It has been already pointed out that the subarachnoid space, with the ventricles and central canal of the spinal cord, forms a single system of lymph spaces which intercommunicate. The lymph or cerebro-spinal fluid in these spaces may accumulate, and so lead to various forms of oedema and dropsy of the brain. In this connection it is to be remembered that the skull is a closed cavity, and any increase in the fluid in these spaces implies a decrease in the quantity of blood in the vessels or of the brain substance itself.