studies of the size of the cells in the cerebral cortex. iii. the striate area of man, orang and...

STUDIES O F THE SIZE O F THE CELLS I N THE CEREBRAL CORTEX

111. THE STRIATE AREA O F M A N , ORANG AX'D CEBUS

GERHARDT VON BONIN Departmmt of dnntoniy, Umivei-sity of Illinois, Chicago

POUR FIGURES

IXTRODUCTION

I n the previous commuriicatiori of this series (v. Bonin, '38 b) it was showii that the distribution curve of the motor area showed statistically significant differences from species to species. The coefficient of variation increased with increasing complexity of the brain, and the giant cells of Betz were larger in comparison with the ordinary cells in the brain of mail than in that of lower forms. It appeared as though increased variability of cell size were a sign of higher cerebral development.

The present study on the visual area of three primates iu- tends in the first place to test these conclusions, and in the second place to analyze, by a different method, how far the size of cells depends on their localizations in various depths of the cortex.

The visual area, 17 of Brodmanri ('09) and OC of v. Economo and Koskiiias ( '25) was chosen since its structure is so characteristic that its identification presents no difficulty whatsoever. The stripe of Gennari o r Vicy d'Azyr, visible to the naked eye, call be demonstrated even to a freshman, and has led to the term area striata, introduced, so fa r as the writer is aware, by Elliot Smith ('04). The arrangement of the cells in eight ( o r nine) layers is so well known that it seems superfluous to describe them here once more. To

395

396 GERHARDT VON BONIN

remind the reader that this structure is found only in primates and (in its incipient stages) in some rodents may be similarly superfluous. But a few words should be added about the current interpretations of these layers. Many students of Nissl preparations appear to embrace Brodmann's ( '09) theory that in the striate area the fourth layer has split up into three sublayers. Vogt and Vogt (,19), Rose ('35), Le Gros Clark ( ' as ) , and v. Volkmann ( '28) belong to this group. The nomenclature introduced by Brodmann is indeed almost universally employed. Brodmann's theory, however, has never been accepted by students of Golgi preparations. Ramon y Cajal ('11) and Lorente de N6 ('35) have shown that the cells in Brodmann's layer I V A are mostly small pyramidal cells of exactly the type found in layer 111. In a later study on the visual cortex of the cat, Ramon y Cajal ('23) has again emphasized the essential similarity in the structure of the striate area of cat and man, pointing to the interpretation of Brodmann's I V A as being in fact I I IC. v. Economo and Koskinas ('25) emphasized that even the Nissl picture showed many small pyramids in IVA. While leaving this question unsettled they appear to incline toward the view of the Spanish master.

The striate area contains conspicuously large cells in I V B and V. Meynert was the first to describe them under the general name of So1itarzellen.l Those in the fourth layer are large stellate cells, the shape of which has been minutely described by Ramon y Cajal ('11). Fibers from the optic radiation end directly around their perikarya. The large solitary cells in the fifth layer are pyramidal cells, the basal den- drites of which form a dense protoplasmic plexus.

That the striate area is the visuo-sensory area is now universally admitted. It has been shown for many species that the optic radiation from the lateral geniculate body ends in this area. Polyak ('32) and Brouwer ( ' 3 6 ) and their co- workers have done the most important work in this field.

'Le Gros Clark ( ' 2 5 ) again has called attention to the fact that Meynert himself did not clearly differentiate between the large stellate cells of I V B and the large pyramidal cells of V.

SIZE OF CELLS I N STRIATE AREA 397

A separate parastriate area (18, OB) was recognized by Elliot Smith ( '07) who introduced that term and by Brodmann ('09). I t is built according to the six-layer pattern characteristic for the isocortex. In non-primates it seems to be absent. Near the border between striate and parastriate area, the latter is characterized by the presence of very large pyramidal cells in layer IT1 C. The functional significance of this magno-cellular margin is f a r from clear. Since larqe pyramidal cells are generally associated with motor function, the view is widely held that these cells are in some way con- nected with eye movements.

Evidence from monkeys, such as that of Levinsohn ('09), or that collected by Vogt and Vogt ('07) seems to support this view,2 but clinical observations are far from being con- firmatory. According to Foerster ('36) stimulation of this area in man fails to elicit eye movements. This author as well as Penfield and Boldrey ('37) could evoke eye movements only from area OA (Brodmann's area 19).3

When viewing slides under the microscope, the margo magnocellularis appears to be more pronounced in man than in the orang or cebus. In man the large pyramidal cells stand out more clearly; they appear to be more numerous and spread over a wider band than in the primates. In some way this large-celled margin is evidently due to the formative in- fluence of the striate area, as observations of isolated islands of OB within OC bearing giant cells (Solis, '36) indicate. I t is for this reason that the parastriate margin, to be referred to as OBy, has been included in the present study. To wall off or shield the striate area from the surrounding cortex may be its primary function.

The location of the striate area in the depth and on both sides of the calcarinc fissure, as well as its sharp boundary

a Levinsohn's points y6c< belong very probably t o the area OBy. 'Penfield and Boldrey ('37) report tha t upon stimulation of the occipital

cortex one of their patients had the sensation of light and moved his eyes toward the light spot in his victual field. I n their opinion the adversive movements elicited from the striate area in animals may find their explanation in a similar way.


against the parastriate area, OB of v. Ecoiiomo and Koskinas ( ' a s ) , are too well known to require further description. Its extent and position has recently been studied by Ngowyang ('37). For man the most exhaustive description of its cyto- architecture has probably been given by v. Economo and Roskinas ( '25). For the orang, Filimonoff ( '33) gave a de- tailed account, and for cebus a paper by the present writer ( '38a) may be consulted.

TIIE DISTRIBUTION CURVES

The measurements on which this study is based were taken on one brain of each species: the human brain and that of the orang were fixed in neutral formalin, embedded in cel- loidin and stained with cresyl violet following Weil's ('33) technique. The writer is indebted to the late Professor Jaffe for the human brain, and to Dr. Pilot for kindly placing the brain of an orang at his disposal. The animal had died about 24 hours prior to the post-mortem. The brain of the cebus was fixed in alcohol, cut in celloidiii and stained with thionin. The sections wcre lent by Dr. Kliiver to whom the writer wishes to express his gratitude. All observations were made on parts of the cortex on its free surface.

To thank his chief, I)r. 0. F. Kampmeier, for his generous support is once more a pleasure for the author.

The statistical constants of the distribution curves of nuclear volume for the two areas OC and OBy are given in table 1 (for explanation of the symbols compare Y. Uonin, '38 b, and references given there). All curves are significantly different from the 'normal' curve as a glance at the constants gl and g2 will show. Both for OC and for OBy the average size of the iiuclei decreases as we go down the table. It is a corollary to this fact that the cell density is greatest in Cebus (compare Mayer, '12, and v. Bonin, '38 a ) .

I n all three species, the mean for OC is smaller than that for OBy. The difference between the two areas is greatest for man and smallest for Cebus. We obtain the following values :

SIZE OF CELLS I N STRIATE AREA

185.223.31

109.123.04 122.223.59 _ _ _ _ -

399

76.922.48

70.122.15 69.6k2.54 _____

Mean of OBy in per cent of mean of OC: Man 137% Orang 112% Cebus 104%

The difference in cell size between the two adjacent areas decreases as we come to simpler brains. On the other hand, we find that the coefficients of variation show no clear-cut trend, contrary to what we found in the study on the motor area. For OC and OBy, man has the lowest coefficients, then comes Cebus, then the orang.

TABLE 1 Stutivtical constunlu of striate ureu ( O C ) and parastriate margin ( O B y )

nuclear volumes in ps

OC (646) I 134.822.39 60.721.69 145.021.48 OBy (480)

Orang OC (532) OBy (376)

OC (871) OBr (534)

~

Cebus

41.5-Cl.55 -. - -

64.322.66 57.022.71

55.021.67 61.3-r-2.48

1.8220.096 ' 6.84f0.192 2.60rt0.111 ,14.5020.222

I

-- 1 3.11&0.106 15.1020.211 2.0420.126 ' 6.23k0.251 -~

1.5720.083 3.6920.166 2.25f0.106 I 8.7420.211

M = mean; u = standard deviation; V = coefficient of variation; g, and g, = higher parameters.

Almost all the differences between the parameters g , and g , are significant, those between the values of gl for man and Cebus in the case of OC, and f o r orang arid Cebus in the case of OBy being the only exceptions. The mean nuclear volumes of the special cells in area OC and OBy are given in table 2. The size of the stellate cells show surprisingly little variation from species to species. This result can be accepted unre- servedly for man and orang since both brains were treated according to the same technique, but would need further check- ing for Cebus, the brain of which was fixed differently. The solitary pyramidal cells, on the other hand, show a distinct trend: they are largest in man, smaller in the orang and


smallest in Cebus. There is a clearly significant difference between man and Cebus (169.5 t 38.4 $). For man and orang we find a difference of 91.6 r+ 42.6 F ~ , for orang and Cebus 77.9 35 v3. For both of these values we find a probability of between 0.05 and 0.02 to have arisen by chance alone. In view of the clear-cut trend just pointed out it seems per- missible to accept these differences as real. When compared with the mean size of all cells, the solitary pyramidal cells of man and orang are both 3.8 times as large as the ordinary cells while those of Cebus are 4.25 times as large. The nuclear volumes of the large pyramidal cells in OBy shows the same trend as those of OC V ; only in an accentuated manner.

TABLE 2

Nuclear w lume of special cells (in f i3)

O W i oc 1 I

Man Orang Cebus

Giant stellate ( IVB)

315.3k11.4 321.1k24.1 325.4428.9

Solitary pyramidal

507.1k32.3 4 1 5 5 2 2 7 . 7 337.6-t20.7

(V)

Large pyramidal

820.6238.1 515.0f52.2 341.2f19.1

(111)

There are significant differences between all three species. F o r man and orang we find 315.6 64.6 v3 for orang and Cebus 173.8 -c- 55.6 p3. Compared to the mean nuclear volume of all cells (table l), the large pyramidal cells of man are 4.4 times their size while those of the orang and Cebus are only 4.2 times their size. The difference between man and the two primates is so small, particularly in view of the large sampling errors of the large cells that no great value can be attached to it.

It is of some interest to test Bok's ('36) formula for cellular volume in this connection. Assuming the large pyramidal cells to be three times as high as they are broad, putting, in other words h = 6 r where h denotes height and r the radius of the cell, and considering them to be perfect cones the

SIZE O F CELLS I N STRIATE AREA 401

volume of which is given by v =+-r2h; we obtain, in conjunc- tion with Bok's formula v =-&* O1 (where N denotes nuclear volume) the following figures :

h d = 2 r

Man 66 22 Orang 48 16 Cebus 36 12

F o r man, v. Economo and Koskinas ( '25, p. 618) give i:<g IJ

in very good agreement with our figures. On the whole then, the study of the distribution curves and

the size of the large cells has confirmed our previous result that there are statistically significant differences between homologous areas of various species thus jeopardizing any at- tempt to arrive a t a criterion of homology by this method. I t has shown further that the striate area of man and its large-celled margin show less variability than that of some lower forms in contrast to its motor area. The difference in mean cell size between OC and OBy, however, arranges the species in a reasonable order. There is finally a surprising equality of the nuclear volume of the giant stellate cells in all three species.

The trend in the variability discovered here need not sur- prise us too much. I n the first place, even the motor cortices of man and Cebus showed no difference in their variabilities. But apart from that, it is perfectly possible for motor and sensory fields to have opposite phylogenetic trends in this quantitative measurement. It should be borne in mind that a sensory area-and the striate area is perhaps the most highly developed sensory area 4-will tend to become largely re- ceptor. This process can, of course, never come to comple- tion. The sensory impulses must go to other parts of the cortex, and there are, as Biemond ('30) and others have shown, corticofugal (projection) fibers in the optic radiation. v. Weizsacker's ('33) views and his conception of the 'Gestalt- kreis' may be alluded to. But large and massive projection

Thus Carnap ( '28) , e.g., gires the optic sense the highest number of di- mensions: (DZ) = 5.

402 GERHARDT TON BONIN

fibers evidently become of less importance with increasing cerebral differentiation. It is interesting to note, e.g., that eye movements can be elicited from the striate area of lower forms, but not from that of man.j The motor cortex, on the other hand, receives even in man sensory impulses. It forms part of the field of radiation of the lateral thalamic nucleus (compare Polyak, '32; Walker, '38) and thus subserves both sensory and motor functions. The increase of variation in the motor cortex and the decrease in the striate area may, therefore, be perfectly compatible with each other.

THE SIZE AND DEPTH CURVES

The correlation of the distance of the cells from the cortical surface and of their nuclear volume appeared to afford some further interest. It was studied in detail by Bok ('36). He could show a very simple and beautiful correlation: while in the upper layers of the cortex (I1 and 111) the nuclear volume increases proportional to the depth, in the lower layers IV, V and VI the nuclear volume increases with the square of the depth. Bok measured 1546 ganglion cells 'mostly' in the area temporalis superior posterior (TA,). He emphasizes the importance of making the measurements on a cross section exactly perpendicular to the surface and on a part of the cortex which is not curved.

The mere inspection of microscopical sections through the cortex shows that cells differ in size from area to area, and that i t is improbable that the depth and size curve will be the same for all areas. The first point has, in the writer's opinion, been sufficiently substantiated by the present series of studies. The second point, however, has not yet come within the scope of these studies. It is the object of this com- munication to carry quantitative methods one step further by following Bok's procedure. The tracings of the nuclei for the present study were therefore made in such a way as to record their relative position to each other. To go through the whole depth of the cortex required as much as fifteen or eighteen

'See, however, footnote 3.

SlZE O F CELLS I N STRIATE AREA 403

Nan Orang

121.52 6.1 73 .32 4.1 160 .92 9.6 93 .62 5.7 163.82 8.6 109 .42 9.4 154.62 9.0 101 .62 5.5 117.22 7.5 100.62 5.5 120.4% 4.1 97 .12 6.8 134.7f12.6 j101.32 7.5 123.3210.0 109.5210.4 110 .32 9.0 137.9213.7 110.9f 6.2 133.4213.8

96.52 4.1 107.2k12.4 100.02 4.3 103.32 6.7 148.2220.6 95.6+ 6.3 141.92 8.0 79.0210.2 137 .92 6.0 134.1236.2 167.82 9.5 138.2216.5 165.2210.7 122.5218.0 188.5218.6 160.2231.5 201.1214.6 141.6221.1 220.6229.3 153.2216.9

sheets of paper. When all cells from layer VI to the super- ficial border of layer I1 were traced, the sheets were arranged in proper position, and the whole depth of the cortex divided in twenty parts. For each twentieth the number of nuclei in the various size classes were recorded, and a chart for each area was constructed from all the surveys made. Three cross surveys were taken in all instances. The total number of cells

Cebus ~ Man ’ 52.92 3.2 117 .82 6.0 71.32 6.5 132 .72 5.6 84 .22 7.6 164.6210.3 60.6& 4.9 163 .82 8.3 135.3211.0 114.32 8.5 71.32 7.3 170.7216.4 107.72 7.6 121.5216.0 71 .02 4.9 224.0216.3 150.0k10.2 75.32 6.5 81 .32 6.2 179.6f16.2 173.2216.7 90.3k12.0 71.3+ 5.4 171.5215.4 170.4223.1 62.02 4.6 84.22 5.7 191.3229.8 139 .92 8.8 66 .52 4.1 73 .52 4.2 188.8f46.5 189.6223.3 65 .92 6.3 71 .82 3.6 107.72 7.6 173.2221.7 58 .52 4.0 7 5 . 0 2 4.8 9 7 . 7 2 4.3 190.5230.2 76.42 8.7 75 .02 6.7 120.42 7.8 99 .12 8.2 95.1211.0 97 .42 7.4 140 .82 7.6 86.22 6.9 69.22 6.9 97.42 9.6 163.5227.4 66.92 6.0 7 8 . 7 2 7.5

114.9% 8.7 192.8217.2 123.8k18.0 71.82 7.6 89.92 7.3 169.2213.1 110.9211.4 79.32 9.8 84 .22 7.3 212.3218.0 133.3215.6 79 .62 7.7 93.72 5.7 193.4217.6 133.9220.6 90 .42 7.9 99.1213.5 188.8216.8 l l6.6k12.7 63 .62 7.4

IXTERVAL

1 2 3 4 5 6 7 8 9

30 11 12 13 14 15 16 17 18 19 20

TABLE 3

Y e a n s and standard errors for each level of depth

oc I OBY

for each area are given in table 1. The means for each inter- val of depth (beginning at the border between layer I and I1 were recorded (see table 3) , and the diagrams given in figures 1 and 2 constructed. These ‘curves’ shall be referred to as ‘size and depth curves.’

The size and depth curve of OC for man (fig. 1) starts out fairly low, to rise steeply in layer 111. Then there is a dip, due to the small cells in IV A, a rise in IV B, another still deeper dip in 1V C, and a rise in V followed by a still further


rise in VI. The small dips at this end of the curve can be disregarded, since they may merely be due to the errors of random sampling (compare table 3) . But it is interesting to observe that the cells in IVC are smaller than in IVA. Be- tween columns 5 and ll we find a difference) of 20.7 2 8.5 p3. The probability for this difference to have arisen by chance alone is between 0.02 and 0.01. To test this further we compute the standard errors pooling 5 and 6 on the one, and 11 and 12 on the other hand. We obtain 118.5 t 5.21 p3 and 98.3 -t 2.98 p3 respectively which leads to. a clearly significant difference.

The curve for the orang shows the same general charac- teristics. Valleys and peaks alternate in about the same rhythm as in man although in the orang they are slightly shifted toward the depth. This may merely be due to local variations in the part of the striate area selected for this survey; it would need, at any rate, further observations be- fore any weight could be attached to this fact. In the orang, too, the nuclear volume in IV A is larger than in IV C, but statistically this difference is not significant. We find here between column 5 and column 13 a difference of 18.1 -+ 12.3 p3. There is a similar upward trend in the deeper layers in the orang as in man although the curve appears less steep.

The curve of Cebus does not show that, family resemblance which is so clearly present in the case of man and orang. The peak caused by layer I11 is much less pronounced. The nuclear volume of IV A is smaller than that of I V C, although the difference is clearly not significant. The behavior in IV B is somewhat erratic; we feel inclined to ascribe this to the errors of random sampling. It certainly does not show the distinct peak evident in the curves of both man and the orang. I n the deeper parts (V and VI) the curve is quite irregular. The upward trend found in man and orang appears at first sight to be missing.

Turning to OBy, the curve for man rises to a sharp peak in 6, to be followed by a smaller one in 9. Between the dip in 8 and the higher peak in 6 there is a difference of 52.5

406 GERHARDT VON BONIS

22.4p3, a figure which may have arisen by chance alone in less than 2 out of 100 cases. Layer IV causes a deep dip in 11, 12 and 13 after which the curve rises just as in OC to fall off only slightly in 19 and 20. But the sampling errors are here so large that no great stress can be laid on this. The curve is naturally much simpler than that of OC, although it does not exhibit the simple regularity found by Bok ( '36). The curve for the orang is very similar to that for man. It shows the same two peaks in 111, a dip in layer IV, even somewhat deeper than in man, and a similar, although less pronounced rise in the deeper part of the cortex. The two peaks in layer I11 in the orang are not statistically as significant as they are in man. The difference hetween column 7 and 9 is 33.3 18.9 p3 and that between column 10 and 9, 49.7 t 24.9 p3. Neither of these values points to a clearly significant difference as a short calculation will show. Although the curve fo r Cebus shows similar trends, they are less pronounced than in man and the orang. The double peak in I11 is there, but the dip between them is quite insignificant. Again, there is practically no rise in V and VI. The values from 12 t o 20 could be fitted to a straight line running almost horizontally.

The behavior of V and VI shows probably the most characteristic evolutionary trend of any character revealed in these curves. This can be tested numerically: We can fit a straight line to these observations by the method of least squares and determine the angle which this line forms with the abscissa. This angle will be referred to as the steepness of size gradient. The equation for a straight line being of the form x = a + by, the slope j, of the line is given by the coefficient b, and we have tg$ = b.

The coefficient b has been worked out by the method of least squares, and the values for the angle j , are given in table 4, together with the standard errors of the ordinates.

While the fit of straight lines is none too good, as was to be expected, the steepness of size gradient shows both for OC and OBY a very definite trend: it increases as we go from Cebus to orang and to man.


It is an obvious problem to find out whether this observa- tion is capable of generalization, and, if so, what its functional significance is.

It is interesting to observe that the striate area of man is more completely surrounded by, and 'based on,' large cells than those of other primates. We must assume that excitations spread in the cortex from cell to cell over measurable areas (i.e.? areas not infinitesimal in comparison to the whole cortex), and we must further assume, as Lorente de N6 ('38) has discussed recently that a cortical cell will be excited only when a large number of impulses impinge upon it within a short period of time. It is further clear that large cells will need a larger number of synapses to become excited in order

TABLE 4

Steepness o f size-gradient and standard error of ordinates p i n layer P and PI

I 1 OBY ~~~ ~

l or - -- ---_ - __ . -~ * lL I *

Man 1 i 13.3 I 50040' I I,"., Orang ' 43" 10' ' 19.4 1 46" 10' 1 14.2 rebus 1 22" 0' I 17.1 18. 20' I 11.0

to bring them to a discharge. I t would then follow that the large cells would form a sort of wall, capable of killing waves of excitations running toward them in the cortex and thus isolating the striate area. The motor function of some of these large cells (that of the somato-motor area, e.g.) may be entirely secondary. While this view is, of course, purely speculative, it, may suggest ways of approach to the problems of size distinctly beyond the purely descriptive facts with which these studies have so fa r been concerned.

But to come back to the task of finding out facts about the size of cells: there can be no doubt that the 'size and depth curves' supply information not given by the distribution curves discussed in the first part of this and in the previous papers. They reveal one thing very clearly: that purely as far as size is concerned, homologous areas differ in different species, that these curves can therefore not be used as a

408 GERHABDT VON BONIN

simple criterion of homology. Their interest rather appears to lie in the fact that they show evolutionary changes unsus- pected by former merely descriptive studies. The steepness of the size gradients in the lower layers is an example in point. The method as developed in this paper, has, however, one serious shortcoming: after all the labor of constructing the size and depth curves, after weeks of computations, we end up by comparing them not so much according to a rigorous numerical method as by simple inspection and description. Rigorous numerical methods are precluded by the fact that the various layers are of unequal thickness in different animals. To compute the ‘size gradients’ in the manner in which this was done for layer V and VI partially overcomes this difficulty, but is still far from the ideal method.

Further progress might be made by standardizing these curves in the following way: We can compute the mean and the standard deviation of all columns, and then determine the deviation of the mean of each column in terms of the ‘inter- columnar’ standard deviation. This will bring all the curves on a common baseline, and reduce them to a common ‘yard- stick’ thus facilitating their comparison. The inter-columnar standard deviation will moreover give another measure for the variation prevalent in a given area. We find for the inter- columnar means, standard deviations and coefficients of variation :

Inter-columnar m a n s and variations OC Y a V

Man 144.3 32.93 23.0

Cebus 81.0 14.15 17.4 OBY M a P Man 164.6 34.0 20.7 Orang 127.6 37.0 29.0 Cebus 82.6 19.8 24.0

Orang 114.6 23.51 20.0

Now the coefficients of variation of OC, but not those of OBy, arrange the three species in the order of the complexity of their brain.

Inspecting the curves (figs. 3 and 4) the greater similarity between man and orang against the Cebus is again immedi-

TH

SIZE OF CELLS IN STRIATE AREA

- - + + 0 I 4

L I I I I I I 1 1 I I I I I I I I I I I I T - I I I 1 - 1 1 1 I

+ & -i J 0 - & +

E .JOURNAL O F COMPAR.4TIVR NFURQIiOGY, VOL 70, XO. 3

409

-r a Ld

W

m

.C

h

41 0 GERHARDT VON BONIN

ately apparent. For OC both show practically the same dips for I V A and IVC, and they are very similar in the deeper part of the curve. Man, however, has relatively large cells in I1 and 111, but has a much less pronounced peak in IV B. For OBy, we find again a somewhat closer resemblance between man and orang than between either of them and Cebus. Although the inter-columnar variation of man is less than that of the orang (the calculations have become too involved for estimates of sampling errors), man shows the deepest dip in layer IV. I n layers V and VI, only the curve for man is con- sistently on the positive side, another expression of the steeper size gradient which was discussed in a previous paragraph.

SUMMARY

Summing up, we can say this about area OC and its magno- cellular margin OBy in man, orang and Cebus: the statistical constants of the distribution curves of nuclear volume differ in almost all instances significantly. The average nuclear volume is largest in man, smaller in orang and least in the Cebus. The giant stellate cells show practically the same nuclear volume in all three species. The solitary large pyramidal cells are largest in man, of lesser size in the orang and of least size in the Cebus, but their relative size is about the same in all three forms. The variation is less for man than for the primates. The average size of the nuclear volume for OBy follows the same order. I ts giant pyramidal cells are relatively somewhat larger in man than in orang and Cebus. The ratio of mean nuclear volumes of OC to those of OBy decreases as we descend from man to orang and to Cebus. In addition to the simple distribution curves, the correlation between nuclear volume and the distance of the cells from the cortical surface was investigated. The ‘size and depth curves’ show differences between the three species. Those of man and orang resemble each other much more closely than either of them resembles that of the Cebus. Both in OC and OBy the size gradient in layers V and VI shows a distinct evolutionary trend. The method of size and depth curves and its present shortcomings are briefly discussed,

SIZE OF CELLS I N STRIATE AREA 411

LITERATURE CITED

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BOII, S. T. A quantitative analysis of the structure of the oerebral cortex. Ron. Akad. Wet., Verhand, Dee1 XXXV: pp. 1-55.

TON BONIN, G. 1938a The cerebral cortex of the cebus monkey. J. Comp. Neur., vol. 69, pp. 181-227.

Studies of the size of the cells in the cerebral cortex. 11. The motor area of man, cebus and cat. J. Comp. Neur., vol. 69, pp.

BRODMAN~~, K. 1909 Vergleichende Lokalisationslehre der Grosshirnrinde. J. A. Barth, Leipzig.

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CAJAL, 8. R. 1911 Histologie du systhne nerveux. t. 2. Maloine, Paris.

1936

1935 b

1923 Studien iiber die Sehrinde der Katze. J. f . Psychol. u. Neur., Bd. 29, S. 161-181.

CARNAP, R. 1928 Der logisclie Aufbau der Welt. Berlin. CLARK, W. E. LEGROS 1925 The visual cortex of primates. J. Anat., vol. 59,

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