THE FORE-BRAIN OR PROSENCEPHALON 



819 



nences, named gyri or convolutions, and separated by furrows termed fissures and 

 sulci. The furrows are of two kinds, complete and incomplete. The former appear 

 early in fetal life, are few in number, and are produced by infoldings of the entire 

 thickness of the brain wall, and give rise to corresponding elevations in the interior 

 of the ventricle. They comprise the hippocampal fissure, and parts of the calcarine 

 and collateral fissures. The incomplete furrows are very numerous, and only indent 

 the subjacent white substance, without producing any corresponding elevations in 

 the ventricular cavity. 



I The gyri and their intervening fissures and the sulci are fairly, constant in their 

 arrangement ; at the same time they vary within certain limits, not only in different 

 individuals, but on the two hemispheres of the same brain. The convoluted con- 

 dition of the surface permits of a great increase of the gray matter without the 

 sacrifice of much additional space. The number and extent of the gyri, as well 

 as the depth of the intervening furrows, appear to bear a direct relation to the 

 intellectual powers of the individual. 



Certain of the fissures and sulci are utilized for the purpose of dividing the hemi- 

 sphere into lobes, and are therefore termed interlobular ; included under this category 

 are the lateral cerebral, parietooccipital, calcarine, and collateral fissures, the 

 central and cingulate sulci, and the sulcus circularis. 



FIG. 726. Lateral surface of left cerebral hemisphere, viewed from the side. 



The Lateral Cerebral Fissure (fissura cerebri lateralis [Sylvii] ; fissure of Sylvius) (Fig. 

 726) is a well-marked cleft on the inferior and lateral surfaces of the hemisphere, 

 and consists of a short stem which divides into three rami. The stem is situated 

 on the base of the brain, and commences in a depression at the lateral angle of the 

 anterior perforated substance. From this point it extends between the anterior 

 part of the temporal lobe and the orbital surface of the frontal lobe, and reaches 

 the lateral surface of the hemisphere. Here it divides into three rami : an anterior 

 horizontal, an anterior ascending, and a posterior. The anterior horizontal ramus 

 passes forward for about 2.5 cm. into the inferior frontal gyrus, while the anterior 

 ascending ramus extends upward into the same convolution for about an equal 

 distance. The posterior ramus is the longest; it runs backward and slightly upward 

 for about 7 cm., and ends by an upward inflexion in the parietal lobe. 



The Central Sulcus (sulcus centralis [Rolandi] ; fissure of Rolando; central fissure) 





