Although it differs in its chemical constitution from true lymph, the cerebro-spinal fluid seems to functionate as lymph, in addition to acting as a lubricating agent, and playing a part in regulating the vascular supply of the brain. In cases of cerebral hæmorrhage, abscess, tumour, or depressed fracture, room is made up to a certain point for the extraneous matter by displacement of cerebro-spinal fluid. _Vascular supply._--The free anastomosis between the vessels entering into the formation of the circulus arteriosus (circle of Willis) ensures an abundant supply of blood to the brain. The larger arteries run in the sub-arachnoid space and give off branches which ramify in the pia mater before entering the cerebral substance. Within the brain, each artery being more or less terminal, there is no free anastomosis between adjacent vessels, with the result that if any individual artery is obstructed the vitality of the area supplied by it is seriously impaired. The venous arrangements are also peculiar in that the veins are thin-walled and valveless, and open into the rigid, incompressible sinuses which run between the layers of the dura mater. Most of the blood passes to the internal jugular vein, and any increase in the pressure of this vessel is immediately transmitted back to the cerebral veins. As the blood vessels project into a rigid case filled with incompressible material, and as the total _volume_ of blood in the brain is constant (Munro and Kelly), any alteration in the supply of blood to the cerebral tissue must be due to an increased _velocity_ of flow, and this in turn depends upon changes in the aortic and vena cava pressure. Thus, if the aortic pressure rises, more blood will enter the cerebral vessels and will move along more rapidly; while if the pressure in the vena cava rises there is obstruction to the passage of blood in the arteries and diminished velocity of flow. The ebb and flow of cerebro-spinal fluid in and out of the spinal canal may also help to control the pressure. #Nerve Elements.#--The nervous system is composed of a multitude of units, called _neurones_, each neurone consisting of a nucleated cell, with branching protoplasmic processes or _dendrites_ and one _axis-cylinder_ or _axon_. The nutrition of an axis cylinder depends on its continuity with a living cell. If the cell dies, the axis cylinder degenerates. If the axis cylinder is severed at any point, it degenerates beyond that point, and the nucleus of the nerve-cell disintegrates--chromatolysis. The axis cylinder of one cell ends in a number of fine filaments which arborise around another nerve-cell, thus bringing it into physiological, if not anatomical, relationship with the first cell. The termination is called a cell-station or _synapsis_. In this way the various sections of the nervous system are kept in association with one another and with the rest of the body.