ii8o 



HUMAN ANATOMY. 



Choroid plexus 



Lateral 

 ventricle 



The evident purpose of the horizontally directed processes and collaterals being to bring 

 into relation different cortical cells, such association tracts become evident only after the neces- 

 sity for the exercise of the corresponding psychic functions has arisen. Hence in the cortex of 

 young children the strata of horizontal fibres are very feebly developed. With the progressive 

 advance of intellectual capacity, the association paths become correspondingly more marked, 

 according to the suggestive observations of Kaes, the increase continuing beyond even middle 

 life. Whether this augmentation is due to actual increase in the number of association fibres, 

 or, as suggested by Edinger, is dependent upon the further growth and myelination of collaterals 

 already present in an immature condition, is uncertain. 



Local Variations in the Cerebral Cortex. — It has been pointed out,, in 

 prefacing the foregoing description of the structure of the cerebral cortex, that, 

 whilst in the main certain features are common to the cortex wherever well devel- 

 oped, more or less evident variations occur in different localities. Such variations 

 are, for the most part, slight and depend upon the size and number of the nerve-cells 

 and the richness and direction of the nerve-fibres — changes which produce alterations 

 in the relative proportions of the strata. The width of the stratum zonale is almost 

 constant and subject to little modification, being usually well defined from the layer 

 of small pyramidal cells. The layer of the large pyramidal cells, on the contrary, 

 exhibits considerable variation, either in increased thickness, as in the precentral 



gyrus, or in diminished 

 '■ . breadth, as in the occipital 



lobe. The layer of poly- 

 morphic cells is fairly 

 uniform, but within the 

 precentral convolutions 

 is reduced almost to 

 disappearance, although 

 the pyramidal cells of 

 the superimposed (third) 

 layer are here of unusual 

 size. Such variations in 

 the histological features 

 of the cortex are prob- 

 _^ ably correlated with dif- 



Frontal section across left hippocampus and gA-rus dentatus. X 2}4. ICrenceS m the lUnCtlOU 



of its various regions, 

 although the exact relations between such differences are in many cases still obscure. 

 Disregarding the cortical regions which are profoundly modified by their rudi- 

 mentary character, such as the olfactory lobe (page 1152), apart from minor varia- 

 tions in details, the cortex of the greater part of the frontal, parietal, occipital, temporal 

 and lifnbic lobes and of the insula closely corresponds in its structure. That of the 

 motor TRolandic) region, of the calcarine (visual) area of the occipital lobe, and 

 of the hippocampus, dentate gyrus and adjacent part of the hippocampal gyrus, 

 however, presents modifications which call for brief description. 



The Rolandic cortex of the precentral gyrus, particularly towards the upper margin of the 

 hemisphere, of the paracentral lobule and of the adjoining part of the postcentral gyrus — the 

 great cortical motor area of the hemisphere — is distinguished by the great breadth of the layer 

 of large pyramidal cells, the unusual size of the last-named elements and the feeble development 

 of the layer of polymorphic cells. The pyramidal cells collectively tend to larger size as the 

 upper end of the precentral convolution is approached and, in addition, cells of extraordinary 

 dimensions appear. These elements, known as the giant pyramidal cells of Betz, reach their 

 maximum size within the paracentral lobule, where some attain a breadth of .065 mm. or almost 

 double that of the pyramidal elements in other regions. The giant cells are further distinguished 

 by their robust and rounded form, their distribution in small groups of from three to five in the 

 deeper layers of the cortex, and the exceptional thickness of their axones. 



The occipital cortex in the vicinity of the calcarine fissure (Fig. 1013) is distinguished even 

 macroscopically by the clearness of the outer stripe of Baillarger, here called the stripe of 

 Gennari or of Vicq d"" Azyr. The stratum zonale is somewhat smaller than usual, but is 

 exceptionally rich in tangential fibres and fusiform cells. The more superficially placed 

 elements of the second stratum are spindle form rather than pyramidal and give oflf two 



Alveus covering 

 hippocanipu.-i 



Fimbria 

 Gyrus dentatus 



Subiculum 



(Gyrus 



hippocampi) 



