464 THE BRAIN 



the insula to meet the temporal operculum. Its margin 

 forms the upper lip of the posterior branch of the lateral 

 fissure. 



'The frontal operculum (Fig. 186) is the small triangular 

 piece of cerebral cortex between the anterior ascending and 

 horizontal branches of the lateral fissure. It is sometimes 

 termed the pars triangularis. 



The orbital operculum (Fig. 186) is for the most part on 

 the under surface of the hemisphere. It lies below the 

 anterior horizontal limb of the lateral fissure, and passes 

 posteriorly from the orbital aspect of the frontal lobe over 

 the anterior part of the insula. 



Sulcus Centralis (O.T. Fissure of Eolando). The central 

 sulcus takes an oblique course across the lateral surface 

 of the cerebral hemisphere (Fig. 186). Its upper end in 

 many cases cuts the supero-medial border of the hemisphere 

 a short distance posterior to the mid-point between the 

 frontal and occipital poles, whilst its lower end terminates 

 above the middle of the posterior branch of the lateral fissure. 

 When its superior extremity turns round the supero-medial 

 border of the hemisphere it is continued posteriorly, 

 for a short distance, on the medial surface (Fig. 187). 

 Although in its general direction the sulcus centralis is 

 oblique, it is far from being straight. Nearer to its upper 

 than to its lower end it is bent posteriorly so as to form a 

 bay, within which is accommodated a portion of the cerebral 

 cortex which represents the motor area of the opposite upper 

 limb. The upper and lower limits of this bay are termed 

 the upper and lower genua of the fissure. 



Fissura Parieto - occipitalis. The greater part of this 

 fissure is situated on the medial surface of the cerebral hemi- 

 sphere (Fig. 187); only a very small part appears on the 

 lateral face (Fig. 186). 



The lateral part of the parieto-ocdpital fissure (O.T. external 

 parieto -occipital] cuts the supero-medial border of the hemi- 

 sphere, in a transverse direction, from one and a half to two 

 inches anterior to the occipital pole. It is usually not more 

 than half an inch in length, and it is brought to an abrupt 

 termination by an arching convolution which winds round its 

 extremity, and receives the convenient name of arcus parieto- 

 occipitalis (Fig. 186). 



The medial part of the parieto-ocdpital fissure (Fig. 187) 



