THE CENTRAL NERVOUS 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 (d-c, d@, and e¢), irregular 
and incomplete, about the fissure of Sylvius. These arches all 
open ventrad. The first arch (d—c) (that next to the fissure of 
Sylvius) lacks the central part, the keystone, so that a com- 
Fic. 145.—DIAGRAM OF THE SULCI AND GYRI ON THE LATERAL SURFACE 
OF THE HEMISPHERE. 
A, frontal lobe; 2, temporal lobe; C, occipital lobe. «, lateral fissure or fissure 
of Sylvius; 4, sulcus anterior; ¢, sulcus posterior (the broken line connecting the 
ends of @ and 4 serves to show how the first arch is completed to form the ectosylvian 
sulcus, as in the dog ; d, sulcus suprasylvius; e, sulcus lateralis; J. sulcus cruciatus; 
& sulcus coronalis; 2, sulcus ansatus; 7, sulcus rhinalis; 7, sulcus preesylvius; 4, 
sulcus rhinalis posterior. 1, 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; 
11, olfactory bulb. 
plete arch is not formed, but only the two sides of the arch, as 
two separate sulci (6 and c). The one of these craniad of the 
fissure of Sylvius is the sulcus anterior (4); 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 (1) and posterior (2) Sylvian gyri. 
The second arch is formed by the suprasylvian sulcus 
(sulcus suprasylvius) (7). The gyrus between the anterior 
and posterior sulci ventrad and the suprasylvian sulcus dorsad 
