542 



NATURE 



[September 26, 1901 



But these figures do not convey the full extent of the pre- 

 dominance of the occipital lobe in the ape. The anterior border 

 of the lobe grows forwards beyond its proper limits, and pushes 

 its way over the parietal lobe which lies in front, so as to coyer 

 over a portion of it by an overlapping Up termed the occipital 

 operculum. There is not a trace of such an arrangement in the 

 human brain, and even in the anthropoid ape the operculum 

 has become greatly reduced. Indeed, in man there is exactly 

 the reverse "condition. The great size of the parietal lobe is a 

 leading human character, and it has partly gained its predomin- 

 ance by pushing backwards so as to encroach, to some extent, 

 upon the territory which formerly belonged to the occipital lobe.' 

 A great authority^ on the cerebral surface refers to this as a 

 struggle between the two lobes for surface extension of their 

 respective domains. " In the lower apes," he says, " the oc- 

 cipital lobe proves the victor : it bulges over the parietal lobe 

 as far as the first annectant gyrus. Already, in the orang, the 

 occipital operculum has suffered a great reduction ; and in man 

 the victory is on the side of the parietal lobe which presses on 

 the occipital lobe and begins, on its part, to overlap it." Now 

 that so much information is available in regard to the localisa- 

 tion of function in the cerebral cortex, and Flechsig has stimu- 

 lated our curiosity in regard to his great "association areas" 

 in which the higher intellectual powers of man are believed to 

 reside, it is interesting to speculate upon the causes which have 

 led to the pushing back of the scientific frontier between the 

 occipital and parietal cerebral districts. 



The parietal lobe is divided into an upper and a lower part 

 by a fissure, which takes an oblique course across it. Rudinger,^ 

 who studied the position and inclination of this fissure, came 

 to the conclusion that it presents easily determined differences 

 in accordance with sex, race and the intellectual capacity of the 

 individual. He had the opportunity of studying the brains of 

 quite a number of distinguished men, amongst whom were 

 Bischoff of Bonn, Dollinger of Munich, Tiedemann of Heidel- 

 berg, and Liebig of Munich, and he asserts that the higher the 

 mental endowment of an individual the greater is the relative 

 extent of the upper part of the parietal lobe. 



There is absolutely no foundation for this sweeping assertion. 

 When the evolutionary development of the parietal part of the 

 cerebral cortex is studied exactly the reverse condition becomes 

 manifest. It is the lower part of the parietal lobe which in 

 man, both in its early development and in its after growth, ex- 

 hibits the greatest relative increase. .'\dditional interest is 

 attached to this observation by the fact that recently several 

 independent observers have fixed upon this region as one in 

 which they believe that a marked exuberance of cortical growth 

 may be noted in people of undoubted genius. Thus Retzius 

 has stated that such was the case in the brains of the astronomer 

 Hugo Gylden,'' and the mathematician Sophie Kovalevsky ; ' 

 Hansemann" has described a similar condition in the brain of 

 Helmholtz ; and Gusznian " in the brain of Rudolph Lenz, the 

 musician. Some force is likewise added to this view by Flech- 

 sig, who, in a recent paper,'* has called attention to the fact 

 that within this district there are located two of his so-called 

 " Terminalgebiete," or cortical areas, which attain their func- 

 tional powers at a later period than those which lie around them, 

 and which may therefore be supposed to have specially high 

 work to perform. 



Without in any way desiring to throw doubt upon the obser- 

 vations of these authorities, I think that at the present moment 

 it would be rash to accept, without further evidence, conclusions 

 which have been drawn from the examination of the few brains 

 of eminent men that have been described. There cannot be a 

 doubt that the region in question is one which has extended 



1 It is necessary to emphasise this point, because in WIedersheim's 

 " Structure of Man " we are told that in man there is a preponderance of 

 the occipital lobe, and that the parietal lobe is equally developed in man 

 and anthropoids. 



- Eberstaller, IVkner Meriizinisclu Blatter, iRi^, No. 19. p. 581. 



•' " Beitrage zur Anatomie und Embryologie," als Festgabe, Jscob Henle, 

 1882. 



•* Retzius, Biologische Untet-suchnngen, neue Folge, viii. 1898, "Das 

 Gehirn des Astronomen Hugo Gyld^ns." 



■^ Retzius, Biologische UntersHchiingen, neue Folge, ix. 1900, " Das 

 Gehirn der Mathematikerin Sonja Kovalevsky." 



t Hansemann, Zcitschrift /i,r Psycholo^ie imd Physiologic der Siimes- 

 organe. Band xx. Heft i, 1899, " Ueber das Gehirn von H 

 Helraholtz." 



" Josef Guszman, Anatomischer Anzeigcr, Band xix. Nos. 

 April igoi, " Beitrage zur Rlorphologie der Gehirnoberflache." 



* Flechsig, " Neue Untersuchungen iiber die Markbildui 

 menschlichen Grosshirnlappen," Xeurologisches Centralhlatt, N 



NO. 1665, VOL. 64] 



greatly in the human brain, but the association of high intellect 

 with a special development of the region is a matter on which I 

 must confess I am at present somewhat sceptical. 



But it is not only in a backward direction that the pariet.al 

 lobe in man has extended its territory. It has likewise increased 

 in a downward direction. There are few points more striking 

 than this in the evolution of the cerebral cortex of man. In 

 order that I may be able to make clear the manner in which 

 this increase has been brought about, it will be necessary for 

 me to enter into some detail in connection with the develop- 

 ment of a region of cerebral surface termed the insular dislrn't. 

 The back part of the frontal lobe is also involved in this down- 

 ward extension of surface area, and, such being the case, it may 

 be as well to state that the boundary which has been fixed 

 upon as giving the line of separation between the parietal and 

 frontal districts is purely artificial and arbitrary. It is a demar- 

 cation which has no morphological significance, whilst from a 

 physiological point of view it is distinctly misleading. 



The insular district in the fretal brain is a depressed area of 

 an elongated triangular form. The general surface of the cere- 

 brum occupies, all round about it, a more elevated plane, and 

 thus the insula comes to be bounded by distinct walls, like the 

 sides of a shallow pit dug out in the ground. The upper wall 

 is formed by the lower margins of the frontal and parietal lobes, 

 the lower wall by the upper margin of the tempoial lobe, and 

 the front wall by the frontal lobe. From each of these bounding 

 walls a separate portion of cerebral cortex grows, and these 

 gradually creep over the surface of the insula so as to overlap it, 

 and eventually completely cover it over and exclude it from the 

 surface, in the same way that the lips overlap the teeth and 

 gums. That which grows from above is called the fronto- 

 parietal operculum, while that which grows from below is termed 

 the temporal operculum. These appear very early, and are 

 responsible for closing over more than the hinder three-fourths 

 of the insula. The lower or temporal operculum is in the first 

 instance more rapid in its growth than the upper or fronto- 

 parietal operculum, and thus it comes about that when their 

 margins meet more of the insula is covered by the former than 

 by the latter. So far the development is apparently precisely 

 similar to what occurs in the ape. The slit or fissure formed 

 by the approximation of the margins of these two opercula is 

 called the Sylvian fissure, and it constitutes a natural lower 

 boundary for the parietal and frontal lobes which lie above it. 

 At first, from the more energetic growth of the lower temporal 

 operculum, this fissure slants very obliquely upwards and back- 

 wards, and is very similar in direction to the corresponding 

 fi.ssure in the brain of the ape. But in the human brain this 

 condition is only temporary. Now begins that downward move- 

 ment of the parietal lobe and back part of the frontal lobe to 

 which reference has been made. The upper or fronto-parietal 

 operculum, in the later stages of f etal life and the earlier months 

 of infancy, enters into a growth antagonism with the lower or 

 temporal operculum, and in this it proves the victor. The 

 margins of the two opercula are tightly pressed together, and, 

 slowly but surely, the fronto-parietal operculum gains ground, 

 pressing down the temporal operculum, and thus extending the 

 territory of the frontal and parietal districts. This is a striking 

 process in the brain development of man, and it results in a 

 depression of the Sylvian fissure or the lower frontier line of the 

 frontal and parietal lobes. Further, to judge from the oblique 

 direction of the Sylvian fissure in the brain of the ape, the 

 process is peculiar to man ; in the simian brain there is no 

 corresponding increase in the area of cerebral cortex under 

 consideration. 



I do not think that it is difficult to account for this important 

 expansion of the cerebral surface. In the fore part of the region 

 involved are placed the groups of motor centres which control 

 the muscular movements of the more important parts of the 

 body. These occupy a broad strip of the surface which stretches 

 across the whole depth of the district concerned. Within this 

 are the centres for the arm and hand, for the face, the mouth 

 and the throat, and likewise, to some extent, the centre for 

 speech. In man certain of these have undoubtedly undergone 

 marked expansion. The skilled movements of the hands, as 

 shown in the use of tools, in writing, and so on, have not been 

 acquired without an increase in the brain mechanism by which 

 these are guided. So important, indeed, is the part played by 

 the human hand as an agent of the mind, and so perfectly is it 

 adjusted with rf^erence to this office, that there are many who 

 think that the first great start which man obtained on the path 



