STRUCTURAL ARRANGEMENTS OF CEREBRUM 427 



fibres only form a small proportion of the total number making up the 

 bundle. 



(c) The psalterium or hippocampal commissure is a thin lamina formed 

 of transverse fibres filling up the small triangular space on the under surface 

 of the hinder part of the corpus callosum formed by the divergence of the 

 posterior pillars of the fornix. Like the anterior commissure, the hippo- 

 campal commissure is closely associated with the sense of smell. Its fibres 

 arise from the pyramidal cells in the 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 convolutions 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. 

 219), 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 den- 

 drites 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. 



