Dec. 14, 1876] 



NATURE 



143 



the orang, in the latter the frontal lobes are more com- 

 pressed, giving a pointed form to the frontal extremity, 

 and the occipital lobes are larger and more rounded, so 

 that the figure is pear-shaped rather than ovoid ; but the 

 vault is decidedly more lofty and better arched ; also the 

 orbital concavity is less marked in the orang, so that any 

 dtticiency in the lateral development of the frontal lobes 

 might very well be compensated by their downward ex- 

 tension. The chimpanzee and gorilla, however, exhibit a 

 very great resemblance in shape and proportions, though 

 the former has somewhat more compressed frontal lobes, 

 a grea'er development of the occipital region, and appa- 

 rently greater width, so that the cast looks more globular 

 than that of the gorilla. 



The corpus callosutn is of good length, but rather thin ; 

 its proportion, taking the length of the brain as 100, is 

 41, and its average thickness appears about one-twelfth 

 of that ; in the chimpanzee the length is 39 and the 

 thickness one-eleventh ; in the orang, 44. ; and in man, 

 40 ; thickness, one-thirteenth. 



The convolutions are strongly marked ; in a general 

 view they are slightly more subdivided than in the chim- 

 panzee, but in complexity and asymmetry the orang ex- 

 ceeds the gorilla to about the same extent. 



Outer Surface of tJie Hemisphere. — The posterior 

 branch of the Sylvian fissure extends upwards and ends 

 in the usual bifurcation, nearly at the junction of the 

 middle and posterior thirds of the hemisphere, and a'most 

 half of the height fron the lower margin ; judged by this, 

 the fissure is more oblique than in man, less so than in 

 either orang or chimpanzee. The short anterior limb is 

 very faintly marked in front of the insula, but its ending 

 is distinct, bifurcated, on the outer side of the frontal lobe. 

 The insula has its fore part uncovered in the bottom of 

 the fissure. The external parieto-occipital fissure travels 

 over the outer surface to within a very short distance of 

 the lower margin of the hemisphere ; its hinder margin, 

 prolonged forwards, gives rise to a convex operculum in 

 about its lower two-thirds, very much resembling that of 

 the chimpanzee, although somewhat more sinuous. The 

 fissure ot Rolando is very oblique, the lower end is re- 

 markably forward, being actually in front of the tip of the 

 temporo-sphenoidal lobe (probably part of this is due to 

 the position of the brain), and the upper end reaches the 

 longitudinal fissure behind the centre of the hemisphere ; 

 the angle formed by the two fissures is very little more 

 than a right-angle, 95°. 



The length of the hemisphere being 100, the distances 

 in a horizontal line from the anterior extremity to the 

 upper end of the fissure of Rolando, i.e., the extreme 

 length of frontal lobe, being a, thence to the parieto- 

 occipital fissure, length of parietal lobe, b, and from that 

 to the hinder extremity, occipital lobe, c, we get — 



The length of the frontal and the smallness of the 

 occipital lobes are especially noteworthy. 



The convolutions of the frontal lobe present their 

 typical arrangement : the ascending frontal is very 

 simple, following in easy curves the fissure of Rolando, 

 and marked only by one slight indentation opposite the 

 superior frontal sulcus ; it is bounded in front for the 

 lower half by the prae-central sulcus (Ecker) from which 

 the distinct and strongly-bent inferior frontal sulcus runs 

 'forwards to the tip of the lobe. The superior frontal 

 sulcus has its characteristic T shape, the top of the T 

 being placed vertically in front of the upper half of the 

 foregoing convolution, and the second limb sent forwards 

 almost straight for about two-thirds of the distance to the 



anterior extremity between the upper and middle frontal 

 convolutions. Of the three horizontal gyri, the upper 

 springs by a narrow root from the ascending frontal close 

 to the margin, and shows plainly the indications of the 

 longitudinal division into two ; the middle is much nar- 

 rower, connected to the ascending by a pedicle between 

 the prse-central and superior frontal sulci ; and the inferior 

 frontal is well developed, arching over the anterior limb 

 of the Sylvian fissure, and considerably folded. Com- 

 pared with the chimpanzee, the upper is narrow, the middle 

 and lower larger and more subdivided. The orang 

 throughout the whole lobe exhibits a greater richness of 

 convolution. 



The convolutions of the parietal lobe are very de- 

 finitely and strictly marked off, and at the same time they 

 are decidedly the most developed of the whole brain, far 

 exceeding the chimpanzee, and not inferior to the orang. 

 The intra-parietal sulcus, springing from the Sylvian 

 fissure near its hinder end, runs forwards and then up- 

 wards, round the front of the supra-marginal lobule, 

 parallel to the fissure of Rolando, as far as the centre of 

 that, when it turns backwards at an obtuse angle, con- 

 tinues, approaching slightly the longitudinal fissure, and 

 debouches into the external parieto-occipital fissure ; from 

 the angle which it forms, the customary prolongation, 

 giving off two or three smaller branches, is sent upwards 



Fif/..\ 



Inner surface of the brain of the Gorilla, /.p. internal parieto-occipital 

 fissure ; f.c. Calcarine fissure ; s^.cm. Calloso-marginal ssure. 



in continuation of the ascending portion, and dividing the 

 ascending parietal convolution from the parietal lobule 

 almost completely ; the interruption to this fissure aboiit 

 its bend which is seen so often m human brains I have 

 observed only in the orang. The ascending parietal 

 convolution is much more developed than the ascending 

 frontal, and it presents a ma'^ked indication of a longitu- 

 dinal fission ; the lower end has the usual triangular ex- 

 pansion split into two (superior marginal convolution of 

 Gratiolet). The parietal lobule is large and divided into 

 an outer and an inner portion, the latter showing further 

 subdivisions, in this condition approaching more to the 

 human brain than either orang or chimpanyee. The 

 supra-marginal lobule is more developed than in either 

 chimpanzee or orang, and divided into three portions 

 by a triradiate sulcus, but its proportion aiid the 

 amplitude of its gyri are much inferior to the human 

 brain. The angular convolution springs from the upper 

 end of the supra-marginal lobule by a narrow bent piece ; 

 the descending branch here differs from the chimpanzee 

 and orang in being cut off from the middle temporo- 

 spheroidal convolution and running backwards into the 

 middle occipital convolution, constituting the anterior 

 boundary of the external parieto-occipital fissure. 

 The occipital obe is by no means richly convoluted, 



