720 [Recess, 



of potash or soda. Sometimes, as in certain parts of the posterior 

 lobe itself, one can scarcely make out more than seven layers, there 

 being only one broad layer of arciform fibres running along the grey 

 layer outside the white central stem. It is an error to call the layers 

 containing these arciform fibres (for I shall so name them) the white 

 layers of the convolution, for they are always interspersed with 

 numerous cells, with processes of which they are continuous. In 

 some parts of the brain (on the vertex for instance) the second (from 

 the centre) of the arciform bands of fibres is very broad and strong, 

 and thickly interspersed with large and small cells of different 

 shapes. These arciform fibres of the convolutions run in different 

 planes, transversely, obliquely, and longitudinally. Where a con- 

 volution bends round upon itself at a right angle, a section made 

 at the angle contains them in abundance; but here the separate 

 fibres forming the arciform bands are very short, being cut in their 

 passage. The curved arciform fibres, then, establish an infinite 

 number of communications in all directions between different parts 

 of each convolution, between different convolutions, and between 

 these and the central white substance. I have already shown that 

 the more superficial layer of grey substance contains numerous 

 arciform fibres, but finer and less strongly marked. 



But the convolutions at the extremity of the posterior lobe differ 

 from the rest, not only in the greater distinctness of their several 

 laminae, but also in the appearance of some of their cells. On ad- 

 vancing forward, the convolutions contain a great number of cells of 

 a much larger kind. In a section, for instance, taken from a convo- 

 lution at the vertex, and in a vertical line passing through the optic 

 thalamus, the greater number of the cells differ but little from those 

 at the extremity of the posterior lobe ; but amongst these cells, in 

 the two inner bands of arciform fibres, and the grey layer between 

 them, I found a number of much larger, triangular, oval, and pyra- 

 midal cells scattered about at variable intervals. The pyramidal 

 cells are very peculiar. Their bases are quadrangular, directed 

 toward the central white substance, and give off four or more pro- 

 cesses, which run partly toward the centre to be continuous with 

 fibres radiating from the central stem, and partly parallel with the 

 surface of the convolution, to be continuous with arciform fibres. 

 The processes may frequently be seen to subdivide into minute 



