THE CENTRAL NERVOUS SYSTEM 463 



atlanto-occipital membrane into the ample subarachnoid space lying over 

 the fourth ventricle. Another method is to introduce a glass cannula through 

 a slit in the sheath of a nerve root up into the subarachnoid cavity of the 

 spinal canal. In man it may be obtained in small quantities for examination 

 by introducing a hollow needle directly into the spinal canal in the lumbar 

 region between the laminse of the vertebrae. On introducing a cannula 

 into the subarachnoid space, the fluid may spurt out, showing that it is under 

 a certain pressure (about 100 mm. H 2 0). After the first rush the fluid begins 

 to drop away, at first rapidly, but more slowly -with lapse of time. If 

 the fluid be allowed to drain off for some hours, signs of interference with the 

 functions of the central nervous system are evinced. The cerebro- spinal 

 fluid appears to be formed chiefly in the neighbourhood of the choroid plexus. 

 Although its composition would suggest that it was merely a transudation 

 from the blood, the amount formed does not seem to run parallel with the 

 pressure in the capillaries of the brain. Moreover, it has been shown by 

 Dixon and Halliburton that a considerable increase in the flow of cerebro- 

 spinal fluid may be brought about by injecting an extract of the choroid 

 plexus itself. It has therefore been concluded that this fluid is really a 

 secretion by the modified ependyma cells covering the fringes of the choroid 

 plexus. 



Although the method of formation of the cerebro-spinal fluid is still not 

 clear, there is no question that its removal from the subarachnoid space is 

 brought about by simple physical factors. The subarachnoid space com- 

 municates with the ventricles by means of openings in the roof of the fourth 

 ventricle. The pressure of the fluid is ordinarily about equal to the pressure 

 in the venous sinuses of the cranium. If salt solution be injected into the 

 subarachnoid space, it escapes with extreme ease, and it is found that its 

 channel of escape is into the veins and especially into the venous sinuses of 

 the dura mater. Its removal by these sinuses is facilitated by the existence 

 of peculiar structures, known as the Pacchionian bodies. Each of these 

 bodies is a bulbous protrusion of the arachnoid membrane into a blood 

 sinus. It remains connected with the arachnoid by a narrow pedicle, 

 through which a continuation of the sub-arachnoid space is prolonged into 

 the interior of the sinus. It is therefore a little sac of arachnoid membrane 

 separated from the blood stream only by an invagination of the endothelium 

 lining the sinus. Filtration of the cerebro-spinal fluid will occur into the 

 venous sinuses whenever the pressure of the fluid rises above that of the 

 blood in the sinus. The fluid can also escape, but with greater difficulty, 

 along the sheaths of the spinal nerve roots, by which it will pass into the 

 lymphatics outside the spinal canal. 



THE NUTRITION OF THE BRAIN 



The grey matter of the brain is very richly supplied with blood 

 vessels. Any interference with the blood flow through the brain rapidly 

 checks the functions of the central nervous system in consequence of 



