THE TELENCEPHALON. 



1 1 37 



Fig. 9S2. 



The form and relations of the fissure of Sylvius are so dependent upon the growth of the 

 surrounding parts, that a sketch of the development of this region of the hemisphere is necessary 

 for an understanding of the significance of this conspicuous sulcus. During the third fcetal 

 month the lateral surface of the cerebral hemisphere 

 presents a crescentic depressed area, the fossa Sylvii, 

 whose floor corresponds to the itisu/a or island of Reil. 

 The latter is seen in the adult brain, on separating the 

 margins of the Sylvian fissure, as a sunken area which is 

 completely hidden by the overhanging parts, the opercula 

 insulae, of the surrounding lobes (Fig. 990). During the 

 fifth month the former shallow crescentic Sylvian fossa 

 gives place to a more definitely walled triangular depres- 

 sion, which, during the succeeding month, begins to be 

 enclosed by the formation of the opercula. The details of 

 this process have been carefully studied by Cunningham' 

 and more recently by Retzius.- The opercula which 

 bound the triangular fossa, named from the regions 

 which contribute them and at first three in number, are 

 the upper or parieto-frontal, the lower or temporal, and 

 the anterior or orbital. The upper and lower walls first 



cjme in contact and thereby form the posterior limb of the Sylvian fissure. Later the angle 

 between the upper and front walls of the fossa becomes modified and is finally obliterated by the 

 appearance of a wedge-shaped projection, later the J'rontal operciilmn, which insinuates itself 



between the adjacent end 

 Fig. 983. 



Inferior precentral Rolandic fissure 



Sylvian 

 fossa 



Temporal 

 lobe 



Left hemisphere of brain of five months 

 foetus ; three-fourths natural size. 



Inferior frontal 



Parieto- 

 frontal 

 operculum 



Frontal 

 operculum 



Orbital 

 operculum 



Olfactory bulb 



Insula 



Interparietal 



Sylvian fissure Superior temporal sulcus 



Lateral surface of left hemisphere of eight months fcetus ; insula is partly covered 

 by opercula; three-fourths natural size. {Retzius.) 



of the parieto-frontal and 

 the orbital opercula. The 

 orbital and particularly the 

 frontal operculum are late 

 in their differentiation and 

 growth, and . not until 

 towards the second year 

 after birth do they come 

 into apposition with each 

 other and the remaining 

 opercula to complete the 

 curtain that overhangs the 

 insula. Along with the 

 closure of the front part of 

 the Sylvian fossa, the dif- 

 ferentiation of the anterior 

 limbs of the fissure pro- 

 gresses, since upon the 

 adequate growth of the frontal operculum depends the production of a distinct pars triangularis 

 and of two sejiarate anterior branches. Faulty development of this intermediate part of the 

 opercular wall accounts for the Y or I form, as well as the occasional absence, of the anterior limbs. 



The central fissure (sulcus centralis), or fissure of Rolando, extends 

 transversely across the upper half of the convex dorsal surface of the hemisphere and 

 therefore, w^ith the bordering precentral and postcentral convolutions, interrupts the 

 general longitudinal course of the gyri and sulci. Bearing this peculiarity in mind, 

 the fissure is readily identified even in brains exhibiting an elaborate and complex 

 modelling. It begins above on the supero-mesial margin of the hemisphere, a short 

 distance behind the middle of the border, and descends with a slight general forward 

 obliquity to the vicinity of the posterior limit of the fissure of Sylvius, above whose 

 mid-point it usually ends. Its upper extremity usually extends over the supero- 

 mesial border of the hemisphere and, passing obliquely backward, cuts for a short 

 distance into the marginal gyrus of the mesial surface (Fig. 987). Its lower ex- 

 tremity usually ends short of the Sylvian fissure, but occasionally (rarely) opens 

 into this cleft. It constitutes a very definite boundary on the external surface of the 

 hemisphere betw^een the frontal and parietal lobes. Although passing obliquely 

 downward and forward, the course of the central fissure is by no means straight 



^ Contribution to the Surface Anatomy of the Cerebral Hemispheres, Irish Academy, 1892. 

 *Das Menschenhirn, 1896. 



72 



