LAYERS OF CELLS IN THE CORTEX. 



169 



posed of fusiform and irregular cells. The fusiform corpuscles have a definite arrange- 

 ment, being placed for the most part vertically at the summit of a gyrus ; but parallel to 

 the surface in the sulci, where they correspond in direction to the arcuate fibres passing 

 from one convolution to another ; they are said to be connected with these fibres. 



Fig. 117. CELLS FROM THE CEREBRAL 



CORTEX, SHOWN BY GOLGl's METHOD. 



(G. Retzius.) 



n, n, neuroglia-cells ; p,p, pyramids ; 

 a,a, axis-cylinder processes of pyramids 

 giving off collaterals. 



Beneath the last layer is the 

 medullary centre, with which it gra- 

 dually blends. The fibres of the 

 white substance, as they radiate into 

 the grey matter, become finer. They 

 are mostly continuous with the axis- 

 cylinder processes of the pyramidal 

 cells, the collaterals of those forming 

 two plexuses of medullated fibres 

 which lie, the one at the base of the 

 3rd layer, the other between this 

 and the 2nd layer. These plexuses 

 (inner and outer white plexuses of 

 W. Krause) are probably the cause, 

 of the lines of Baillarger seen with 

 the naked eye in a section of the 

 grey cortex of a fresh brain. 



In the Sylvian fissure the fusi- 

 form cells are more abundant' than 

 elsewhere, and from their number 

 in the claustrum the fifth layer has 

 been termed by Meynert the " claus- 

 tral formation." They are also very 

 abundant in the amygdaloid nucleus, 

 which is indeed chiefly formed by a 

 thickening of the deepest layer of 

 the cerebral cortex. The cornu 

 ammonis on the other hand is formed 

 almost exclusively of the large pyra- 

 midal cells, and the layer in which 

 these occur (third layer) has, in 

 like manner, been termed the " for- 

 mation of the cornu ammonis." 



The axis-cylinder processes of the 

 pyramids when they reach the medul- 

 lary centre, pass either as association-fibres to other parts of the cortex of the same 

 hemisphere, or as commissural fibres to the corpus callosum, and through this to the 

 opposite hemisphere, or as projection-fibres to the corpus striatum and optic 

 thalamus, or by way of the internal capsule to the midbrain, bulb and spinal cord. 

 The junction with fibres of the association-bundles may be T-shaped, in other words 

 they may bifurcate and pass in opposite directions underneath the cortex. Eventually 

 they turn into the cortex again and end by free arborisation amongst its cells. More- 



