212 PROGRESS OF MICROSCOPICAL SCIENCE. Ru^McT^^^^^ 
that are rather larger. It is only here and there that the cells are 
collected into elongated groups which give the appearance of radia- 
tions. On its under-side it gradually blends with the central white 
axis of the convolution, into which its cells are scattered for some 
distance. 
The seventh layer is this central white stem or axis of the con- 
volution. On every side it gives off bundles of fibres, which diverge 
in all directions, and in a fanlike manner, toward the surface through 
the several grey layers. As they pass between the elongated and 
radiating groups of cells in the inner grey layers, some of them become 
continuous with the processes of the cells in the same section or plane, 
but others bend round and run horizontally, both in a transverse and 
longitudinal direction (in reference to the course of the entire con- 
volution), and with various degrees of obliquity. "While the hundles 
themselves are by this means reduced in size, their component fibres 
become finer in proportion as they traverse the layers toward the 
surface, in consequence, ap^parently, of branches which they give off to 
be connected with cells in their course. Those which reach the outer 
grey layer are reduced to the finest dimensions, and form a close net- 
work with which the nuclei and cells are in connection. 
Besides these fibres, which diverge from the central white 'axis of 
the convolution, another set, springing from the same source, converge, 
or rather curve inward from opposite sides, to form arches along some 
of the grey layers. These arciform fibres run in different planes — 
transversely, obliquely, and longitudinally — and appear to be partly 
continuous with those of the divergent set which bend round, as already 
stated, to follow a similar course. All these fibres establish an 
infinite number of communications in every direction between different 
parts of each convolution, between different convolutions, and between 
these and the central white substance. 
The other convolutions of the cerebral hemispheres differ from 
those at the extremities of the posterior lobes, not only by the compara- 
tive faintness of their several layers, but also by the appearance of 
some of their cells. We have already seen that, at the extremity of 
the posterior lobe, the cells of all the layers are small, and of nearly 
imiform size, the inner layer only containing some that are a little 
larger. But, on proceeding forward from this point, the convolutions 
are found to contain a number of cells of a much larger hind. A 
section, for instance, taken from a convolution at the vertex, contains 
a number of large, triangular, oval, and pyramidal cells, scattered at 
various intervals through the two inner bands of arciform fibres and 
the grey layer between them, in company with a multitude of smaller 
cells which differ but little from those at the extremity of the posterior 
lobe. [ The pyramidal cells are very peculiar. Their bases are quadran- 
gular, directed toward the central white substance, and each gives 
off four or more processes which run partly toward the centre, to be 
continuous with fibres radiating from the central white axis, and 
partly parallel with the surface of the convolution, to be continuous 
with arciform fibres. The processes frequently subdivide into minute 
branches, which form part of the network between them. The opposite 
