434 



NATURE 



[September 2, 1897 



uniform period of time, so that some acquired their complete 

 functional importance before others. He has now applied the 

 same method of research to the study of the development of the 

 human brain; and has shown that in it also there is a difference 

 in the time of attaining perfect structural development of the 

 nerve-tracts. Further, he has discovered that the nerve-fibres 

 in the cerebrum become myelinated, subsequent to the fibres of 

 the other divisions of the cerebro-spinal nervous axis. When a 

 child is born, very few of the fibres of its cerebrum are myeli- 

 nated, and we have now an anatomical explanation of the 

 reason why an infant has so inactive a brain and is so helpless 

 a creature. It will therefore be of especial interest to deter- 

 mine, whether in those animals which are active as soon as they 

 are born, and which can at once assume the characteristic 

 attitude of the species, the fibres of the cerebrum are completely 

 developed at the time of birth. Flechsig has also shown that 

 the sensory paths myelinate before the motor tracts ; that the 

 paths of transmission of touch, and the other impulses conducted 

 by the dorsal roots of the spinal nerves, are the first to become 

 completely formed, whilst the fibres for auditory impulses are 

 the last. 



Flechsig names the great sensory centre which receives the 

 impulses associated with touch, pain, temperature, muscular 

 sense, &c. , Korperfiililsphdre, the region of general-body- 

 sensation, or the somsesthetic area, as translated by Dr. Barker.^ 

 The tracts conducting these impulses myelinate at successive 

 periods after birth. They pass upwards from the inner and 

 outer capsules and the optic thalamus as three systems.^ Some 

 enter the central convolutions of the Rolandic area, others reach 

 the paracentral lobule, the inferior frontal convolution, the 

 insula, and small parts of the middle and superior frontal con- 

 volutions ; whilst considerable numbers reach the gyrus forni- 

 catus and the hippocampal gyrus, which Ferrier had previously 

 localised as a centre of common or tactile sensibility. 



The Rolandic area, therefore, is not exclusively a motor area, 

 but is a centre associated also with the general sensibility of 

 the body. The motor fibres in it are not myelinated until after 

 the sensory paths have become developed. As the motor paths 

 become structurally complete, they can be traced downwards 

 as the great pyramidal tract from the pyramidal nerve-cells in 

 this area, from which they arise, into the spinal cord, where 

 they come into close relation with the nerve-cells in the anterior 

 horn of grey matter, from which the nerve axial fibres proceed 

 that are distributed to the voluntary muscles. 



Flechsig's observations agree with those of previous observers 

 in placing the visual centre in the occipital lobe ; the auditory 

 centre in and near the superior temporal convolution ; and the 

 olfactory centre in the uncinate and hippocampal convolutions. 

 Of the position of the taste centre he does not speak definitely, 

 although he thinks it to be in proximity either to the centre of 

 general sensation, or to the olfactory centre. 



The centres of special sense in the cortex, and the large 

 Rolandic area, which is the centre both for motion and general 

 sensation, do not collectively occupy so much as one-half of the 

 superficial area of the convolutions of the cortex. In all the 

 lobes of the brain — frontal, parietal, occipito-temporal, and 

 insula— convolutions are situated, not directly associated with 

 the reception of sensory impressions, or as centres of motor 

 activity, the function of which is to be otherwise accounted 

 for. These convolutions lie intermediary to the sensory and 

 motor centres. Flechsig has shown that in them myelination 

 of the nerve-fibres does not take place until some weeks after 

 birth, so that they are distinctly later in acquiring their structural 

 perfection and functional activity. As the nerve-fibres become 

 differentiated, they are seen to pass from the sense-centres into 

 these intermediate convolutions, so as to connect adjacent 

 centres together, and bring them into association with each 

 other. 3 Hence he has called them the association centres, the 



1 Johns Hopkins Bulletin, No. 70, January 1897. 



2 Drs. Ferrier and Aldren Turner communicated to the Royal Society of 

 London a few weeks ago {Proc. Roy Soc, June 17, 1897) an account of an 

 elaborate research on the tracts which convey general and special sensi- 

 bility to the cerebral cortex of monkeys. Their results were obtained by 

 the aid of destructive lesions and thestudy of the consecutive degenerations 

 in the nerve-tracts. From the brief abstract in the Proceeding's, their re- 

 search, though conducted by a different method, harmonises with the ob- 

 servations of Flechsig on the human brain, in regard to the course and 

 connections of the great thalamic cortico-petal .sensory fibres. They have 

 also traced association fibres in connection with both the visual and auditory 

 systems. 



3 The term association fibres was introduced a number of years ago to 

 express fibres of the cerebrum which connect together parts of the cortex 

 in the same hemisphere. Flechsig's fibres belong to this system. 



function of which is to connect together centres and convolutions 

 otherwise disconnected.' 



We have now, therefore, direct anatomical evidence, based 

 upon differences jn their stages of development, that, in addition 

 to the sensory and motor areas in the cortex of the human 

 brain, a third division — the association centres — is to be 

 distinguished. 



If we compare the cerebrum in man and the apes, we find 

 those convolutions which constitute the motor and sensory 

 centres distinctly marked in both. An ape, like a man, can 

 see, hear, taste, smell and touch ; it also exhibits great muscular 

 activity and variety of movement. It possesses, therefore, 

 similar fundamental centres of sensation and motion, which are 

 situated in areas of the cortex, resembling in arrangement and 

 relative position, though much smaller in size than, the corre- 

 sponding convolutions in the adult human brain. It is not 

 unlikely, though the subject needs additional research, that the 

 minute structure of these centres resembles that of man, though, 

 from the comparatively restricted area of grey matter in the ape, 

 the neurones will necessarily be much fewer in number. 



In the cerebrum of a new-born infant, whilst the motor and 

 sensory convolutions are distinct, the convolutions for the 

 association areas, though present, are comparatively simple, 

 and do not po.ssess as many windings as are to be seen in the 

 brain of a Chimpanzee not more than three or four years old. 



Again, if we compare the brain of the Bushwoman, miscalled 

 the Hottentot Venus, figured by Gratiolet and by Bischoff, or 

 the one studied by Mr. John Marshall, with that of the 

 philosopher Gauss, figured by Rudolph Wagner, we also re- 

 cognise the convolutions in which the motor and sensory areas 

 are situated. In all these brains they have a comparative 

 simplicity of form and arrangement which enables one readily 

 to discriminate them. When we turn, however, to the associa- 

 tion areas in the three tiers of convolutions in the frontal lobe, 

 and in the parieto-occipital and occipito-temporal regions where 

 the bridging or annectant convolutions are placed, we cannot 

 fail to observe that in a highly developed brain, like that of 

 Gauss, the association convolutions have a complexity in 

 arrangement, and an extent of cortical surface much more 

 marked than in the Bushwoman, and to a still greater degree 

 than in the ape. The naked-eye anatomy of the brain therefore 

 obviously points to the conclusion that these association areas 

 are of great physiological importance. 



The problem which has now to be solved is the determin- 

 ation of their function. Prolonged investigation into the de- 

 velopment and comparative histology of the brain will be 

 necessary before we can reach a sound anatomical basis on 

 which to found satisfactory conclusions. It will especially be 

 necessary to study the successive periods of development of 

 the nerve-fibre tracts in the cerebrum of apes and other mammals, 

 as well as the magnitude and intimate structure of the associa- 

 tion areas in relation to that of the motor and sensory areas in 

 the same species. 



Flechsig, however, has not hesitated to ascribe to the associa- 

 tion centres functions of the highest order. He believes them 

 to be parts of the cerebral cortex engaged in the manifestations 

 of the higher intelligence, such as memory, judgment, and re- 

 flection ; but in the present state of our knowledge such con- 

 clusions are of course quite speculative. 



It is not unlikely, however, that the impulses which are 

 conveyed by the intermediate nerve-tracts, either on the one 

 hand, from the sense centres to the association centres, or on 

 the other, from the association centres to the sensory and motor 

 centres, are neither motor nor sensory impulses, but a form of 

 nerve energy, determined by the terminal connections and con- 

 tacts of the nerve-fibres. It is possible that the association 

 centres, with the intermediate connecting tracts, may serve to 

 harmonise and control the centres for the reception of sensory 

 impressions that we translate into consciousness, with those 

 which excite motor activity, so as to give to the brain a com- 

 pleteness and perfection of structural mechanism, which without 

 them it could not have possessed. 



We know that an animal is guided by its instincts, through 

 which it provides for its individual wants, and fulfils its place 

 in nature. In man, on the other hand, the instinctive acts are 

 under the influence of the rea.son and intelligence, and it is 



1 The association centres had previously been referred to by other ob- 

 servers as "silent portions" of the cortex, not responding to electrical 

 stimulus. Their possible function had beeii discussed by Prof. Calderwood 

 in " Relations of Mind and Brain," 2nd edit., 1884. 



NO. 1453. VOL. 56] 



