Sulcal pattern of the anterior parahippocampal gyrus in the human adult.

The sulcal pattern of the human entorhinal region, located in the anterior portion of the parahippocampal gyrus, was investigated in 184 brain hemispheres of both sexes between the ages of 19 and 85 years. A new classification of the sulcal pattern is introduced with reference to the extension of the collateral and rhinal sulci. Both sulci were found in all the investigated hemispheres. The collateral sulcus is a continuation of the rhinal sulcus in 41.9%. The rhinal sulcus represents the anterolateral border of the entorhinal region. By defining four branches of the rhinal sulcus all the possible ramifications occurring in the investigated hemispheres could be described. In 19.0% of the cases, a well developed entorhinal sulcus emerges in the central portions of the entorhinal region. An incipient entorhinal sulcus is evident in 29.6% and the sulcus is absent in 51.4% of the investigated cases. The intrarhinal nick, resulting from the impression of the free edge of the cerebellar tent, is seen as a dip in 35.1% and as a notch in 34.5%. It is absent in 30.4% of the cases. A comparison between the right and left hemispheres, as well as between the female and male sexes, revealed only minor differences of the sulcal pattern.