206 PROCEEDINGS OF THE ACADEMY OF [1892. 



part of the third or inferior frontal convolution of the brain of Man, 

 is shown by the fact that the convolution in question not only "sur- 

 rounds the end of the anterior vertical branch of the Sylvian fissure, 

 but its inferior lower part passes as an arched convolution under 

 the operculum into the island of Reil. As a confirmation of this 

 view it will be observed that the pre-central fissure, into which the 

 second frontal fissure runs, PI. XI, fig. 2 d, passes downward between 

 the anterior vertical branch of the fissure of Sylvius and the central 

 fissure. The frontal lobes of the Gorilla differ, however, from those 

 of Man in their anterior poi'tion terminating in a point. The 

 inferior or third frontal convolution in the brain of the Gorilla 

 differs, especially from the corresponding convolution in Man, in not 

 only being relatively smaller but in its orbital surface being hollowed 

 out to such an extent that the portion of the inferior frontal con- 

 volution which in man surrounds or is below the anterior branch of 

 the fissure of Sylvius, is absent. Indeed this should be so if, as we 

 have just supposed, the anterior horizontal branch of the fissure of 

 Sylvius is absent. It is hardly to be expected, therefore, that in 

 addition to the olfactory fissure both the orbital fissure and the 

 solco cruciform of Rolando' should be present in the brain of the 

 Gorilla, especially as the fissures on the orbital surface of the frontal 

 lobe in the brain of Man are variable in form. Only one fissure, 

 in addition to the olfactory, is present in the orbital surface of 

 the frontal lobe of the Gorilla's brain, and that resembles in both 

 hemispheres rather the cruciform than the orbital fissure of Man. 



If the interpretation just offered of the convolutions of the 

 frontal lobe of the brain of the Gorilla be correct, it follows that the 

 frontal lobe of the brain of the monkeys below the Gibbon must 

 consist essentially of only two convolutions, the superior and 

 middle frontal, the inferior frontal convolution being in them 

 but little developed, as in Macacxis, or absent altogether as in 

 Cercopithecus. If such be the case then the convolution in monkeys 

 described by Gratiolet^ as being the inferior or third frontal 

 must be regarded as being the middle or second frontal con- 

 volution, the inferior frontal convolution being but little, if at 

 all developed. The view just oflJered, advanced also by BischofF,* 

 as to the nature of the convolutions of the frontal lobe in the 



^Memorie della R. Accad. delle Scienze di Torino, 1829, T. XXXV. 

 ■'' Memoire sur les plis cerebraux de 1' Homme el des Primates. 

 ^Beiirage zur Anatomic des Hylobates leuciscu-, \>. 78, Miinclien, 1870. 



