AMERICAN NEGRO BRAIN 205 
92 per cent of the hemispheres this rhinal element is indicated 
either by a complete separation or by a bridging gyrus at the 
point of union with the central element of the complex (sulcus 
. collateralis). In the detached condition the element is seen to 
be made up of two parts joining at a wide but distinct angle. 
This angle corresponds in position to the shallow groove which 
marks the original course of the complete rhinal fissure. The 
continuation of this groove is the posterior limb of this rhinal 
element of the sulcus collateralis complex, and hence only this 
posterior limb is strictly homologous to a portion of the fissura 
rhinalis. The antero-lateral limb is an accessory branch brought 
into existence by the same growth process which produced the 
obliteration of the central portion of the fissura rhinalis. It 
would seem to be the most natural mechanical result of this 
growth process and its development is almost in direct propor- 
tion to the development of the bridging gyrus rhinencephalo- 
fusiformis. The proof of this statement is disclosed by an 
examination of the examples as they are arranged in figure 7; 
here we have a transition from a typical complete fissura rhinalis 
through the successive stages to the condition of an apparent 
continuous sulcus collateralis having no relation to the incisura 
temporalis. 
The posterior part of the so-called sulcus collateralis is complex 
and difficult to interpret. Two separate modes of termination 
can be identified in this series of brains; the more common is a 
shallow communication by a long postero-lateral arm with a 
transverse sulcus in the region of the gyrus lingualis. There is, 
however, the greatest irregularity in the transverse sulcus and 
it evidently has a very secondary relation to the collateralis 
complex. The other mode of termination is by a superior arm 
which runs parallel to the sulcus calcarinus at a distance of 
about 10 mm., it crosses the mesial border of the hemisphere 
and frequently joins the calcarine-retrocalcarine angle, as seen 
in figures 2 and 6. In several hemispheres both of the above 
elements are present and then the termination is represented 
by a bifurcation; the frequency is 12.6 per cent. 
