298 SCIENCE PROGRESS 



together with the supporting connective tissue elements, neu- 

 roglia cells, blood vessels, and lymphatics, form the central 

 nervous system. Functionally speaking there are three systems 

 of neurones in the brain and spinal cord : (i) afferent pro- 

 jection system ; (2) efferent projection system ; (3) association 

 system (Plate I, fig. 1). 



The Convolntional Pattern of the Brain. — If we look at a 

 human brain we see that the surface of the hemispheres 

 exhibits a number of folds and fissures giving rise to a pattern 

 which I will speak of as the convolutional pattern (Plate II, 

 fig. 1). A section through any of these folds or fissures shows 

 that the external surface or cortex, as it is called, is of a 

 pinkish grey appearance contrasting with the dead white of 

 the subjacent part of the brain. Now a microscopic examina- 

 tion of the grey matter and the white matter explains why 

 there should be this difference in colour. When highly magnified 

 a thin section appropriately stained by dyes shows the grey 

 matter to consist of innumerable ganglion cells to and from 

 which conducting fibres proceed. The microscopic architecture 

 of the grey cortex exhibits a cell and fibre structure of extra- 

 ordinary complexity. The diagram (Plate III, fig. 2) of a section of 

 an adult brain is to illustrate this cell and fibre architecture. You 

 observe that the cells are arranged in six layers, and there are 

 also layers of fibres, some of which run horizontally and some 

 have a radial direction. The horizontal conduct association 

 impulses. Although there is a general similarity in the cell 

 and fibre structure of the cortex of the brain, yet the whole 

 surface of the brain can be mapped out into territories of 

 different cell and fibre architecture (Plate II, fig. 2) ; and physio- 

 logy and medical science teach that there is a corresponding 

 difference in function. 



I have remarked that the grey cortex has a pinkish colour 

 because (relatively to the white matter) the blood supply is very 

 abundant. Now the subcortical matter is white because the 

 nervous processes of the cells of the grey matter are sur- 

 rounded with a sheath of myelin or phosphoretted fatty 

 substance. The bio-chemical processes incidental to all nervous 

 action, therefore to the mental activity of the brain, take place 

 in the cell structure of the neurone. The cortex is the seat of_ 

 consciousness and mental activity, and the functions of the 

 cortex require a continuous supply of oxygenated blood. Un- 



