360 NERYOUS 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 (e). 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, /z), 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 ( i ). The latter curves about the 

 cruciate sulcus (f) (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 (g). 

 Connected usually with the end of the lateral sulcus (e) and 

 running nearly parallel with the cruciate sulcus is the short 

 sulcus ansatus (h). 



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 (z) ; 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 prsesylvian 

 sulcus (/). The portion of the hemisphere craniad of this 

 sulcus is the orbital gyrus (9). 



The sulcus rhinalis (z) is continuous caudad with the 



