September 26, 1901] 



NATURE 



54; 



which has led to his higher development was given by the setting 

 of the upper limb free from the duty of acting as an organ of 

 support and locomotion. It is an old saying " that man is the 

 wisest of animals because of his hands." Without endorsing to 

 iis full extent this view, I think that it cannot be a matter for 

 surprise that the district of the cerebral corte.x in man in which 

 the arm-centres reside shows a manifest increase in its extent. 



In the same region of cerebral cortex, but at a lower level, 

 there are also situated the centres which are responsible for 

 facial expression. In the ape there is a considerable degree of 

 facial play ; but this is chiefly confined to the region of the 

 lips : and the muscles of the face, although present in greater 

 mass, show comparatively little of the differentiation which is 

 characteristic of the lighter and more feeble muscles in the 

 face of man. And then as to the effect produced : These 

 human muscles are capable of reflecting every fleeting emotion, 

 every charge of mind, and by the lines and furrows their con- 

 stant use indelibly fix on the countenance the character and 

 disposition of an individual can to some extent be read. As the 

 power of communication between primitive men became 

 gradually established, facial movements were no doubt largely 

 used, not only for the purpose of giving expression to simple 

 emotions, such as anger or joy, but also for giving point and 

 force to the faltering speech of our early progenitors by reflect- 

 ing other conditions of mind. The acquisition of this power 

 as well as the higher and more varied powers of vocalisation 

 must necessarily have been accompanied by an increase of 

 ceiebral cortex in the region under consideration. And in this 

 connection it is a point welt worthy of note that the area of 

 cortex mapped out in the human brain ' as controlling the 

 muscles of the face, mouth, and throat is as large, if not larger 

 than that allotted to the arm and hand," and yet it is question- 

 able if all the muscles under the sway of the former would weigh 

 as much as one of the larger muscles (say the triceps) of the arm. 

 This is sufticient to show that it is not muscle power which 

 determines the extent of the motor areas in the cerebral cortex. 

 It is the degree of refinement in the movements required, as 

 well as the degree of variety in muscle combinations, which 

 apparently determines the amount of ground covered by a motor 

 centre. 



Still, the increase in the amount of cerebral cortex in man due 

 to the greater refinement of movement acquired by different 

 groups of muscles is relatively small in comparison with the 

 increase which has occurred in other regions from which no 

 motor fibres are sent out, and which therefore have no direct 

 connection with muscles. 



The remarkable conclusions arrived at by Flechsig, although 

 not confirmed and accepted in all their details, have tended 

 greatly to clear up much that was obscure in the relations of 

 the different districts of cerebral cortex. More particularly has 

 he been able to apportion out more accurately the different 

 values to be attached to the several areas of the cerebral surface. 

 He has shown that fully two thirds of the cortex in the human 

 brain constitute what he terms " association centres. " Within 

 these the higher intellectual manifestations of the brain have 

 their origin, and judgment and memory have their seat. They 

 are therefore to be regarded as the psychic centres of the 

 cerebral cortex. 



Now, it requires a very slight acquaintance with the cerebral 

 surface to perceive that the great and leading peculiarity of the 

 human brain is the wide extent of these higher association 

 centres of Flechsig. Except in connection with new faculties, 

 such as speech, there has been relatively no striking increase in 

 the extent of the motor areas in man as compared with the 

 cortex of the ape or the idiot, but the expansion of the associa- 

 tion areas is enormous and the increase in the frontal region 

 and the back part of the parietal region is particularly well 

 marked. It is this parietal extension of surface which is chiefly 

 responsible for the pushing down of the lower frontier of the 

 parietal lobe and the consequent enlargement of its territory. 



I have already referred to the views which have been recently 

 urged by several independent observers, that in the men who 



1 See diagram in Schiifer's article on the "Cerebral Cortex " in his recent 

 work on physiology. 



- The comparison only refers to surface area, and this is not an absolutely 

 true criterion of the relative amount of cortex in each region. The arm- 

 centre has a large amount of cortex stowed away within the fissure of 

 Rolando in the shape of inter-locking gyri which is not taken into account 

 in a measurement confined to the superficial surface area. Still, this does 

 not to any great degree detract from the argument which follows, seeing 

 that the discrepancy is still sufficiently marked. 



NO. 1665, VOL. 64] 



have been distinguished during life by the possession of excep- 

 tional intellectual power, this region has shown a very special 

 development. 



It i<; a curious circumstance, and one which is worthy of con- 

 sideration, that in the left cerebral hemisphere the Sylvian 

 fissure or the lower boundary of the parietal lobe is more de- 

 pressed than in the right hemisphere, and, as a result of this, 

 the surface area occupied by the parietal lobe is greater on the 

 left side of the brain than on the right side. To the physiolfgist 

 it is a matter of every-day knowledge that the left cereijral 

 hemisphere shows in certain directions a marked functional pre- 

 eminence. Through it the movements of the right arm and 

 right side of the body are controlled and regulated. Within it 

 is situated also the active speech centre. This does not imply 

 that there is no speech centre on the right side, but simply that 

 the left cerebral hemisphere has usurped the chief, if not the 

 entire, control of this all-nnportant function, and that from it 

 are sent out the chief part, if not the whole, of the motor 

 incitations which give rise to speech. The significance attached 

 to the dominant power of the left hemisphere receives force froia 

 the now well-established fact that in left-handed individuals the 

 speech function is also transferred over to the right side of i he- 

 brain. To account for this functional preeminence of the left 

 cerebral hemisphere numerous theories have been elabora'ed. 

 The interest attached to the subject is very considerable, but it 

 is impossible on the present occasion to do more than indicate 

 in the briefest manner the three views which have apparently 

 had the widest influence in shaping opinion on this question. 

 They are : (i) that the superiority of the left cerebral hemisphere 

 is due to its greater weight and bulk ; (2) that it may be 

 accounted for by the greater complexity of the convolutions on 

 the left brain and the fact that these make their appearance 

 earlier on the left side than on the right side ; (3) that the 

 explanation lies in the fact that the left side of the brain enjoys 

 greater advantages in regard to its blood supply than the right 

 side. 



Not one of these theories when closely looked into is found 

 to possess the smallest degree of value. Braune ' has shown in 

 the most conclusive manner that if there is any difference in 

 weight between the two hemispheres it is a difference in favour 

 of the right and not of the left hemisphere ; and I may add from 

 my own observations that this is evident at all periods of growth 

 and development. Equally untrustworthy are the views that 

 have been put forward as to the superiority of the left hemi- 

 sphere from the point of view of convolutionary development. I 

 am aware that it is stated that in two or three cases where the 

 brains of left-handed people have been examined this superiority 

 was evident on the right hemisphere. This may have been so ; 

 I can only speak for the large percentage of those who are 

 right-handed ; and I have never been able to satisfy myself that 

 either in the growing or fully developed brain is there any con- 

 stant or marked superiority in this respect of the one side over 

 the other ; and I can corroborate Ecker {Archiv fiir Anlhrop- 

 ologie, 1868, Bd. cxi. ) in his statement that there is no proof 

 that the convolutions appear earlier on the one side than the 

 other. The theory that an explanation is to be found in a more 

 generous blood supply to the left hemisphere is more difticult to 

 combat, because the amount of blood received by each side of 

 the brain depends upon two factors, viz., the physical condi- 

 tions under which the blood-stream is delivered to the two 

 hemispheres and the calibre of the arteries or tubes of supply. 

 Both of these conditions have been stated to be favourable to 

 the left hemisphere. It is a matter of common anatomical 

 knowledge that the' supply pipes to the two sides of the brain 

 are laid down somewhat differently, and that the angles of junc- 

 tion, &c., with the main pipe are not quite the same. Further, 

 it is true that the blood-drains which lead away the blood from 

 the brain are somewhat different on the two sides. Whether 

 this would entail any marked difference in the blood-pressure 

 on the two sides I am not prepared to say. This could only be 

 proved experimentally ; but, taking all the conditions into con- 

 sideration, I am not inclined to attach much importance to the 

 argument It is easy to deal with the loose statements which 

 have been made in regard to the size of leading supply pipe 

 (viz., the internal carotid artery). It passes through a bony 

 canal in the floor of the cranium on its way into the interior of 

 the cranial box. Its size can therefore be accurately gauged by 

 measuring the sectional area of this bony tunnel on each side. 



1 "Das Gewichtsverhaltniss der rechten zur linken Hirnhalfte teim 

 Menschen " {Archiv fnr Aftat.). 



