

STRUCTURAL ARRANGEMENTS OF CEREBRUM 483 



cornu ammonis or hippocampus and pass for the greater part to the 

 cornu ammonis of the opposite side. 



MINUTE STRUCTURE OF THE CEREBRAL CORTEX 

 The cortex of the cerebral hemispheres consists of a layer of grey 

 matter covering a central mass of white fibres. With the growth 

 in size of the brain, which accompanies the development of increased 

 intelligence and powers of adaptation, the necessary increase in 

 cortex is rendered possible by the folding of the surface into convolu- 

 tions and fissures. The chief of these convolutions have already 

 been indicated in the sketch of the anatomy of the brain (Fig. 209). 



On section the grey matter is seen to consist of many layers of 

 nerve-cells embedded in neuroglia and nerve fibres, both medullated 

 and non-medullated. The nerve -cells vary in size and shape ; one 

 kind of cell is, however, typical of this part of the central nervous 

 system. This is the pyramidal cell (Fig. 218), a cone-shaped or pear- 

 shaped cell with one large apical dendrite which runs towards the 

 surface and breaks up in the most superficial layer into a number of 

 branches. Dendrites are also given off from the sides and lower angles 

 of the cell. The axon, which arises from the axon hillock in the middle 

 of the base of the cell, passes downwards into the white matter, giving 

 off collaterals in its course. Some of these axons pass by the corona 

 radiata into the internal capsule and into the crura cerebri, including 

 those which form the pyramidal tracts ; others, or their collaterals, 

 may pass into the adjacent regions of the cortex, or across by the 

 corpus callosum into the opposite hemisphere. 



Although varying in structure at different parts, it is generally 

 possible to distinguish four or five layers in the cortex. 



(1) The most superficial layer, known as the outer fibre lamina, or 

 molecular layer, contains very few cells. It is composed generally 

 of the dendrites of cells from the deeper layers. It contains a few cells 

 which are spindle-shaped and are provided with several processes 

 running parallel to the surface. These are sometimes called association 

 cells. It is probable that afferent fibres, entering the cortex, pass 

 up towards the surface and end for a large part in this molecular layer. 



(2) Below this is a layer of pyramidal cells, the outer cell lamina, 

 which is divided by some observers, e.g. Campbell, into three, viz. : 



(a) The small pyramidal cells. 



(6) Medium-sized pyramidal cells. 



(c) Internal layer of large pyramidal cells. 



(3) Below the pyramidal layer we find a stratum of small cells, 

 most of which are stellate in form. This is known as the stellate or 

 granule layer, or middle cell lamina. 



(4) Internal to the granule layer is the inner fibre lamina. In the 



