THE NER VE KYKTEM 



as a rule, quite ramified, its branches transcribing the superfrontal gyre. In 

 the prefrontal region and ventrad of the genu of the corpus callosum lie one or 

 two fissures, more or less jnirallel to the mesorbital border, and called, respec- 

 tively, the rostral and subrostral fissures (sulci rostrales). 



3. The orbital surface of the frontal lobe is constantly marked by a straight 

 fissure, the olfactory fissure (#ulcus olfactorius], which runs parallel to the mesorbital 

 border and is occupied by the olfactory bulb and tract. It is about 5 cm. in length 

 and demarcates the mesorbital gyre from the remaining orbital gyres. This 

 orbital surface is marked by a fissural system (sulci orbitales] that is usually of 

 zygal type, H-shaped or K-shaped, quadriradiate, or, rarely, triradiate. When the 

 transverse element is sufficiently pronounced it merits the name of transorbital 

 fissure, demarcating the preorbital from the postorbital gyral field. 



GYRES OF THE FRONTAL LOBE (lobus frontalis). 1. Lateral Surface. The pre- 

 central gyre (gyrus central is anterior), one of the chief motor areas of the cerebral 

 cortex, is a moderately sinuous gyre extending from the dorsimesal border to 

 the sylvian fissure and demarcated by the central and the precentral fissures 

 (supercentral + precentral). 



= GYRE 

 F.= FISSURE 



FIG. 675. Fissures and gyres of the mesal surface of the left hemicerebrum. 



The superfrontal gyre is limited laterally by the superfrontal fissure, while it 

 is continuous over the dorsimesal border with its mesal surface. 1 It merges 

 insensibly with the medifrontal gyre in the prefrontal region, while it may be 

 partially subdivided by the paramesal fissure. 



The medifrontal gyre (gyrus frontalis medius) is broader than the preceding, 

 demarcated by the superfrontal and subfrontal fissures, and often marked by the 

 medifrontal fissure in its prefrontal portion. 



The subfrontal gyre (gyrus frontalis inferior} is limited by the subfrontal fissure 

 and the basisylvian + sylvian proper. It is traversed by the presylvian and sub- 

 sylvian rami, embracing the preoperculum or pars triangularis. The gyre is of 

 historic importance since Broca, in 1861, declared it to be the seat of speech con- 

 trol. (See Cerebral Localization.) 



1 There being no fissure at this border, it is improper to give the mesal surface of this gyre a different n:imj 

 <*. e., "marginal gyrus" of the authors). 



