AMERICAN NEGRO BRAIN 197 
of the sulcus frontalis inferior or by a possibly misinterpreted 
ramus ascendens fissura Sylvii. A narrow sunken gyrus between 
the sulcus diagonalis and the sulcus praecentralis inferior is seen 
in several hemispheres and no doubt indicates a transition stage 
in the formation of the former sulcus. 
The gyrus frontalis superior has already been noted on account 
of its great width; it may be added however that it is indented 
by a number of shallow elements which are identified as repre- 
senting a sulcus frontalis mesialis. There is no definite or regular 
arrangement to these elements unless it is a greater tendency to 
a longitudinal direction in the posterior part of the gyrus and a 
transverse direction in the anterior region. In a small number 
of hemispheres a remarkable arrangement of longitudinal and 
transverse fissures suggests some governing factor in the pro- 
duction of these elements. 
The mesial aspect of the gyrus frontalis superior is usually 
divided into two regions, a superior and a rostral, by a prominent 
anterior branch of the sulcus cinguli or by a transverse sulcus 
in the region, often in communication with the sulcus rostralis. 
The superior region is very variable in width and fissuration, 
dependent to a considerable extent upon the form and width of 
the lateral aspect of the gyrus frontalis superior. The sulci 
in this region are more frequently arranged in transverse series 
but may form an irregular longitudinal sulcus as seen in figure 2. 
The rostral region contains two fairly constant sulci, the sulcus 
rostralis and the sulcus subrostralis. In a small number of 
hemispheres an additional parallel sulcus is present; this is also 
seen in figure 2. 
The orbital surface presents no distinguishing features; the 
most common form is a posterior transverse sulcus about 15 mm. 
in length which terminates at each extremity in a prominent 
posterior limb, and two or three anterior sagittal sulci which 
frequently communicate with it. A gyrus orbitalis transversus 
is so formed which is frequently incised by one or two branches 
from the Sylvian fossa. 
The sulci retrocentralis and interparietalis will be considered 
together after the plan of Retzius, but it is very doubtful whether 
