512 VETERINARY PHYSIOLOGY 



from the cerebro-spinal fluid of the brain to the blood ; but 

 in the lower part of the spinal cord they do not pass out freely, 

 and therefore anaesthetics like cocaine may be injected into the 

 subarachnoid space in this region to ansesthetise the cord. 



This outward passage into the blood seems to show that 

 the fluid secreted by the choroid plexus is carried away in 

 the blood stream. It may escape along certain of the cranial 

 nerves, more especially along the olfactory nerve. This 

 channel is of importance in allowing the entrance of certain 

 micro-organisms such as those of infective poliomyelitis and 

 of cerebro-spinal meningitis in man. 



It is also probably passed into the blood of the dural 

 sinuses by the arachnoidal villi which project into these, and 

 which are covered by curious collections of cells, and from 

 the perivascular spaces into the capillaries which they 

 surround. 



Excessive secretion or blocking of the lateral ventricles 

 may cause an increase in the fluid, a rise of pressure and 

 cerebral symptoms. The fluid naturally escapes from the 

 fourth ventricle by the foramen of Magendie and the foramen 

 of Luschka into the subarachnoid space. 



4. Quantity. — The quantity of fluid contained is small, 

 in man about 130 c.cms. 



5, Functions. — (i) The cerebro-spinal fluid, filling all the 

 spaces in the brain, equalises pressure throughout the 

 cerebro-spinal system and acts as a water cushion, especially at 

 the base of the brain, protecting the medulla against shock, 

 (ii) In the perivascular lymphatics it acts as a support to 

 the thin- walled blood-vessels. (iii) It also acts as an 

 adjusting mechanism in variations of blood supply to the 

 central nervous system, (iv) It plays the part of a middle- 

 man between the blood and the nerve cells. 



