THE CENTRAL NERYOUS SYSTEM. 



359 



of Sylvius is short but deep, and is formed eariy during 

 development. In man it covers an area, the insula, at its 

 bottom, which may be seen by separating the sides of the 

 fissure. In the cat the insula is rudimentary and can be 

 demonstrated only with difficulty. 



The lateral surface of the hemisphere is marked by fissures 

 which form three concentric arches (bc, d, and e), irregular 

 and incomplete, about the fissure of Sylvius. These arches all 

 open ventrad. The first arch (bc} (that next to the fissure of 

 Sylvius) lacks the central part, the keystone, so that a com- 



A 



FIG. 145. DIAGRAM OF THE SULCI AND GYRI ON THE LATERAL SURFACE 



OF THE HEMISPHERE. 



A. frontal lobe; B, temporal lobe; C, occipital lobe. a. lateral fissure or fissure 

 of Sylvius; b, sulcus anterior; c, sulcus posterior (the broken line connecting the 

 ends of a and b serves to show how the first arch is completed to form the ectosylvian 

 sulcus, as in the dog ; d, sulcus suprasylvius ; e. sulcus lateralis; f. sulcus cruciatus; 

 g, sulcus coronalis; A, sulcus ansatus; i, sulcus rhinalis; j, sulcus prsesylvius; k, 

 sulcus rhinalis posterior, i, anterior Sylvian gyrus; 2, posterior Sylvian gyrus; 3, 

 gyrus ectosylvius; 4, gyrus suprasylvius; 5, gyrus marginalis; 6, gyrus compositus 

 posterior; 7, sigmoid gyrus; 8, olfactory tract; 9, orbital gyrus; 10, lobus pyriformis; 

 II. olfactory bulb. 



plete arch is not formed, but only the two sides of the arch, as 

 t\vo separate sulci (b and c). The one of these craniad of the 

 fissure of Sylvius is the sulcus anterior (b) ; the other is the 

 sulcus posterior (c). In the dog this arch is usually complete 

 and the sulcus forming it is frequently known as the ectosylvian 

 sulcus (sulcus ectosylvius). 



The gyri included between the fissure of Sylvius on the one 

 hand and the anterior and posterior sulci on the other are 

 known as the anterior ( i ) and posterior ( 2 ) Sylvian gyri. 



The second arch is formed by the suprasylvian sulcus 

 (sulcus suprasylvius) (d). The gyrus between the anterior 

 and posterior sulci ventrad and the suprasylvian sulcus dorsad 



