RO CTULE OF THE CEREBRAL CORTEX. 543 
convolution it is always thicker than at the bottom of a furrow. The maximum 
thickness of cortex (about 6 mm.) is attained in the upper parts of the two central 
convolutions, whilst the minimum (about 2°5 mm.) may be observed in the region 
of the occipital pele. The amount of gray cortex differs considerably in different 
individuals, ard appreciably diminishes in old age. It is also stated, but upon very 
imperfect evidence, that it is relatively more abundant in the male than in the 
female. 
In structure, likewise, marked differences may be noted in the gray cortex 
of different rezions. In certain localities this is quite apparent to the naked eye 
when sections we made through it, but the relation which this bears to the functions 
displayed hy tiese different regions is, to a great extent, unknown. Further, there 
are no sharp t’ansitions in structure. One form of cortex passes gradually and in- 
sensibly into te variety of cortex which is distinctive of an adjoining region, and 
throughout tl whole mass a general ground type may be recognised. It is only 
to those geneal structural features which more or less characterise the entire 
cortical layer hat we shall be able to refer. 
When sectons are made through the fresh brain, and the cut surface is closely 
inspected, it wil usually be apparent that the cortex is indistinctly stratified. On 
the outside thre is a thin, whitish layer, and beneath this the gray matter presents 
two strata of ery nearly equal thickness, viz. : a middle, gray-coloured stratum and 
an inner, yelbwish-red stratum. Between the two latter layers a narrow white 
band is, in may places, visible. This is termed the outer band of Baillarger. When 
the layers indated above are present, four strata superimposed on each other are 
recognised ; bt in certain regions, as, for instance, in the anterior central convolu- 
tion, a second vhite streak traverses the deep or inner gray layer and divides it into 
two. ‘This is 2rmed the inner white band of Baillarger, and, when it is present, the 
gray cortex beomes divided obscurely into six alternating white and gray layers. 
The cuter and of Baillarger is strongly marked in the region of the calcarine 
fissure and givs a characteristic appearance to this portion of the cortex. In this 
locality 15 receves the name of the band of Vicq d’Azyr (Fig. 400, p. 538). 
To objain afull understanding of the minute structure of the cerebral cortex many 
different nethds must be employed, and it is only by combining the several separate 
pictures vhich we thus afforded that the end in view is, in some measure, reached. 
The sratifiation indicated above has little bearing upon the more essential points of 
the intinate sructure of the cortex. The three white layers are brought about by an 
ageregaton of fibres running in a tangential direction, or, in other words, in a direction 
parallel } thesurface of the convolution. 
Ner’e-cels.— According to the arrangement and the characters presented by the 
nerve-cels whch are met with at different depths, it is now usual to recognise four layers 
in the ortex These are: (1) the stratum zonale ; (2) the layer of small pyramidal 
cells ; (¢) thelayer of large pyramidal cells ; and (4) the layer of polymorphic cells. 
As te pyramidal cells are specially characteristic of the cerebral cortex, we shall 
describe the two layers which contain them first. The difference between these two 
layers Irgely depends upon the difference in the size of the constituent cells. Taken 
togethe thesecond and third layers constitute the chief part of the cortex ; they merge 
insensiby int each other, and, in the parietal and frontal lobes, the layer of large pyramidal 
cells is ne thickest of all the layers. In both of these strata the pyramidal cells present 
the sam fora, and apparently also similar connexions. 
A pramdal cell has a triangular outline. Its apex is directed towards the surface 
of the onvdution and is drawn out into a long, tapering, apical, dendritic process ; its 
base is urnel towards the medullary centre of the gyrus, and from this (usually from the 
centre)t sleider axon proceeds. Numerous lateral dendrites are given off from both sides 
of the cll-baly, and particularly from the two basal corners. The apical dendrite varies in 
length,.ccording to the depth at which the cell is placed. In every case it passes straight 
toward the surface of the convolution. Every here and there fine lateral branches come 
off fronit, «nd ultimately it enters the stratum zonale, where, close to the surface, it ends 
by bresing up into a large number of fine terminal filaments, which spread out horizon- 
tally inevery direction and interlace closely with the corresponding filaments of other 
pyramial cells and with the other elements of this layer. 
Thaxon of the cell descends, gives off collaterals, assumes a medullary sheath, and 
enters 1e central white core of the gyrus as a nerve fibre. 
