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have almost 1.4 times greater density of cells. For these allied 

 American Monkeys the density of cells, in function of the body 

 weight, is, therefore, proportionate to the cephalisalion : the greater 

 the quantity of brain, in function of the body weight, the greater 

 the density of cells. 



A Cebus, however, of the same body weight as Hylobates syn- 

 dactylus, would have only I wo thirds of the density of cells of (his long- 

 armed ape, though the latter has three fifths of the weight of the brain 

 of the large Cebus-species in question. Here the density of cells is 

 about in inverse ratio to (he cephalisation. 



Of special in(erest is (he comparison of the density of the cells 

 of the Chimpanzee with that of Man, both having about the same 

 body weight. IJammarbekg ') detei-mined the density of cells, in normal 

 human biains, through the whole thickness of the cortex, per 0. 001 

 cubic millimeter, i. e. '/lo of M.ayer's unit of volume, in different 

 cortical areas, some of which may be compared with the areas 

 according to Brodmann. Thus in the lobus occipitalis the area striata 

 or field 17 of Brodmann. Calculated to the same unit of volume as 

 that of Mayer we find here 386 cells, whereas Mayer's chimpanzee 

 has 2888, i. e. 7.5 times as many. Thus 152 in the area gigantopy- 

 ramidalis or Brodmann's field 4, as against a density for that chim- 

 panzee of 1172, i.e. 7.7 times as many. In the area frontalis 

 agranularis or field 6 the density for Man is only 111, as against 

 1136 or 102 times as much for the chimpanzee examined by Mayer '). 



As, according to Brodmann's') measurements, the entire surface of the 

 cortex in the Chimpanzee is about a third of that in Man, the regio 



^) G. Hammakbekg, Studiën übei* Klinik und Pathologie der Idiotie, nebst Unter- 

 suchungen iiber die normale Anatomie der Hirnrinde. Upsala 1895. 



2) In the part of the gyrus frontalis superior, which belongs to the area frontalis 

 agranularis or Brodmann's sixth field 1 have estimated the density of cells in the 

 deepest half of the third layer, and likewise in the deepest half of the fourth layer 

 of the part of the lobus occipitalis, which corresponds to the area striata or 

 Brodmann's seventeenth field, from drawings (Table I, Ing. 2 and Table II, Fig, 4)-, 

 namely according to the ratio of the other densities drawn and also calculated by 

 Hammakberg. Thus was also evaluated the density of cells of the insignificant 

 first layer in the part of the gyrus centralis anterior, which belongs lo the area 

 giganlopyramidalis or Brodmann's fourth field. 



Hammarberg calculated the number of cells per unity of volume from 10 suc- 

 cessive sections, each of a thickness of 0.01 mm., or 5 sections of a thickness of 

 0.02 mm., Mayer on the other hand from only a single section of a thickness of 

 0.01 mm. As also parts of cells are counted, the number must be slightly greater 

 according to the latter method. Judging by Hammarberg's drawings the difference 

 can, however, not be considerable. 



'^) K. Brodmann, Neue Forschungsergebnisse der Grosshirnrindenanatomie, rait 



