360 NERVOUS SYSTEM. 
is the ectosylvian gyrus (3) (because dorsad of the ectosylvian 
sulcus). ‘ 
The third arched sulcus, next to the medial margin of the 
hemisphere, is variable: it forms the lateral sulcus (¢). The 
gyrus lying between the suprasylvian and lateral sulci is the 
suprasylvian gyrus (4) (because dorsad of the. suprasylvian 
sulcus). The lateral sulcus is sometimes broken into two (as 
in Fig. 137, 4), the two ends overlapping so as to'leave a 
small gyrus between the two ends. 
The gyrus dorsomediad of the lateral sulcus, extending 
onto the medial surface of the-hemisphere as far as the splenial 
sulcus, is the marginal gyrus (5). 
The four sets of gyri included between these three sets of 
arched sulci and the fissure of Sylvius unite caudad and craniad 
to form single gyri. The caudal one is the gyrus compositus 
posterior (6). The cranial one (less regular than the caudal 
one) is the sigmoid gyrus (7). The latter curves about the 
cruciate sulcus (/) (described below). 
In the cranial part of the lateral surface of the hemisphere 
are certain sulci and gyri which do not belong to the system 
above described. The cruciate sulcus (/) is a short transverse 
sulcus passing from the lateral surface of the hemisphere onto 
its medial surface, where it extends caudad about a centimeter. 
Curving around the lateral end of this, separated from it by 
part of the sigmoid gyrus, is the short coronal sulcus (,). 
Connected usually with the end of the lateral sulcus (¢) and 
running nearly parallel with the cruciate sulcus is the short 
sulcus ansatus (/). 
At its ventral end the fissure of Sylvius (a) joins a longi- 
tudinal groove passing craniad and caudad from the point of 
junction. That part which passes craniad is the sulcus 
rhinalis (7); it forms the dorsolateral boundary of the olfactory 
tract (8). Passing dorsocraniad from the sulcus rhinalis is a 
sulcus which separates a very small cranial lobe from the rest 
of the hemisphere; this is the supraorbital or presylvian 
sulcus (7). The portion of the hemisphere craniad of this 
sulcus is the orbital gyrus (9). 
The sulcus rhinalis (2) is continuous caudad with the 
