142 



THE CEREBRAL HEMISPHERES. 



The anterior horizontal and anterior ascending limbs of the fissure subdivide this operculum 

 into three unequal parts, an upper or fronto-parietal (pars fronto-pai'ietalis), a middle or 

 frontal (pars triangularis, "cap" of Broca), and a lower or orbital (pars orbitalis) (fig. 101, 

 a, b, c~). All these are now often spoken of as opercula. The convolution bounding the 

 Sylvian fissure below is also spoken of as the temporal operculum, so that four opercula are 

 thus enumerated. They are formed in the embryo by the manner in which the mantle comes 

 to overlap the central lobe. The temporal and fronto-parietal appear first, above and below the 

 Sylvian fossa of the embryo : the frontal and orbital develope much later. The meeting of 

 the four opercula determines the formation of the three limbs of the Sylvian fissure (fig. 104). 



The Sylvian fissure is usually longer in the left than in the right hemisphere ; the difference 

 being due to the greater development of the frontal operculum on the left side (fig. 106) 



Fig, 104. DIAGRAMS ILLUSTRATING THE MANNER IN WHICH THE INSULAR REGION is COVERED IN BY 

 THE VARIOUS OPERCULA. (Cunningham.) 



The part printed black represents uncovered part of the Sylvian fossa of the embryo, the unshaded 

 portions represent the different opercula. 



P.P., fronto-parietal operculum ; T., temporal operculum ; F., frontal operculum (pars triangularis 

 of third frontal gyrus) ; 0, orbital operculum ; p. h., posterior limb of fissure ; a. a., anterior ascending 

 limb ; aji., anterior horizontal limb. 



In I, the fronto-parietal and temporal opercula are beginning to grow over the fossa ; in II, the 

 formation of all four opercula is advanced, the pars triangularis being well marked, and the anterior 

 limb U-shaped. In III, the pars triangularis is only slightly developed, and the anterior limb is a Y ; 

 in IV the pars triangularis is absent and the anterior limb is simple (I-form). 



(localisation of speech centre, Broca). The horizontal and ascending limbs of the anterior 

 branch of the Sylvian fissure often arise from a common stem (Y-condition, fig. 101 and fig. 

 106, right side), the pars triangularis being then less developed than usual. This part may 

 even not form a distinct opercular projection, in which case the anterior limb of the Sylvian 

 fissure is undivided (I-condition). 



The anterior and ascending limbs arose by a common stem from the main fissure (Y form), 

 in 32 p. c. of the hemispheres examined by Cunningham ; independently (V and U form) in 

 37 '5 p. c. ; and as a single limb (I form) in 30 p. c. There are sometimes two secondary 

 extensions of the Sylvian fissure on to the orbital lobe (orbital limbs), but these are less deep 

 and have not the same morphological importance as the others. 



The posterior limb about corresponds in the adult to the level of the squamous suture, but 

 in children younger than fifteen years it is rather above this level. The angle which the 

 direction of the posterior limb forms with a line perpendicular to the superior border of the 

 hemisphere (Sylvian angle), varies considerably, but is more acute on the whole in the child 

 than in the adult (Cunningham). 



In the brain of some monkeys, the posterior limb of the Sylvian fissure joins (superficially) 

 the parallel fissure, which it gradually approaches, and the two are continued as an apparently 



