THE CENTRAL NERVOUS SYSTEM. 97 



that these cells accumulate an excess of oxygen during sleep and 

 gradually use it up during the day. However that may be, there is 

 evidence to show that the blood-flow is side-tracked during sleep, so that 

 the brain receives a smaller supply than it does in the waking hours. 

 If a limb be enclosed in a plethysmograph, it is found that the volume of 

 the limb increases during sleep owing to dilatation of the blood-vessels. 

 As a result, the supply of blood to the brain is diminished. Howell 

 suggests that the dilatation of peripheral blood-vessels may be due 

 to fatigue of the vaso-motor centre, but the ultimate cause is still 

 uncertain. Probably the lessened blood supply is secondary to the 

 diminution in functional activity. 



THE CEREBRO-SPINAL FLUID. 



The cerebro-spinal fluid forms a water cushion by which the brain 

 and spinal cord are protected from jarring shocks during any sudden 

 movement of the body ; it may also contain substances which influence 

 the functional activity of the nervous tissues. It occupies the space 

 between the membranes of the brain and spinal cord, and fills the ventri- 

 cular cavities of the brain and the central canal of the cord. The fluid 

 lying in the ventricular cavities communicates with that which fills the 

 space between the membranes at the foramen of Majendie and at two 

 other foramina, one of which lies at each side of the recess of the fourth 

 ventricle. 



The cerebro-spinal fluid resembles lymph, but is much less con- 

 centrated than that fluid. It is clear and limpid, and has a specific 

 gravity of about 1007. It contains traces of proteins, and its inorganic 

 constituents correspond with those of blood plasma. 



If the dura mater is punctured, the cerebro-spinal fluid escapes from 

 the opening, showing that it is under a certain degree of pressure, 

 which can be measured by inserting a cannula between two vertebrae 

 and connecting it with a manometer. It is found that the pressure 

 corresponds roughly with the venous blood pressure, and that it varies 

 to a slight extent with variations in arterial and venous pressure. 



If the cerebro-spinal fluid is allowed to escape, it is rapidly replaced. 

 This is shown in cases where the escape continues for some time, either 

 in conditions experimentally produced in animals, or in such accidental 

 circumstances as fracture of the base of the skull in man. A loss of 

 100 c.c. or more per hour has been known to continue for weeks in a 

 human being. 



The cerebro-spinal fluid is derived from the^choroid plexus, and 

 Dixon and Halliburton have recently shown that it s is formed by a 

 process of secretion. The normal stimulus to secretion is a hormone 



7 



