1899.] on Structure of the Brain in Belaiion to its Functions. 127 



similar plan, only in man they are relatively more numerous, more 

 varied and more complex. 



If a brain be sliced into, it will be seen that the centre is white, 

 and that it has a mantle or rind of a greyish colour. This grey 

 mantle consists of countless millions of cells with their processes. 

 The photograph I am showing is of a tiny speck of grey substance of 

 the brain highly magnified. You observe the little pyramids of 

 protoplasm with a nucleus in the middle sending out branches in all 

 directions, these processes forming apparently a delicate network. 

 The whole of the surface of the brain is covered by myriads of these 

 little cells arranged in five layers (Figs. 3a and 3b). Streaming in 

 among these cells of the grey matter, and derived from other cells in 

 the deeper structures of the brain, are innumerable fibres, while a still 

 larger number of fibres may be seen lower down, and these are in 

 great part the fibres which stream out from the grey matter to form 

 the white substance (Fig. 1). 



The nervous units are all of similar general structure in the 

 brains of all vertebrates. 



I will describe now the structure of one of these simple nervous 

 units. It consists of a little lump of protoplasm, the cell body ; in 

 the centre of which, as in the centre of other cells, is a nucleus and 

 nucleolus ; starting out from the cell in all directions are numbers of 

 processes like the branches of a tree, these branching processes are 

 termed protoplasmic processes ; they were once believed only to possess, 

 like the roots of a tree, the power of absorbing nutrition and carrying 

 the same to the cell to be utilised ; we now know, also, that they serve 

 to conduct nervous currents. There is, however, only one process of 

 the cell which becomes a nerve fibre and arises as a cone-like swelling 

 at the centre of the base of the little pyramid. After a short distance 

 this fibre becomes insulated by a sheath of fatty substance termed 

 myelin. Nervous currents when transmitted along the nervous ele- 

 ments are collected by the protoplasmic processes and transmitted to 

 the cell, and thence outwards by the process which will become the 

 core of a nerve fibre, so that the direction of the molecular vibration 

 is always the same. Although there are countless myriads of these 

 nervous units which are termed neurones, forming an inextricable net- 

 work, yet when stained by the chromate of silver method invented by 

 Professor Golgi, the innumerable delicate processes of these nerve 

 cells are found not to be in continuity. Like the trees of a forest, 

 there is contiguity but not continuity ; each one is independent, and 

 has an independent life and structure (Fig. 2). 



The nervous system consists then of systems, groups, communities 

 and clusters of these nervous elements called neurones, mingled with 

 supporting tissue and blood vessels for their nutrition. The amount 

 of blood in the grey matter is very much more abundant than in the 

 white matter, indicating that the chemical changes incidental to vital 

 activity take place there, rather than in the white matter. The 

 processes of mind are inseparably connected with the functional 



