426 PHYSIOLOGY 



fibres form only 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, inclining 

 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 fiv% layers in the cortex. 



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

 mokcular layer, contains very few cells. It is composed generally of the den- 

 <lrit.es 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 

 lor si large part in this molecular layer. 



(2) Below this is a layer of pyramidal cells, the outer cell lainina> which 

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



(a) The small pyramidal cells. 



(b) Medium-sized pyramidal cells. 



(c) Internal layer of large pyramidal cells. 



