1892.] NATURAL SCIENCES 6f PHILADELPHIA. 205 



concealed. The operculum in the brain of the Gorilla, as in that of 

 jNIan, is formed partly by the lower ends of the two central convolu- 

 tions where they pass into each other, and partly by portions of the 

 third frontal convolution and lower parietal lobule. The fissure of 

 Sylvius, with its posterior branch, separates the frontal and parietal 

 lobes from the temporal lobe. The central fissure, or fissure of 

 Rolando, PI. XI, figs. 1, 2 R, invariably present in the human brain as 

 well as in that of most monkeys, is well marked in the brain of the 

 Oorilla. Beginning on the upper surface of the hemisphere, slightly 

 posterior to the middle line, it passes obliquely forward and down- 

 ward to terminate near the upper border of the posterior branch of 

 the Sylvian fissure, and is larger in the left than in the right hemi- 

 sphere. The central fissure divides quite naturally in the Gorilla 

 the frontal from the parietal lobes upon the upper surface 

 of the hemisphere. The central fissure in its whole length is 

 bordered, as in Man, by two convolutions, the anterior and posterior 

 central convolutions, PI. XI, fig. 2 a, b. The former we regard as 

 belonging to the frontal, the latter to the parietal lobes. It has 

 already been mentioned that the lower ends of the two central convo- 

 lutions, where they pass into each other around the end of the central 

 fissure, enter into the formation of the operculum. 



While this is the case in the left hemisphere, it is not strictly 

 so in the right one, since the central fissure is not only shorter 

 on the right side than on the left, but also on account of the pre- 

 central fissure on the right side being longer than on the left, it 

 passes down in front of the central fissure and almost reaches the 

 posterior branch of the Sylvian fissure. The anterior central convolu- 

 tion, PI. XI, figs. 1, 2 a, may be considered as giving origin at different 

 levels from above downward to the superior, middle and inferior 

 frontal convolutions, PI. XI, figs. 2 c, d, e. The superior or first 

 frontal convolution, PI. XI, fig. 2 c, is separated from the middle 

 or second frontal convolution, PI. XI, fig. 2 d, by the first frontal 

 fissure, and the second frontal convolution, PI. XI, fig. 2 d, from 

 the inferior or third frontal convolution, PI. XI, fig. 2 e, by the 

 second frontal fissure. It is impossible to say whether the con- 

 volution, which, passing downward and forward and bending 

 around reaches the orbital surface of the frontal lobe, should be re- 

 garded as the continuation of the first or second frontal convolution 

 or not. That the inferior part of the frontal lobe of the brain of the 

 Gorilla, Pi. XI, fig. 2 e, should be regarded as homologous with at least 



