex base toward the median line, and extending 
laterally to the insertion of the tela choroidea inferior, and into the 
recessus lateralis. Its superior or frontal portion is usually covered by 
the striae medullares, with the consequent irregular elevations. Its 
inferior end extends caudally as far as the area postrema, and forms 
the lateral boundary of the ala cinerea. The acustic area is more promi- 
nent in the foetus, and forms a projection or tubercle, which Schwalbe 
designated the “ Acustic tubercle.” Later Dejerine (Anatomie des Cen- 
tres Nerveux, 1901, Tome Deuxieme, p. 498) has suggested the limit- 
ing of this name to the nucleus of termination of the cochlear nerve, 
which properly corresponds to the tuberculum acusticum of mammals 
other than man. 
Regarding the superior part of the floor of the ventricle, Retzius has 
found little that is new. He describes a “ Fovea mediana ”—a depres- 
sion of the median sulcus slightly frontal to Arnold’s eminentia teres. 
The superior fovea and locus coeruleus in his description do not differ 
from what we have already learned from Arnold, Stilling, and Clarke. 
