CORTICAL LOCALISATION AND FURROW FORMATIOK GEOFFREY JEFFERSON

Victoria, B. C. (Late Demonstrator in Anatomy, University of Manchesler)

ONE FIGURE

The study of cortical localisation, whether by the more popular microscopical methods of Brodmann, Campbell, Bolton, Mauss and others, or along the macroscopical lines laid down by Elliot Smith, has raised many points of great interest. Not the least important of these is the question of the relationship of the cerebral furrows to the various specialised areas. Anatomists are divided into two camps: Those who believe that the furrows are produced purely by growth antagonism and those who claim chief importance for the influence of local differentiation. It will be shown below that both of these factors have a marked influence, and it will be our present endeavor to examine just what value each of the two has, first in the primate brain and then in lower brains.

With regard in the first instance to cortical differentiation. It will be seen that this factor is unsatisfactory as the sole pro- ducer of furrows, for it does not always lead to furrow formation. Brodmann (4) has recently published surveys (fig. 1) of two of the lower monkeys (Hapale jacchus and Lemur niger), both of which have neopallial surfaces showing distinct histological areas. Yet neither of them has many furrows; Hapale jacchus has no sulcus centralis, for instance, although the animal possesses sharply defined motor and sensory areas, and this furrow is but dimly fore-shadowed in the Lemur named. If cortical special- isation was sufficient in itself to produce furrows, then any animal which had more than one area on its brain (and what animal has not?) should have corresponding furrows to limit the boundaries of those areas, or to be enfolded axially along them, to use Elliot Smith’s terminology. As we have just seen- and many other examples could be cited-this is not the case.

291

292 GEOFFREY JEFFERSON

Further, the maximum and minimum thickness of the gray matter of man are about 4.0 mm. and 1.25 mm. respectively. So that the maximum depth of the furrow that could run be- tween contiguous areas, if specialisation were the only cause of furrows, would be 2.75 mni. This in itself is enough to show that other factors besides cortical specialisation are involved in the production of the sulci.

WitJh regard next to growth nntsgonism: It is evident that growth antagonism must play an all the areas would be spread out,

important part, otherwise evenly upon the surface of

17

Fig. 1 (‘oi.tical n i : i p of Ahrm(Jsct, Hapale jac.c:hns jI\. 13rodinttnn ( S ) ] . Shows that. specialisat ion nlonp is iiot sufficbient t o prodiicc furrows. When these areas hayo iric,rcasetl in extent, and new rm’s h n v ~ been added, furrows will have to a p p c : ~ . Their sites raii drcntly be foiwastc:cl on this map. :ts t.he edges of the :~r(tns corresl)ond r e ina rh l j l y with i h r position of thc fiirrovs in higher brains.

the brain, and fissures w-ould be absent. This state of affnirs is almost possibk in the lower monkeys whose cortical surfaces have but few areas to accommodate, and accordingly in their case the brain is to a large extent smooth. In the human brain, where place has to be found for upwards of forty distinctly different histological areas, the displaying of all these areas in toto is impossible, as an enormous cranium would he necessary. Only some 36 per cent of the cortical areas are exhibited on the surface of the human brain: 64 per cent are hidden in the fur- rows (4). This means that the cranium would have to be three

CORTICAL LOCALISATION 293

times the size that it is at present if man were to have a smooth brain, and would necessitate a head so much heavier that the whole skeleton would have to be remodelled to support it. The figures just given show the immense importance of growth antagonism as a causative factor in the production of the cere- bral furrows. But growth antagonism between t'he expanding cortex and its fibro-osseous capsule is not sufficient in itself to account for the whole story of furrow formation in the primate brain, in which the furrows on the whole are remarkably con- stant. If the formation of sulci depended alone on the crumpling of a plastic surface to accommodate itself to a limited space, the furrows so formed would be not necessarily the same in any two brains. The constancy of position, length, inclination, and direction, of the cerebral sulci of man and the anthropoids denotes a mode of origin less fortuitous than this.

It is evident, then, that the necessity for the appearance of furrows at all arises from growth antagonism, but we must look to architectural changes in the gray matter as the guiding influence in the placing of the furrows. It is of course true that the edges of the areas and the sulci do not always corre- spond with mathematical precision. On the other hand, the remarkably close relationship between the two is one of the most striking facts that the cortical maps have brought out. Vntil comparatively recently the furrows were taught to students with great zeal as more important than the gyri. We now real- ize that the two are causally interdependent. When we have more cortical maps, so that an average can be taken and bound- aries finally decided upon, it may well be that we shall be able to map out the various areas free-hand from the sulci done. This is to a large extent possible to-day, as in the case of motor, sensory, visual and parietal areas.

GROWTH ANTAGONISM

This is a phrase which needs some explanation. Literally it means antagonism between (in this case) a rapidly expanding cortex and its limiting capsule. But we shall not be viewing it in t.he proper perspective unless we abtempt to realize how

294 GEOFFREY JEFFERSON

it has come about that there should be any antagonism a t all between the brain and the skull. Antagonism, as will be seen below, is practically absent in Prosimiae, but is present to a marked degree in man owing to the many new areas he has acquired. Hence a better phrase would be ‘evolutionary antag- onism’, particularly as there is much evidence to show that the human skull and brain are markedly in sympathy,’ as it were, as regards growth in any individual case although the space allowed the brain is small. So that ‘growth antagonism’ is a misleading term in some respects. It must be remembered that there are two integral factors involved in growth antagonism: (1) Rapid cortical expansion; (2) Limited intra-cranial space. Either of these two factors is capable of profoundly influencing the amount of furrow formation and convolution. But i t seems as if Nature had regulated these factors for each order. As we shall see, it seems as if the intra-cranial space was relatively set for all the primates, whilst the amount of cortical expansion is gradually increased. In other orders different arrangements are made. All histological evidence as embodied in cortical maps, supports the fact that the human brain is richer in areas than that of any other animal. Difference of histological structure and difference of function seem to be closely allied. For as we trace the brains of the animal scale upwards and new functions are seen to be acquired we find that the gray matter increases in extent, in area, and not in thickness. Brodmann has shown that in certain Prosimiae whose brains are poor in special areas the amount of cerebral surface to be accommodated and the amount of room provided are not much discrepant, so that, only 7 per cent of the total surface has to be infolded in furrows. In the anthropoids so many new areas to house new functions have arisen that the discrepancy is much greater, whilst in man the quantity of brain surface in excess of the accommodation has risen to be two-thirds of the whole (see table 1). Thus man’s brain is more highly convoluted than that of any of the primates, because the increased area of neopallium

cephalp, and the reverse. I refer here to cases of arrested cerebral detelopment and coincident micro-

CORTICAL LOCALISATION 285

lodging the newly acquired functions has not been accom- panied by a proportionate increase in thc intra-cranial space provided.

A study of Brodmann’s interesting table (4) (see next page) in full shows a remarkably gradual and progressive diminution in convolutioning as we descend through the primates to the lowest monkeys. It is therefore probable that the factors in furrow for- mation are the same and are constant throughout the primates, otherwise the downward procession would not be so orderly. The probable cause of this is to be found in the fact that in the primates the cranial capacity remains relatively more or less con- stant, whilst the surface area to be accommodated gradually increases from the Lemurs to man, so that more and more fur- rows arise.

The new areas that make their appearance in the anthropoids are the association areas. In 1013 the writer (1) (2) (3) pointed out that the parietal field was a new acquisition in human and anthropoid brains and therefore that the furrows traversing this area were new formations too and could have no homologies on lower brains. Ingalls (6) has since confirmed this. The pro- simian neopallium is almost entirely composed of the motor and the various sensory areas, each surrounded or contiguous with its corresponding “psychic” area (Flechsig and Bolton). Whilst in the higher brains these areas become widely separated nota- bly by the development of the posterior association field. In spite of the appearance of this large new mea behind the cen- tral siilcus the furrow is placed further back (more caudally) on the human brain than on the lower ones. Brodniann (5) has shown this to be due to the great increase in extent of the regio frontalis (proper). So that with these new areas to be accommodated it is not surprising that the human brain should be more highly convoluted than the lower, since the intra- cranial space has not increased coincidentally with it. It must be granted that when furrows do appear t!iey tend to do so at the edges of the specinlised areas, as Elliot Smith ( 7 ) was tJhc first to point out. Difference of structure necessarily denotes a difference of texture, as we know in civil life from our experi-

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ence of fabrics. So that when furrows must occur, their position, length, and direction are determined by differences of archi- tecture in the gray matter. 13ut the depth of the furrows, and what is synonymous with this-the amount of cortex buried in the sulci, is determined by the amount of growth antagonism present. In other wordh, in the primates furrows are necessarily produced because a great number of cortical areas have to be housed in a small space and the depth of the furrows will depend on the amount of discrepancy between the volume of the occu- pier and the space to be occupied.

I t must be remembered that a furrow is not an entity in itself any more than dyspepsia is a disease. Both are symptoms- the furrow of evolutionary changes in the cortex. It will he quite evident in the light of modern kno~vledge that the much discussed (6) temporary furrows are post-mortem artifacts, as Hochstetter and Elliot Smit,h have shown.

So much for furrow formation in the primates. A glance a t thc accompanying table will reveal a very curious

fact, that there are brains more rich in sulci and gyri than man’s, and this in animals much further removed from him than his ancestral primates. Thus certain of the carnivora, ungulates and cetacea, have brains distinguished by an extraordinary luxuriance of convolution. ,Ind it is very confounding at first to observe that the elephant, horse, and probably the whale (Brodniann) have more convolutions than man. It is impossible to concede that the brains of these animals can be so rich in areas as many of the lower apes, riot to mention man, in spite of the so-called sagacity of the elephant, horse and dog. Corti- cal rnaps of these animals are not yet forthcoming,* since atten- tion has been very naturally fixed first on the primates. Yet the only logical inference that can be drawn in the case of the genera now under discussion is that furrow formation in their case must depend on factors different from those just laid down for the primates. And since cortical specialisation has pre- sumably not reached a high point in them, we must look to the

YBiothn,z~~n refers to surveys of the dog’s brain by himself and by his col- eague Scharff i(3). p. 1791. He gives no reference.

CORTICAL LOCALISATION 299

second factor iH growth antagonism-restricted intracranial space-as the chief cause. The skulls of all these animals named are adapted to special purposes-that of the carnivora for the seizing and killing of prey, necessitating very powerful temporal muscles with great buttresses of bone to give wide origin to them. In the ungulates the head is modified for the cutting of food from the surfa.ce of the ground and for providing a large area for the grinding of herbs.

In the cetacea the head end of the body has to be kept small to enable the animal to plough the water with the least possible resistance, whilst at the same time these animals possess an enormous body weight. which has to be represented in their brains. So that it is not surprising that the amount of con- volutioning in the cetacea should be, as i t is, greater than in the carnivora and most of the primates. In the primates the existence of the prehensile paw allows the skeleton of the face to be reduced to a minimum so that more space can be allowed the brain, and the weight of the skull still be within supportable limits.

CONCLUSIONS

1. That to say that furrow formation depends alone either on cortical specialisation or on growth antagonism is not to be sufficiently explicit.

2. That furrow formation depends primarily on evolutionary antagonism between the neopallium which is constantly ac- quiring new areas, and its fibro-osseous capsule, the skull.

3. That the furrows t8hus originated tend to appear a t the edges of areas possessing cyto-architectural differences. 4. That in the primate brain furrow formation depends on

constant factors throughout the order. 5. That the highly convoluted brains of carnivora, unpulata

and cetacea argues a mode of origin different from that existing in the primates.

300 (:EOFFREY JEFFERSON

REFERENCES

(1) JEFFERSON, GEOFFREY 1913 a 1 he interparietal sulcus. Brit. Journ. of Anat., pol. 47, July.

(2) JEFFERSON, GEOFFREY 1913 b A note on the superior postcentral sulcus. Anatom. Anzeiger, Band 44.

(3) JEFFERSON, GEOFFREY 1914 The parietal area. Review of Keurol. and Psychiatry, February.

(4) BRODMAXN, 6. 1913 Xeue Forschungsergebnisse der Grosshirnrinden- anatomie mit besonderer Beriicksichtigung anthropologischer Fragen. Gesell. Deutsch. Naturfors. u. h-zte.

(5) BRODMANN, K. 1912 Neue Ergebnisse uber die vergleichende histolog. Lokalisation der Grosshirnrinde mit besond. Berucksich. d. Stirn- hirns.

(6) INGALLS, N . W. 1914 Parietal region in the primate brain. Journ. of Cornparati\ e Neurology, June.

(7) ELLIOT SMITH, G. 1913 Section on “The central nervous system.” Cun- ningham’s text-book of anatomy. 4th Edn. Also, 1907 S e w topo- graphical survey of human cerebral cortex. Brit. Journ. of Anat., iol . 41.

Verhand. d. Anatom. Gesell., April.