408 J. J. KEEGAN 



The sulci precentrales are typical of Cunningham's ('92) de- 

 scription in the right hemisphere, but in the left hemisphere the 

 superficial interrupting gyrus occurs lower, apparently between 

 the two rami of the sulcus precentralis inferior. The sulcus 

 diagonalis is incorporated in the ramus verticalis in both hemi- 

 spheres, thus reducing the posterior area of the gyrus frontalis 

 inferior to a small area in the floor of the sulcus precentralis 

 inferior. The factor which would account for this might be an 

 overgrowth of the motor area of the mouth region in the gyrus 

 precentralis inferior or of the motor speech center in the gyrus 

 frontalis inferior. Cole ('10) has attempted to relate the sub- 

 mergence of this area or gyrus to a simian condition and quotes 

 Kohlbriigge and Bolk as supporting his contention that the 

 sulcus diagonalis represents a detached portion of the simian 

 sulcus precentralis inferior. 



The sulcus frontalis superior is well defined in both hemispheres 

 and superficially continuous from the s. precentralis superior to 

 the s. fronto-marginalis, maintaining about an even distance 

 from the mesial border throughout. A prominent bridging gyrus 

 near the posterior extremity permits the interpretation of the 

 continuity of the posterior middle frontal region with the ante- 

 rior superior frontal region as illustrated in E. Smith's chart. 

 Appleton ('10) considers the tendency towards extra segmenta- 

 tion of the s. frontalis superior as a lowly character. 



The sulcus fronto-marginalis is better defined in the left hemi- 

 sphere than in the right but is too irregular in both to permit of 

 much comparison. 



The sulcus frontahs inferior is a high arcuate fissure on both 

 sides in full communication with the s. precentralis inferior. Its 

 length and its distance from the fissura cerebri lateralis are each 

 about a centimeter greater in the right hemisphere. This gives 

 rise to a larger gyrus frontalis inferior on the right side which 

 would be contrary to expectation if the speech center had any 

 relation to the size of this gyrus. 



The sulcus frontalis medius is a very subordinate system of 

 shallow sulci, according to the interpretation of the s. frontalis 

 superior. The greater width of the gyrus frontalis medius of 



