THE CEREBRUM 401 



It begins at the substantia perforata anterior. Thence it 

 passes laterally, forming a deep cleft between the temporal 

 pole and the orbital area of the inferior surface of the 

 hemisphere. When it reaches the lateral surface of the 

 hemisphere, the fissure immediately divides into three 

 radiating branches (i) the ramus posterior ; (2) the ramus 

 anterior horizontalis ; and (3) the ramus anterior ascendens. 



The posterior branch (Figs. 138, 152) is the longest and 

 the most important of the three. It extends backwards, with 

 a slight upward inclination, for a distance of 7.5 cm. (three 

 inches} or more, between the temporal lobe, which is placed 

 below it, and the frontal and parietal lobes, which lie above it. 

 Finally, it turns upwards, into the parietal lobe, in the form 

 of an ascending terminal piece (Figs. 138, 152). 



The anterior horizontal branch (Fig. 152) runs forwards in 

 the frontal lobe, for a distance of about 19 mm. (three-quarters 

 of an inch), immediately above and parallel to the posterior 

 part of the superciliary margin of the hemisphere. 



The anterior ascending branch (Figs. 138, 152) passes 

 upwards, with a slight anterior inclination, into the lower 

 part of the lateral surface of the frontal lobe for a distance 

 of about 25 mm. (one inch). In many cases the two anterior 

 limbs spring from a common stem of variable length. 



Sulcus Circularis (O.T. Limiting Sulcus of Reil). If the 

 lips of the posterior ramus of the lateral fissure are gently 

 pulled asunder, the insula (O.T. island of Reil) will be seen 

 at the bottom of the cleft which is termed the lateral fossa 

 (Fig. 157). It is surrounded by a sulcus, called the circular 

 sulcus, which is separable into three parts, viz., an upper 

 part, bounding the insula above, a lower part, marking it 

 off below, and an anterior part limiting it anteriorly. The 

 insula thus mapped out is somewhat triangular in outline, 

 and over its surface ramify branches of the middle cerebral 

 artery. 



Opercula Insulae. The present is a good time to study 

 the manner in which the insula is shut off from the surface 

 of the hemisphere. When the lateral fissure is held widely 

 open, it will be noted that the insula is overlaid by portions 

 of cerebral cortex which appear as if they were undermined. 

 The overlying portions are called the opercula insula, and it is 

 their opposed margins which form the boundaries of the 

 lateral fissure. The opercula are four in number, and are 



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