THE CEREBRAL 



919 



hemisphere, particularly the lenticular nucleus, from whose ectal surface the insular 

 cortex is but little removed. As will be learned at a later stage, few if any pro- 

 jection fibres pass to and from the island of Reil; its function is almost wholly 

 associative for adjacent parts of the cereb'ral mantle. The island of Reil there- 

 fore becomes buried beneath the more energetically growing and bulging parts 

 immediately around it. There is at first a slight fossa (observable in the tenth 

 to twelfth week) which, as development proceeds, and as the overhanging opercula 

 encroach upon the island of Reil, becomes more deeply situated as a cleft-like 

 depression until at birth the fossa has become a fissure, with the island of Reil 

 perhaps slightly exposed near its cephalic extremity, where the incomplete apposi- 

 tion of the opercula leaves a triangular space. This space is usually obliterated 

 in childhood, but is commonly met with in certain races (negro, Australian) 

 and in brains showing developmental defects or arrest. The mechanics of the 

 formation of the surface outline of the sylvian fissure by the apposition of the 

 growing and plastic opercula may be understood by a reference to Fig. 678. 



INTERLOCKING 

 QYRI 



.PRECENTRAL 

 GYRE 



POSTCENTRAL 

 GYRE 



Fin. 673. Central fissure fully opened up, so as to exhibit the interlocking gyres. 



The central fissure (fissure of Rolando [sulcus centralis]) is situated at about the 

 middle of the convex surface, and, coursing obliquely laterofrontad, divides this 

 surface into approximately equal parts, intervening between the frontal and parietal 

 lobes. It may be traced from a point at or near the dorsimesal border, about 



1 cm. (|- inch) caudad of the mid-point of the occipitofrontal arc. It then runs 

 sinuously laterofrontad to within a short distance of the sylvian fissure, about 



2 cm. (i inch) caudad of the sylvian point; its line of general direction makes an 

 angle of about 70 degrees w r ith the median line (Rolandic angle). If measured 

 along its sinuosities, its length averages 10.5 cm. (4 inches). Its curved course 

 may be analyzed into five alternate curves (sometimes more or less), of which 

 three are convex frontad and two caudad. It is rarely very much branched and 

 does not often anastomose with neighboring fissures. Its dorsal end bears a 

 constant relation to the caudal limb of the paracentral, frontad of which it can be 

 found as a hook-like curve (Fig. 675). If the lips of the central fissure be divari- 

 cated, interdigitating sub-gyres are commonly seen in its depths (Fig. 673). 

 These interlocking gyres are often fused to a greater or less degree, and a total 

 interruption of the fissure has, in rare instances, been observed. The central 

 figure develops at about the end of the fifth month of intrauterine life, not as 

 a single integer, but as the result of the union of two segments a short dorsal and 

 a longer ventral segment. As development proceeds these segments eventually 





