THE CEREBRAL HEMISPHERES 



921 



in the more complex configuration of the prefrontal region; (2) the medifrontal 

 fissure (side us front alls mcdius) situated in the prefrontal part of the medifrontal 

 gyre, rarely extending throughout, and usually ending cephalad in a widely spread 

 bifurcation which constitutes the orbitofrontal fissure when independent. The 

 medifrontal fissure is usually very much ramified and frequently anastomoses 

 with neighboring fissures. The fissure is a characteristic of human and anthro- 

 poid brains only. 



By the occurrence of either or both paramesal and medifrontal fissures, the 

 ordinary three-tier type of frontal lobe is converted into a four-tier and five-tier 

 type; the latter more often in the brains of the more highly intellectual a feature 

 which is concomitant with the comparatively late phyletic and embryonic develop- 

 ment of the two secondary fissures described. 



SUPERCENTRALF. 



. = FISSURE 

 , = GYRE 

 R. = RAMUS 



FIG. 674. Fissures and gyres of the lateral surface of the left hemicerebrum. 



Other, less important, fissures are: (1) the inflected fissure (fissura inflexa), 

 incising the dorsimesal border between the central fissure and the cephalic limb 

 of the paracentral; (2) the radiate fissure, near the lateral orbitofrontal border; (3) 

 the transprecentral, a short oblique piece ventrad of the central and usually dipping 

 into the sylvian cleft; and (4) the diagonal fissure between the presylvian ramus and 

 the ventral end of the central, and often confluent with the precentral (Fig. 674). 



2. The mesal surface of the frontal lobe is bounded by the dorsimesal border, 

 the mesorbital border, and the callosal fissure An arcuate fissure or system 

 of fissures intermediate between the dorsimesal margin and the supercallosal . 

 fissure divides this surface into the superfrontal gyre, mesal aspect, and the callosal 

 gyre. The name "callosomarginal" was usually applied to this fissure, but an 

 examination of many brains reveals a certain integrality of fissural parts, which 

 are not always connected. One constant segment from its relations with the 

 central fissure is called the paracentral fissure, composed of a main stem with a 

 cephalic and a caudal limb, embracing the paracentral gyre. Frontad thereof 

 extends the supercallosal fissure, often in two segments, running a concentric 

 course between the arched dorsimesal border and the genu of the callosum. 

 The supercallosal may be confluent with the paracentral. The supercallosal is, 



